Báo cáo khoa học: "Some aspects of phytohormonal participation in the control of cambial activity and xylogenesis in tree stems"

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Some aspects of phytohormonal participation in the control of cambial activity and xylogenesis in tree stems S.Lachaud Laboratoire de Biologie et de Physiologie V6g6tale (URA81), Station de Beau-Site, 25, Biologique rue du Faubourg St-Cyprien, 86000 Poitiers, France wood-latewood transition is associated in Introduction time in Abies basalmea with the largest decrease in the IAA level (Sundberg et Early investigations concerning the regula- aL, 1987). However, according to these tory role of phytohormones in cambial ac- authors, the duration of the cambial activi- tivity were based on the assumption that a ty period appears to be independent of clear correlation exists between hormonal auxin content, and the regulation of this level and response. More recently, precise duration is still poorly understood. During measurements of endogenous hormone the rest period, the IAA level often levels using rigorous techniques have remains relatively high in the cambial often shown the importance of phytohor- zone; treatment with exogenous IAA can- monal intervention being challenged not then induce the resumption of cambial rather than elucidating how these sub- activity. Thus, the responsiveness of cam- stances might actually regulate cambial bial cells to auxin varies with the season. growth. This paper, which summarizes a Their ability to respond, marking the end review in preparation, refers to some of cambial rest, is recovered after expo- recent findings and hypotheses about 3 and chilling temperature (Riding to sure important questions. Little, 1984). Does the seasonal variation in the cam- bial cells’ sensitivity to IAA result from How can auxin (IAA) regulate seasonal changes in their ability to transport auxin? variations in cambial activity and xylo- Several authors have observed a decline genesis? in IAA transport in autumn, but they have different interpretations of the cause. to Little (1981), this change During the period of cambial activity, the According intensity of cell production and some fea- after the cessation of xylem pro- occurs tures of the resulting wood (radial enlarge- duction in A. balsamea, so it cannot account for the onset of cambial rest. ment, vessel development) are often posi- tively correlated with the auxin level in the Other authors describe important qualita- tive changes in the pattern of IAA trans- cambial zone. Furthermore, the early-
port. In Fagus silvatica, the IAA pulse that that occur in cambial cells during the ac- is typical of polar transport in active tivity-dormancy transition (Riding and Lit- cambium (Lachaud and Bonnemain, tle, 1984; Catesson, 1988). In October, 1982) is less intense in September and these cells do not divide, although they disappears from October to December in are metabolic;!lly active; membrane trans- diffusive profiles (Fig. 1 A); its progressive port proceeds then mainly by exo- and renewal starting in late winter can be cor- endocytosis. A renewal of endo-mem- related with different steps of cambial branes during this period might be asso- reactivation (Fig. 1 B). ciated with a seasonal inactivation of The search for an explanation of the auxin receptor and carrier proteins. Later variation in cambial response to IAA may on, the breaking of rest may occur when yield results by paying attention to the the conditions of active membrane trans- important structural-functional changes port are regained.
nous and poorly lignified fibers and of a Is abscisic acid (ABA) involved in the few small vessels, is mainly attributed to regulation of cambial activity and xylo- the presence of a lateral gradient in auxin genesis during the annual cycle? concentration, auxin transport occurring preferentially towards the lower half of the Exogenous ABA can reduce wood produc- bent stem. This hypothesis is supported tion and radial enlargement of tracheids in by experiments showing that exogenous conifers, particularly at the end of summer. IAA induces or suppresses tension wood Latewood differentiation and the cessation formation, when applied to the lower and of cambial activity have often been at- upper sides of an inclined stem, respec- tributed to a high endogenous ABA level tively. Reports concerning the intervention in the cambial zone. However, recent of other phytohormones in this process measurements (Little and Wareing, 1981) somewhat conflicting. are show that ABA peaking in late summer is rather incidental and drought-induced. Several observations indicate that ten- During winter, a decrease in the ABA sion wood induction is a complex process. level, often associated with an increase in The response to gravity, in terms of lateral auxin transport and tension wood forma- conjugated ABA, is frequently reported, tion, is much more important in intact trees but these changes are not clearly correlat- than in isolated branches (Lachaud, ed with the breaking of cambial dormancy (Little and Wareing, 1981Moreover, ABA 1987). Is tension wood formation mediat- ed mainly by asymmetrical auxin distribu- content increases again in reactivating cambium, for example, in the trunk of tion or by changes in cell properties on Pinus contorta (Savidge and Wareing, both sides of the bent stem? Recent ex- 1984), and in young elongating shoots. In periments indicate that proton efflux is enhanced on the lower side of leaning actively growing and well-watered stems, herbaceous stems. At the same time, cal- the cell sensitivity to this inhibitor seems to be low (Powell, 1982). Moreover, ABA cium ions enter the cytoplasm by opening channels, which might then activate IAA mainly appears to enhance stress adapta- tion rather than to regulate active growth. carriers (Pickard, 1985). A new approach to answering the question of tension wood Because its participation in the control of the seasonal variation in cambial activity formation may result from these findings. cannot be explained by simple concentra- tion changes, the role of ABA in this pro- cess remains questionable. Recent data Conclusion suggest that ABA may reach its target sites if it leaks out of the most alkaline cell compartments, but its possible receptors The recent evolution of the phytohormone are unknown in the cambial zone. concept and the considerable progress realized in cytophysiology and biochemis- try prompt the following remarks about the Is the formation of tension wood, on regulation of cambial dynamics: 1) the the upper side of leaning dicot stems, properties of cambial cells, particularly the induced by an asymmetrical lateral dis- pattern of membrane transport, may tribution of phytohormones? change during the annual growth cycle of the tree; 2) the sensitivity of cambial cells to a phytohormone may be low if the This particular xylogenesis, characterized regulator is compartmentalized or if its by the differentiation of numerous gelati-
Little C.H.A. & Wareing P.F. (1981) Control of receptors are seasonally missing or cambial activity and dormancy in Picea sit- modified; 3) in an active cambial cell, phy- chensis by indol-3-ylacetic and abscisic acids. tohormones may regulate the intensity of Can. J. Bot. 59, 1480-1493 sink activity rather than its duration. Pickard B.G. (19.35) Early events in geotropism of seedling shoots. Annu. Rev. Plant Physiol. 36, 55-75 Powell L.E. (1982) Shoot growth in woody References and possible participation of abscisic plants acid. In: Plant Growth Substances. (Wareing P.F., ed.), Academic Press, New York, pp. 363- Catesson A.M. (1988) Cambial cytology and 372 biochemistry. In: Radial Growth of Plants. (Iqbal M., ed.), Research Studies Press, Taunton, Riding R.T. & Little C.H.A. (1984) Anatomy and U.K., in press histochemistry of Abies balsamea cambial zone cells during the onset and breaking of dorman- Lachaud S. (1987) Xylogen6se chez les dicoty- cy. Can. J. Bot. 6.2, 2570-2579 16dones arborescentes. V. Formation du bois de tension et transport de I’acide indole ac6tique Savidge R.A. & Wareing P.F. (1984) Seasonal triti6 chez le hdtre. Can. J. Bot. 65, 1253-1258 cambial activity and xylem development in Pinus contorta in relation to endogenous indol- Lachaud S. & Bonnemain J.L. (1982) Xyloge- 3-yl-acetic and ( acid levels. Can. J. S)-abscisic nese chez les dicotyiddones arborescentes. 111. For. Res. 14, 676-682 Transport de I’auxine et activité cambiale dans les jeunes tiges de h6tre. Can. J. Bot. 60, 869- Sundberg B., Little C.H.A, Riding R.T. & Sand- 876 berg G. (1987) Levels of endogenous indole-3- acetic acid in the vascular cambium region of Little C.H.A. (1981) Effect of cambial dormancy Abies balsames! trees during the activity- state on the transport of [i-!4C]indol-3-ylacetic rest-quiescence transition. Physiol. Plant. 71, acid in Abies balsamea shoots. Can. J. Bot. 59, 163-170 342-348