Báo cáo khoa học: "Maturation of woody plants: review of metabolic and genomic aspects"

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Original article Maturation of woody plants: review of metabolic and genomic aspects a V Haffner F Lardet, MP Carron L Enjalric, IRCA/CIRAD, Laboratoire de Culture in Vitro, avenue du Val de Montferrand, BP 5035, 34032 Montpellier, France (Received 23 November 1990; accepted 15 July 1991) Summary — The first part of this review consists of an evaluation of the bibliographic data on matu- ration studies in woody plants. It reports on the existing knowledge and the remaining questions re- lating to the events which control the transition phase between the juvenile and the mature phase, as well as the causes of the relative stability of these 2 phases. The physiology and molecular biolo- gy aspects are then considered for listing biochemical markers of maturation in woody plants. These markers occur as part of the primary and secondary metabolism (mineral and carbon nutrition, growth regulators, polyamines, phenolic compounds, peroxidase activity) and gene expression (nu- cleic acids, transcription, proteic synthesis). The results considered show that maturation is accom- panied by variations in different - more or less linked - parameters. The discussion on the interven- tion of these parameters in the control of maturation and their use as maturation criteria shows that determination of the mature state should be multifactorial. These considerations point to a new "sys- tem" approach to physiology, based on the relations between the different metabolic systems of plants, and designed for the correlative study of tree development. This approach is intented to fur- ther the understanding of the phenomenon in question, and the determination of reliable maturation criteria. juvenility / maturation / criterion / metabolism / genetic expression Résumé — Maturation chez les plantes ligneuses : synthèse sur les aspects métaboliques et La première partie de cette revue consiste en l’évaluation des données bibliographi- génomiques. ques concernant les études sur la maturation chez les ligneux. Elle rapporte l’ensemble des acquis et des questions concernant les événements physiologiques contrôlant la transition du stade juvé- nile au stade mature, ainsi que les causes de la relative stabilité de ces 2 phases. Les domaines de la physiologie et de la biologie moléculaire sont ensuite considérés afin d’inventorier les marqueurs biochimiques de la maturation des plantes ligneuses. Ces marqueurs interviennent dans le cadre des métabolismes primaires et secondaires (nutrition minérale et carbonée, régulateurs de crois- sance, polyamines, composés phénoliques, peroxydases) et de l’expression du génome (acides nu- cléiques, transcription, synthèse protéique). Les résultats considérés montrent que la maturation Abbreviations: IAA, Indole-acetic acid; GAs, gibberellins; CKs, cytokinins; NAA, naphthalene-acetic acid; ABA, abscisic acid; ATP, adenosine triphosphate; NTP, nucleotide triphosphate; GDP, guano- sine diphosphate; GTP, guanosine triphosphate; RNA, ribonucleic acid; DNA, desoxyribonucleic acid.
s’accompagne de variations au niveau de certains paramètres plus ou moins liés entre eux. L’inter- vention possible de ces paramètres dans le contrôle du processus de la maturation et leur utilisation comme critères de maturation sont discutés. Il apparaît que la détermination de l’état mature d’une plante ligneuse serait multifactorielle. Ces considérations débouchent sur l’évocation d’une nouvelle approche de la physiologie, de type «système», basée sur les relations existant entre les différents systèmes métaboliques des plantes, et préconisée pour l’étude corrélative du développement des arbres. Cette démarche est proposée pour avancer dans la compréhension du phénomène et la dé- termination de critères fiables de la maturation. juvénilité / maturation / critère / métabolisme / expression génétique THE MATURATION PHENOMENON which is considered as the means of maxi- mum rejuvenation for trees (Bonga, 1982). Many other woody species exhibit this pro- It has been known for many years that cess, which has been reviewed by several measurement of plant age chronological authors (Borchert, 1976a; Fortainer and does not allow universal interpretation of Jonkers, 1976; Chouard, 1977; Bonga, the different phases of physiological devel- 1982; Hackett, 1985; Zimmermann et al, opment in plants, and that plant aging has 1985; Greenwood, 1987). 2 different aspects: physiological aging (senescence), which corresponds to the increase in size and/or structural complexi- Distinction between growth ty of the plant (Borchert, 1976a), and onto- and maturation genetic aging, which is localized in the meristem, at the level of the individual cell or of the entire meristem (Hackett, 1985). Most of the studies on this subject are de- Maturation is a developmental process, scriptive; comprehensive studies are still described in woody plants in particular, rare and the events which regulate matura- and characterized by a reduction in the tion are therefore not yet known (Green- growth rate and rooting aptitude of wood et al, 1989). The authors have dis- cuttings, by changes in morphological pa- whether cussed (Hackett, 1985) rameters and by the onset of flowering. maturation corresponds to ontogenetic The reliability of this last parameter is dis- (Greenwood, 1984) or to physiological ag- cussed due to its dependence on environ- ing (Borchert, 1976a). These 2 processes should affect both plant development and mental conditions (Wareing, 1971).The determine its lifespan (Fortainer and Jonk- usual plant used to illustrate the matura- ers, 1976). But the general definition of the tional phenomenon is Hedera helix, which juvenile and mature phases as a "full- has often been used to study this process vigor" phase (Assman, 1970), where annu- since Doorenbos (1954) described the al growth increment reaches a maximum morphogenetic changes between the juve- value followed by a mature phase where nile and mature phases of this plant. The cuttings of Hedera helix retain the morpho- annual growth increment declines and then stabilizes does not allow determination of logical and physiological characteristics of the mother plant (Doorenbos, 1965). In the role of tree size or maturation state as contrast, the mature characteristics are a basis for these 2 phases (Greenwood, 1989). Greenwood et al (1989) reported eliminated during the formation of zygotic that while increasing size and complexity or nucellar embryos (Borchert, 1976a),
may affect the onset of phase change in difficulty in propagating some species required for the (Franclet, 1979; Araucaria, Sequoia). This Loblolly pine, they are not maintenance of mature shoot growth char- is in agreement with the genetic determina- acteristics resulting from changes in the tion of maturation in the meristematic cells, cells of the apical meristem. instead of the correlative control of the phenomenon by other differentiated parts Actually, annual growth increment of the plant. However, the use of in vitro should be in part determined by the matu- methods has resulted in the propagation of ration state of the tree, which in turn is a some species which were recalcitrant to function of size (Greenwood, 1989). This classical propagation methods: 7-month definition leads to a discussion on the rela- old Eucalyptus (De Fossard et al, 1973), tion between juvenile state and vigor, wild cherry tree (Riffaud and Cornu, 1981) which have often been associated (Green- and Hevea (Dublin et al, 1991).This led wood, 1984; Legocka, 1989). In Eastern Hackett (1983) to consider the level of ju- larch, the vigor of a shoot, measured by venility or maturation of a plant as an equi- the proportional growth increment, is asso- librium than can be reversed under certain ciated with a quantitative contribution on conditions instead of an irreversible state. the part of the root system, depending on Thus, the efficiency of the propagation the distance between the shoot and the technique must be questioned in any dis- root (Greenwood, 1989). On the other cussion on the propagation aptitude of one hand, the mature characteristics, such as plant. Nonetheless, there are few methods growth increments and chlorophyll content, for regeneration of plants from tissues of foliar morphology and reproduction compe- coniferous trees once they have passed tence (Greenwood et al, 1989) are asso- the embryonic or seedling stage (Green- ciated with an inability of the shoot de- wood, 1987). Some recent studies even pending on its age, to use the root system show that tissue culture plantlets derived inputs (Greenwood, 1989). This is in from embryonic tissue of pines behave like agreement with the hypothesis that the mature trees (McKeand, 1985; Green- system receiving the signals for vegetative wood, 1987). Then, we do not yet know programming of the meristem is located whether maturation occurs in all the woody within the meristem itself, but that meri- and non-woody plants or whether the phe- stem function is also sensitive to signals nomenon is characteristic of only the received from the environment and from woody plants, even if its expression can elsewhere in the plant (Sussex, 1989). greatly differ between certain woody spe- Thus, whether the cause of maturation cies (for example, coniferous and other is self-programming of the meristem or sig- woody plants often do not behave in a sim- nals from the other parts of the plant, the ilar manner). expression of maturation occurs through The intrinsic determination of the meris- changes in the activity of the apical meri- tematic cells in the mature phase should stem (Borchert, 1976a) and cambia (Bon- be either biochemical or biophysical, and ga, 1982). should correspond to a different ability of the cells to react to developmental signals Determination of the mature state emitted by differentiated parts of the plant. in the meristem This change in competence of the meriste- matic cells persists in the absence of an in- Maturation has long been considered as itiating stimulus and could be related to the irreversible process with regard to the number of divisions the apex has under- an
(Hackett, 1980). In animals, there is cell division, whereas areas of weak mitot- gone ample evidence that the decline in ability ic activity may age more slowly. The areas of cells to divide with increasing age is a of weak mitotic activity would be very controlled developmental phenomenon, stable genetically because of their low mu- proportional to the number of cell divisions tation rate, related to the small number of (Greenwood, 1984). Sussex (1976) dis- cell divisions. cussed the existence of systems measur- As a matter of fact, Charlesworth (1989) ing the developmental time, associated supports the idea that long-lived plant spe- with the cell cycle of plants, and Green- cies show high mutation rates because of wood et al (1989) agreed with the possibili- the great number of divisions before gam- ty that a developmental time-clock resides ete formation. Plants have characteristics in the meristem, although their data on allowing the accumulation of somatic muta- Loblolly pine do not directly indicate that tions: lack of a separated germline, open maturation is proportional to the amount of system of growth, flexible meristem organi- mitotic activity that has occurred in the api- zation and the fact that most somatic muta- cal meristem. In the same manner, Bonga tions are not immediately life-threatening (1982) reported that the differences in de- (Klekowsky, 1988). This led Klekowsky gree of juvenility between different shoot and Godfrey (1989) to recall that this is apical meristems in the tree could be relat- one reason for the accumulation of muta- ed to the number of cell divisions that sep- tions in the meristematic initials as the arate each meristem from the original em- plant ages. bryo shoot apex. So the question of genetic stability in Thus, plants could assume some loss in plants concerns the mechanisms available juvenility in each successive division, but to reduce the impact of mutational load. only up to a point (Bonga, 1982). As a The critical point is whether mutant cells matter of fact, even though mature charac- are maintained in apical or even in cambial teristics may be transmitted through the meristems or whether the mutant cells are first generation of vegetative propagules, lost to tissues and organs that soon be- later generations do not become progres- come metabolically moribund (Klekowsky, sively more mature. Furthermore, the ap- 1988). Many characteristics of the apical plication of certain technics of propagation, meristems can affect the loss or fixation of especially in vitro methods, to some spe- somatic mutations: number of cell divisions cies leads to rejuvenation (Mullins et al, undergone by the initials per node of 1979 in Vitis). growth, organization in "méristème d’at- tente" or in "tunica-corpus" or unstratified organization, number of periclinal divisions Genetic stability of the juvenile resulting in the movement of cells between or mature state the different layers of the meristem, and the changes in these parameters during growth (Klekowsky, 1988). The genetic stability of the juvenile or ma- ture phase in certain species may be due Apical meristems also have characteris- to the more or less important presence of tics that can modify the effective mutation weak mitotic activity cells in the main meri- rate. Mutation rate per biological time unit stem (Bonga, 1982). The areas of strong is in part a function of the number of times mitotic activity may age more quickly, un- a genome has been replicated and chro- maturation with each matids divided during the biological time dergoing progressive
type of active substances in plant metabo- unit. Thus, the maintenance of cell pools within the meristem which seldom divide lism are affected by environmental factors, but which give rise to meiocytes (such as and the levels of these substances differ the "méristème d’attente") may reduce mu- between juvenile and adult plants (Zimmer- tation rates (Klekowsky, 1988). mann et al, 1985). The analysis can even reach the molecular level concerning Consequently, the balance between the mechanisms of gene expression in plants appearance, the fixation or the loss of so- (Bon, 1988c). The evolution of these pa- matic mutations within the apical meristem rameters during maturation and their possi- could interfere in the determination of the ble use as criteria are reviewed below. juvenile or mature phase in the apical meri- stem, the persistence of these phases through generations of vegetative propaga- Carbohydrates and other parameters tion, the reversion of the mature phase to of carbon metabolism the juvenile phase for certain vegetatively propagated species especially by in vitro culture, and the differences in juvenile In 1967, Wareing and Seth showed that state between different parts of one plant. carbohydrate synthesis varies during matu- Finally, one of the most significant losses ration. More recently, maturational varia- of mutation occurs during sexual reproduc- tions of cellulose and lignins have been tion (Klekowsky, 1988) which is also the characterized in Pinus radiata (Uprichard means of maximum rejuvenation in trees. and Llyod, 1980). Observations on leaves of Sequoiadendron giganteum (Monteuuis Then, if genes control the ontogeny and and Genestier, 1989) have shown that pa- the final form of an organism, both the spe- cific pattern of ontogeny and the final form rietal polysaccharides of the mesophyll, of an organism may have repercussions particularly cellulose and hemicellulose, in- upon the integrity of the genes (Klekowsky crease in mature trees. et al, 1989). The objective of comprehen- from variations in Apart carbohydrate sive studies of maturation should concern levels, maturation is characterized by occurrences during sexual rejuvenation changes in the levels of many enzymes (Bonga, 1982). (Zimmermann et al, 1985), generally relat- ed to carbon metabolism (amylase, cata- oxidase, alkaline and lase, cytochrome c METABOLIC CRITERIA acid phosphatases, ascorbic acid oxidase). OF MATURATION The juvenile phase of Hedera helix is char- acterized by weaker photosynthetic activity (Bauer and Bauer, 1980), associated with Morphological, physiological, and histocy- a reduction in the activity of the photosyn- tological parameters have been used in thetic apparatus (activity of the ribulose the descriptive of juvenility in many spe- 1,5-diphosphate carboxylase) and with an- cies (Wareing and Frydman, 1976; Zim- atomical features of the leaves (reduction mermann et al, 1985). Biochemical param- of stomatal frequency, number of chloro- eters related to general metabolism or/and plasts per cell, leaf thinness). Lastly, dur- genetic expression could allow a quantita- ing the growing phase of hybrid walnut, the tive approach to the phenomenon. As a pentose phosphate and the amino acid matter of fact, factors such as the physio- degradation pathway function in a synchro- logical state or histocytological structure of nous and moderate manner in juvenile a plant, the concentration, distribution, and
plants, whereas the 2 pathways function in 1989) and with anthocyanin synthesis asynchronous but accelerated manner (Rembur and Nougarede, 1989), a param- an in adult plants (Drouet et al, 1989). eter that can vary with maturation, as well the relation of the concentrations of zinc as Thus, all the levels of carbon metabo- (Cakmak et al, 1989) and manganese lism seem to be implicated in maturation, (Tomasewski and Thimann, 1966) with so the determination of one or several cri- auxin metabolism, could be in agreement teria connected with this field would be with this. rather long and tedious. Thus, the K/Na and K/Ca ratios inverse- ly evolute during the increase intree size Mineral elements and in the course of meristem maturation. They could then be used to distinguish be- tween these 2 maturation processes. The concentrations of chlorine, potassium, and sodium increase with dry weight in buds of mature Picea abies (Von Arnold Polyamines and Roomans, 1983). Sodium increases more quickly than potassium, which is why the K/Na ratio decreases with physiologi- Recently a role in rejuvenation has been cal age (Von Arnold and Roomans, 1983). attributed to polyamines by Georges et al In the same manner, buds of Sequoia (1989), who reported that putrescine and sempervirens have different calcium and spermidine increase when Asparagus is potassium levels depending on their posi- rejuvenated by micropropagation and that tion on the parent plant and the K/Ca ratio there is a close correlation between the decreases with age (Vershoore-Martouzet, level of endogenous polyamines and the length of the rejuvenated phase. On the 1985). The potassium level is known to de- crease in aged tissues in favor of the new contrary, polyamines have been implicated tissues (appeal mechanism) and, in con- in the loss of totipotency during maize tis- trast, the aged tissues accumulate cal- cultures (Tiburcio et al, 1989), and the sue cium. Thus, the K/Na and K/Ca decrease ratio of putrescine to spermine in inter- could be associated with the increased nodes has been found to increase with size of the aged trees rather than with mat- stem age in this plant (Schwartz et al, uration. 1986). On the other hand, in Douglas fir, reju- The results of these studies are con- venation produced by in vitro subculturing fused and do not concern woody plants. is characterized by a decrease in the K/Na But the role of polyamines in the cell cycle and their suggested role in the regulation ratio (Bekkaoui, 1986). More recently, the K/Ca ratio has been recommended for use of senescence and morphogenesis (Gals- as marker of juvenility in Douglas fir and ton and Sawhney, 1990) indicate that poly- amines could be implicated in the regula- eucalyptus cultivated in vivo under very precise conditions, and with other criteria tion of maturation in woody plants. for in vitro culturing (Dechamps, 1986). These data should be in favor of a relation Phenolic compounds and anthocyanins between the increase of these ratios and maturation of the meristem. Furthermore, the interaction of the potassium metabo- Many studies indicate a direct relationship lism with growth regulators (Erdei et al, between phenolic compounds and juvenili-
ty, rejuvenation, and maturation. Qualita- greater extent in phloem. Hy- to an even tive variations of drojuglone glucoside accumulates at the polyphenols occur during plant ontogenesis (Vieitez and Vieitez, beginning of the growing phase in rejuve- 1976). Moreover, the accumulation of hy- nated individuals, while myricitrine accu- droxycinnamic amides is related to the mulates in mature individuals along with flowering and differentiation rate, and there PAL (Claudot, 1989). The authors conclud- is a difference (unconfirmed) between the ed that the organogenetic capacity of dif- juvenile phase, which is lacking in amides, ferent tissues and their activities can be and the mature phase, possessing amides modified during maturation by variations in (Cabanne et al, 1981).The number of phe- the levels of phenol compounds involved in nol compounds increases with maturation different biological processes. Thus, phe- in the chestnut (Garcia et al, 1980). But nol metabolism varies in walnut during ag- phenol compounds cannot be used as ing, so that maturation is qualitatively and morphogenic markers in giant sequoia be- quantitatively characterized by different cause of large clonal variations in poly- phenolic compounds. phenol levels and a lack of synchronization in the physiological states of the plant ma- terial (Monteuuis and Bon, 1986; Bon et al, Hormones 1988). The mature phase appears to be char- Phytohormones are involved in regulating maturation. For instance, maturation phase acterized by inactivation of one or more enzymes involved in biosynthesis of active changes in birch are related to large phyto- polyphenols and flavonoids (Hackett et al, hormone changes in buds and apical part 1989). For instance, if the specific activity of the stem (Galoch, 1985). Furthermore, of phenylalanine ammonia lyase (PAL) in rooting potential, which is directly related mature tissue of Hedera helix is twice that to juvenility, appears to be controlled by observed in juvenile tissues, the accumula- relative phytohormone levels (Gaspar et al, tion of anthocyanins in mature tissue of 1977; Okoro and Grace, 1978; Baz et al, this plant may be due to inactivation of de- 1984a; Berthon et al, 1989; Chin et al, hydroquercetin reductase (DQR). 1989). Severe pruning of walnut tree strongly The action of auxin is related to rejuve- affects the phenol content in new shoots, nation. For instance, auxin has a negative which is similar to that in juvenile individu- effect on plagiotropy in conifers, which is als (Jay-Allemand et al, 1987). In the same associated with maturation (Chaperon, manner, rejuvenation in hybrid walnut has 1979); mature cuttings of Ficus pumila re- also been characterized by 3 ratios of 5 dif- quire twice as much auxin for rooting as ferent polyphenols during the growth peri- juvenile cuttings (Davies, 1984). This indi- od (Jay-Allemand et al, 1988). The study cates that the auxin level decreases in the mature parts of the tree: either the mature also examined 2 phenol compounds, hy- drojuglone glucoside and myricitrine, which meristem supply decreases or/and the show significant differences depending on auxin transport cannot reach the roots be- physiological state in walnut (Jay-Allemand cause of the increased size of the tree. But et al, 1989). The first one is presumed to during maturation the auxin level seems to act as a biological accelerator and the sec- decrease less rapidly than the cytokinin ond as a brake system. Moreover, these 2 level, because Douglas fir maturation is polyphenol markers of hybrid walnut reju- characterized by a decrease in the zeatin/ venation accumulate in sclerenchyma and indole acetic acid (Z/IAA) and zeatin-
riboside/indole acetic acid (ZR/IAA) ratios via root activity related to juvenility or (Maldiney et al, 1986). On the contrary, (Franclet, 1981).CKs induce apex rejuve- maturation of Sequoia sempervirens, char- nation in mature trees of the species Pseu- acterized by a fall in its cloning capacity, is dotsuga menziesii (Bakkaoui, 1986) and accompanied by an increase in the abscis- Picea abies (Tsogas and Bouriquet, 1983), sic acid/indole acetic acid (ABA/IAA) ratio characterized by reactivation. On the con- (Fouret et al, 1986). a relationship has been demonstrat- trary, ed between an increase in CK level and Gibberelins (GAs) are implicated in the lack of rooting capacity in poplar (Okoro morphological reversion of adult leaves of and Grace, 1978). Finally, in conifers, Ben- Hedera helix to the juvenile type (Rogler zyl-adenine (BA) has been implicated in and Dahmus, 1974). The natural gibberel- both promotion and reversal of maturation lic substance GA3 participates in rejuvena- (Greenwood, 1987). With regard to devel- tion (Rogler and Hackett, 1975a; Wareing opment, CK levels decrease in more vigor- and Frydman, 1976; Hackett, 1985). The ous cultivars of apple (Looney et al, 1988). direct effect of GA3 appears to concern el- ongation, depending on the dose, whereas Abscissic acid (ABA) indirectly affects its indirect effect is connected with mor- exerting its action on mature senescence, phological changes related to rejuvenation and older organs by inducing the produc- (Wallerstein and Hackett, 1989). Thus, tion of a senescence factor that controls GA3 seems to induce rejuvenation without ethylene synthesis (Milborrow, 1974). In affecting the persistence of the juvenile addition to its effect on senescence, ABA phase, perhaps in relation with the auxinic is involved in maturation. The mature metabolism (Wallerstein and Hackett, phase is characterized by higher ABA lev- 1989). The role of GAs in maturation is still els than the juvenile phase (Hackett, 1985; under discussion, because although they Galoch, 1985; Fouret, 1987), the Z/ABA reverse the mature phase of Hedera helix and ZR/ABA ratios decrease with matura- (Zimmermann et al, 1985), this group of tion (Maldiney et al, 1986). Rogler and substances promotes flowering in conifers, Hackett (1975a) have reported that the which remains a mature characteristic. GA3/ABA ratio has more importance than Moreover, the primary role of GAs in con- the absolute values of the 2 substances in trolling maturation is questioned by Green- controlling reversion from the adult to the wood et al (1989), because while GAs pro- juvenile phase in Hedera helix and that mote flowering in many conifers, their stabilization of the mature form by ABA probably occurs via regulation of the GAs application often cannot offset a genetic in- disposition of trees to flower. Lastly, with level in the plant (Rogler and Hackett, regard to development, GAs are associat- 1975b). ed with the vigor of apple trees in vivo Thus, even if the role of the root- (Lonney et al, 1988). Vigor has often been produced plant growth regulators (GAs related to juvenility (Greenwood, 1984; and CKs) in the process of maturation is Looney et al, 1988). still confusing (Greenwood et al, 1989), the The relation between cytokinins (CKs) auxins, gibberellins, cytokinins and abscis- and maturation is also still open to discus- sic acid are related to the maturation phe- sion. CKs affect the reactivity and growth nomenon. Furthermore, the ratios between of buds in many species, either directly these 4 phytohormones seem to better de- termine the induction and the stabilization (Von Arnold and Tillberg, 1987; Label et al, 1988; Pilate et al, 1989; Young, 1989) of the phase change than their respective
growth (Poessel et al, 1982). This makes it absolute values. Moreover, their interac- possible to base the selection of cuttings tion and their transport from the synthesis for propagation on their peroxidase content site to the active site probably make them (Quoirin et al, 1974 in Prunus species; Mo- interfere in the physiological as much as in sella-Chancel, 1980 in Prunus persica). the ontogenetic ageing processes. The narrow relation between peroxidasic With regard to rejuvenation, elevated activity and rooting makes this parameter a ethylene levels in the culturing atmosphere good criterion of the physiological state of of Hemerocallis plantlets have been corre- certain species with regard to their propa- lated with transition from the juvenile to the gation capacity, but no relation with the adult phase, which is accompanied by his- meristem maturation has yet been demon- al, 1989). tological changes (Smith et strated in woody plants. Moreover, difficult-to-root petioles of ma- ture Hedera helix produce more ethylene than juvenile petioles (Georges et al, 1989; GENOMIC CRITERIA AND GENETIC Geneve et al, 1990a, b). These authors in- EXPRESSION OF MATURATION dicate that ethylene does not seem to play a significant role in the different rooting re- Nucleic acid composition varies between sponses of juvenile and mature petioles juvenile and mature phases (Zimmer- the treated with naphthalene acetic acid mann et al, 1985). After considerable con- (NAA). In contrast, it appears to have an troversy with regard to desoxyribonucleic inhibitory effect during adventitious root el- acid (DNA) differences, it would appear ongation on juvenile petioles. The role of that the DNA levels of 2c cells do not dif- ethylene in maturation has still been insuf- fere in juvenile and mature tissue of Hede- ficiently studied, but these results indicate ra helix (Zimmermann et al, 1985). On the that these phytohormones should be con- other hand, several authors have found dif- sidered parameter of maturation. as a ferences in total, soluble, and ribosomal ri- bonucleic acid (RNA) levels in the 2 phases (Zimmermann et al, 1985). Wareing and Peroxidasic activity Frydman (1976) have reported quantitative and qualitative RNA variations during mat- considered Peroxidases markers of are as uration of Hedera helix. In Ficus pumila, to- rooting potential (Quoirin et al, 1974; Ranjit tal RNA levels are higher in juvenile indi- et al, 1988; in Prunus; Moncousin and Du- viduals (Davies, 1984). In addition, the creux, 1984 in Cynara scolymus; Berthon RNA levels and cambial activity in this et al, 1987 in Sequoiadendron giganteum; plant increase during maximum rooting, Gus’kov et al, 1988; De Klerk et al, 1989 in and these phenomena are more pro- apple) and some authors also consider nounced in juvenile individuals. Thus, the them to be good biochemical markers of transcription of specific genes should inter- juvenility and rejuvenation of Cynara scoly- fere in the determination of the juvenile mus (Moncousin, 1982; Moncousin and and mature phases. Ducreux, 1984). However, the conclusions In studies of gene expression, it has of Dalet and Cornu (1989) on Prunus avi- been found that DNA coding for ribosomal um do not agree with the other findings. RNA in the 2 forms shows no differences The peroxidase content in plants in redundancy (Dommoney and Timmis, some has been correlated with the potential for 1980; Hackett, 1985). In contrast, it is pos- grafting, micropropagation by cuttings, and sible to isolate cDNA clones specific to the
juvenile and mature phases (Hackett, (Greenwood, 1984). These facts led 1985). It has been proposed that only a Hutchinson et al (1987) to feel they would few genes are active in the mature phase succeed in identifying sequences that were (Zimmermann et al, 1985). Consequently, differentially expressed between juvenile the RNA transcribed from these genes and mature plants. would only represent a small portion of to- On the energetic level, in giant sequoia, tal DNA, suggesting that the molecular ba- the higher RNA/DNA ratio in juvenile apex sis of phase change depends on an altera- during vegetative dormancy is associated tion of the transcription rate of certain DNA with a higher adenosine triphosphate sequences (Zimmermann et al, 1985). (ATP)/nucleotide triphosphate (NTP) ratio This alteration of the transcription rate (Monteuuis and Gendraud, 1987). During could be associated with the methylation growth reactivation, juvenile and mature of cytosine in DNA, since older trees of Pi- apices of giant sequoia show no differen- cea abies showed a greater cytosine ces with regard to the RNA/DNA ratio, methylation than the youngest ones (Wes- whereas the ATP/NTP (Monteuuis and cott, 1987). But this is not the case for La- Gendraud, 1987) and guanosine diphos- rix laricina, in which the morphological fo- phate (GDP)/guanosine triphosphate liar differences related to age were not (GTP) (Bon, 1988a) remain high in the associated with any difference in the cyto- juvenile individuals. The stimulation of pro- sine methylation in DNA (Greenwood et al, tein synthesis in buds of giant sequoia at 1989). However, the methods used would the time of bud burst persists in juvenile not detect methylation of only one or a few buds, but is quickly repressed in mature genes. deficit in energy buds, in correlation with a derivatives such as GTP (Bon, 1988a). Moreover, in Larix laricina, the purifica- tion of RNA did not show any difference and Frydman (1976) and Aghi- Wareing between juvenile and mature trees, sug- on (1978) have observed qualitative and/or gesting that maturation is not the result of quantitative differences in proteins during a general decline in the level of transcrip- maturation in Hedera helix. However, the tion in meristematic tissues (Hutchinson et differences were not great enough to dis- al, 1987). But differential gene expression, tinguish juvenile and adult clones of walnut associated with development, could be (Drouet et al, 1989). Recently, Hackett et masked by variations in genetic back- al (1989) have shown that the juvenile and ground. On the other hand, maturation is mature phases of Hedera helix can be accompanied by variations in the levels of characterized by 2 polypeptides; and Bon chlorophyll and ribulose 1,5-diphosphate and Monteuuis (1987) have reported that carboxylase activity (Bauer and Bauer, rejuvenation of Sequoiadendron gigan- 1980; Hutchinson et al, 1987). Now, if no teum by micrografting is accompanied by a maturation-related expression of the gene decrease in meristem proteins. A mem- of the small subunit of this enzyme has brane protein (J16) specific to juvenile indi- been observed, the chlorophyll a/b binding viduals and individuals rejuvenated by api- protein is differentially expressed in juve- cal micrografting has been detected by 2- nile and mature plants (Hutchinson et al, dimensional electrophoresis (Bon, 1988b). 1987). In addition, the gene coding for the Moreover, culturing of meristem from a cab-protein, which is associated with pho- 100-year-old individual produced protein tosystem II, is expressed differently de- J16 along with juvenile organogenetic pending on the maturation status of larch properties (Bon, 1988c). Two-dimensional
electrophoresis of proteins has also been products or regulatory activities of many used to distinguish vegetative, prefloral, different genes. The question remains of and reproductive apices of Prunus on the whether the maturational changes for one basis of certain polypeptides (Ranjit et al, species vary independently or not. Bor- 1988). chert (1976b) suggested that they vary in- dependently of one another. Results of Thus, the genetic expression seems to Greenwood et al (1989) concerning larch be different in the juvenile and mature do not necessarily support this view, but phases. This is observed by differences in rather suggest that a single process may RNA, proteins and energetic componds affect groups of traits during maturation. In and will soon probably be related to differ- any case, the maturation changes are ex- ences in the transcription rate and genom- pressed through the physiological state of ic state of the 2 phases. Furthermore, mat- the plant, which could not be characterized uration should not only be determined by by a single biochemical parameter. the nucleus, but also by the accumulation of self-replicated DNA, located in the orga- The control of development, ie matura- nelles of the cytoplasm, which could be tion, is often considered to be a process transmitted over many cell generations that is accessible by a simple experimental (Bonga, 1982). approach, whereas metabolism, cells, tis- sues, and the whole plant form systems with many interconnections or "networks" CONCLUSION (Trewavas, 1986). Data concerning the control of these systems are limited and very controversial. Trewavas (1986) The present review of criteria for matura- showed a new "systems" approach to de- tion shows that expression of the juvenile velopmental physiology. The relationships or mature phase of a plant might be con- between metabolic networks form the ba- trolled by different related factors. The car- sis of this approach, which soon was sug- bon, mineral and phytohormonal metabo- gested in a correlative study of maturation lisms and the secondary metabolism are in which a "systems" theory was applied to implicated in the regulation of maturation in (Borchert, 1976b). trees woody plants, at the levels of physiological aging of the tree as well as maturation of An illustration of the "systems" approach the meristem. The variations in all these is given by the fact that at least the mineral parameters are the result of the genetic ex- elements and especially the phytohor- pression (protein synthesis, transcription, mones appear to be implicated in both genes), in which the basis of the matura- physiological aging of the tree and ontoge- tion phenomenon is found. However, the netic aging of the meristem. The meristem phenomenon is complex in woody plants can be considered as a "system", whose (Monteuuis, 1988): it depends on a physio- behavior depends on its self-programming logical context, can vary and even be reit- and on the signals received from the other erated (Nozeran, 1978) and can differ in plant organs (Sussex, 1989). Hence, deter- various plant parts (Chaperon, 1979). mination of the maturation status of a There is a temptation to simplify the whole or fractionated plant and the possi- study of maturation by trying to obtain a bility of predicting its behavior under partic- single biochemical marker of juvenility un- ular conditions will probably depend on the related to physiological markers (Bon, combined development of several criteria 1988c). But the control mechanisms for which remain consistent with physiological maturation probably the result of the criteria. are
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