Structure and functions of the vascular cambium.

Following a general outline on the functioning of the cambium, the authors review the data acquired over the last 20 years. The interest is focused on: i) the ultrastructural characteristics of the two kinds of initials; ii) the early structural, metabolic and molecular modifications occurring during the first stages of daughter cell differentiation into either xylem or phloem; iii) the complex rhythmic changes of structure, metabolism and activity undergone by cambial cells during the seasonal cycle; iv) the characteristics and control of the cessation of cambial activity in autumn and of its reactivation in spring; v) the main research approaches in cell and molecular biology presently open to the students of the cambial meristem.     

Free and Bound Gibberellin Activities and Ent-kaurene Synthesis during Induction of Cold Hardiness in Black Locust Seedlings

The role of gibberellin in the development of cold hardiness in black locust (Robinia pseudoacacia L.) seedlings was investigated. Free and bound gibberellin activities were followed during induction of cold hardiness using ethyl acetate partitioning and pH variation, with subsequent paper chromatographic fractionation and gibberellin bioassay. While total gibberellin activity decreased during the induction of cold hardiness in black locust seedlings, no convincing evidence was found to support conversion of free gibberellin to a bound form. However, bound gibberellin activity did appear to be more stable than did free gibberellin activity during the final stages of cold hardening at freezing temperatures. Gibberellin synthesis was followed using ¹⁴C-mevalonate conversion to ent-kaurene in a cell-free extract of the tissue. Ent-kaurene synthesis decreased during cold hardening with no detectable synthesis in fully hardened seedlings. However, since growth cessation precedes development of cold hardiness, decreased gibberellin synthesis and corresponding trends in free and bound fractions might have been expected, and a cause and effect relationship is difficult to establish. Even so, a decline in one step in gibberellin synthesis and a greater stability of bound than free gibberelin activity are associated with induction of cold hardiness in black locust seedlings.     

ABP1 expression regulated by IAA and ABA is associated with the cambium periodicity in Eucommia ulmoides Oliv

A cDNA clone of Eucommia ulmoides Oliv. encoding auxin binding protein 1 (ABP1), one of the putative receptors of auxin, was isolated, and the seasonal expression of ABP1 in relation to IAA and ABA annual variation was investigated by different technical approaches including RT-PCR, real-time PCR, northern blotting, western blotting, and immunolocalization. In the cambial region, ABP1 expression at both the protein and the mRNA level was found to be high, low, and remarkably scarce in the active, quiescent, and resting stages, respectively, during cambium periodicity. The signal abundance of ABP1 follows the opposite pattern to ABA accumulation and correlates with auxin responsiveness of the cambial tissues, suggesting a role for ABP1 in mediating auxin-dependent regulation of cambial activation in the activity-dormancy cycle. This paper attempts to explain why IAA would 'boost' the reactivation of a quiescent cambium, and not that of a resting cambium. Results also show that ABP1 expression is improved by IAA, while inhibited by ABA.     

提高植物抗寒性的机理研究进展

低温胁迫是世界范围内影响植物产量和品质的主要非生物胁迫.植物抗寒生理生态研究是比较活跃和发展很快的领域.文章综述了提高植物抗寒性机理的研究进展.大量科学研究和生产实践表明,气象因素与植物自身因素是影响植物抗寒性的关键因素,前者主要是温度、光周期和水分,后者主要是植物的遗传学基础、生长时期、发育水平以及低温胁迫下细胞的抗氧化能力.保证植物抗寒基因充分表达对提高植物抗寒性有重要意义.植物抗寒性的遗传机制与调控主要通过5条路径实现:丰富多样的植物低温诱导蛋白,低温转录因子DREB/CBF可同时调控多个植物低温诱导基因的表达,DREB/CBF与辅助因子相互作用调控下游基因表达,Ca2+、ABA及蛋白质磷酸化上游调控低温诱导基因表达,以及不饱和脂肪酸酶基因的表达.基因工程改良植物抗寒性已获重要进展,但距产业化尚有许多开创性的工作要做,目前主要通过导入抗寒调控基因和抗寒功能基因而实现,后者主要是导入抗渗透胁迫相关基因、抗冻蛋白基因、脂肪酸去饱和代谢关键酶基因、SOD等抗氧化系统的基因以及与植物激素调节有关的基因.农林技术对提高植物抗寒性有重大实用价值,其中的不少技术蕴涵着深刻的科学机理,重点评述了抗寒育种、抗砧嫁接、抗寒锻炼、水肥耦合及化学诱导五大技术提高植物抗寒性的作用机理.展望了提高植物抗寒性的研究.     

Low night temperature and inhibition of gibberellin biosynthesis override phytochrome action and induce bud set and cold acclimation, but not dormancy in PHYA overexpressors and wild-type of hybrid aspen

Juvenile trees of temperate and boreal regions cease growth and set buds in autumn in response to short day-lengths (SD) detected by phytochrome. Growth cessation and bud set are prerequisites for the development of winter dormancy and full cold hardiness. In this study we show that the SD-requirement for bud set and cold hardening can be overcome in hybrid aspen (Populus tremula L. × tremuloides Michx.) by low night temperature and inhibition of gibberellin (GA) biosynthesis. Bud set and increased cold hardiness were observed under normally non-inductive long day-length (LD) in wild-type plants, when exposed to low night temperature and paclobutrazol. In addition, the effect of PHYA overexpression could be overcome in transgenic plants, producing bud set and cold acclimation by treatment with: SD, low night temperature and paclobutrazol. After cold acclimation, the degree of bud dormancy was lower for wild-type plants prior treated with LD and transgenic plants (overexpressing PHYA), than SD-treated, wild-type plants. Thus, low night temperature in combination with reduced GA content induced bud set and promoted cold hardiness under normally non-inductive photoperiods in hybrid aspen, but was unable to affect development of dormancy. This might suggest separate signalling pathways from phytochrome regulating the induction of cold/cold hardiness and bud dormancy in hybrid aspen or alternatively, there might be one pathway that fails to complete its action in the transgenic and paclobutrazol treated plants.     

Effect of frost nights and day and night temperature during dormancy induction on frost hardiness, tolerance to cold storage and bud burst in seedlings of Norway spruce

For trees, the ability to obtain and maintain sufficient levels of frost hardiness in late autumn, winter and spring is crucial. We report that temperatures during dormancy induction influence bud set, frost hardiness, tolerance to cold storage, timing of bud burst and spring frost hardiness in seedlings of Norway spruce (Picea abies (L.) Karst.). Bud set occurred later in 12°C than in 21°C, and later in cool nights (7°C) than in constant temperature. One weekly frost night (−2.5°C) improved frost hardiness. Cool nights reduced frost hardiness early, but improved hardiness later during cold acclimation. Buds and stems were slightly hardier in 21°C than in 12°C, while needles were clearly hardier in 12°C. Cold daytime temperature, cool nights and one weekly frost night improved cold storability (0.7°C). Seedlings receiving high daytime temperatures burst buds later, and were less injured by light frost some days after bud burst.     

Gibberellin homeostasis in tobacco is regulated by gibberellin metabolism genes with different gibberellin sensitivity

Gibberellins are phytohormones that regulate growth and development of plants. Gibberellin homeostasis is maintained by feedback regulation of gibberellin metabolism genes. To understand this regulation, we manipulated the gibberellin pathway in tobacco and studied its effects on the morphological phenotype, gibberellin levels and the expression of endogenous gibberellin metabolism genes. The overexpression of a gibberellin 3-oxidase (biosynthesis gene) in tobacco (3ox-OE) induced slight variations in phenotype and active GA(1) levels, but we also found an increase in GA(8) levels (GA(1) inactivation product) and a conspicuous induction of gibberellin 2-oxidases (catabolism genes; NtGA2ox3 and -5), suggesting an important role for these particular genes in the control of gibberellin homeostasis. The effect of simultaneous overexpression of two biosynthesis genes, a gibberellin 3-oxidase and a gibberellin 20-oxidase (20ox/3ox-OE), on phenotype and gibberellin content suggests that gibberellin 3-oxidases are non-limiting enzymes in tobacco, even in a 20ox-OE background. Moreover, the expression analysis of gibberellin metabolism genes in transgenic plants (3ox-OE, 20ox-OE and hybrid 3ox/20ox-OE), and in response to application of different GA(1) concentrations, showed genes with different gibberellin sensitivity. Gibberellin biosynthesis genes (NtGA20ox1 and NtGA3ox1) are negatively feedback regulated mainly by high gibberellin levels. In contrast, gibberellin catabolism genes which are subject to positive feedback regulation are sensitive to high (NtGA2ox1) or to low (NtGA2ox3 and -5) gibberellin concentrations. These two last GA2ox genes seem to play a predominant role in gibberellin homeostasis under mild gibberellin variations, but not under large gibberellin changes, where the biosynthesis genes GA20ox and GA3ox may be more important.     

Ribonuclease activity during induction of cold hardiness in mimosa epicotyl and hypocotyl tissues

Ribonuclease activities in both epicotyl and hypocotyl tissues of Albizzia julibrissin seedlings decreased during induction of cold hardiness. Small decreases in fresh weights during induction of cold hardiness suggest a possible relationship between RNase activities and desiccation. Relative to increases in total protein during induction of cold hardiness, decreases in RNase activities suggest some degree of specificity, possibly invloved in regulation of RNAs and protein synthesis during induction of cold hardiness.     

Ultrastructure of cells in the cambial region during winter hardening and spring dehardening in Salix dasyclados Wim. grown at two nutrient levels

Summary The ultrastructure of cells in the cambial region of Salix dasyclados Wim. (clone 78056) was studied during the development of winter hardiness and the onset of cambial activity in spring. Plants were grown at relative growth rates (R_G) of 8% and 12% respectively, resulting in different nitrogen content in the stems. Frost hardiness of the plants was estimated by standardized freezing tests. Plants with a higher nitrogen status ceased growth later and started re-growth earlier in spring than plants with lower nitrogen content. Differences in ability to withstand low temperatures during autumn and spring were found between plants grown in the two nutrient treatments. During the development of frost hardiness in the autumn, the number of meristematic cells in the cambial region decreased. The cessation of meristematic activity was accompanied by cell wall thickening and ultrastructural changes in the cells. Frost hardiness increased from the ability to survive -6° C in October to survival of -80° C at the beginning of December. From November to February the cambial region comprised a layer of 2–3 thick-walled cells with conspicuous ultrastructural features. Starch accumulated in plastids in September, decreased during November to March and then increased again in accordance with changes of frost hardiness. Onset of cambial activity began between the end of March and the beginning of April, as shown by increased vacuolization of meristematic cells and mitotic activity. By April, the starch content had increased and lipolysis was observed. Frost hardiness had decreased, and plants with low and high nitrogen content were able to survive -15° C and -10° C, respectively. After budburst, all axillary shoot parts were damaged at temperatures below-3° C.Abbreviations Cz cambial zone - ER endoplasmic reticulum - Lb lipid body - m mitochondrion - Mm multimembraneous structure - Ms myelin-like structure - n nucleus - p plastid - Pb protein body - Pc pit cells - Ph phloem - Pd plasmodesmata - Pl plasmalemma - pl protective layer - Pt plasmatubules - Pw primary wall - Sw secondary wall - s starch - t tannins - v vacuole - K vessel - X xylem - Scale bars 1 urn     

Gibberellin metabolism: new insights revealed by the genes

The identification of most of the genes involved in the metabolic pathways for gibberellin hormones has helped us to understand these pathways and their regulation. Many of these enzymes are multifunctional and therefore fewer enzymes than might be expected are required to synthesize the various gibberellin structures. However, several of the enzymes are encoded by multiple genes that are regulated differently, adding unexpected genetic complexity. Several endogenous and environmental factors modify the expression of gibberellin biosynthesis genes, including developmental stage, hormonal status and light. A future challenge will be to dissect the complex, interacting pathways that mediate the regulation of gibberellin metabolism.     

Hormone interactions and regulation of Unifoliata, PsPK2, PsPIN1 and LE gene expression in pea (Pisum sativum) shoot tips

The Unifoliata (Uni) gene plays a major role in development of the compound leaf in pea, but its regulation is unknown. In this study, we examined the effects of plant hormones on the expression of Uni, PsPK2 (the gene for a pea homolog of Arabidopsis PID, a regulator of PIN1 targeting), PsPIN1 (the major gene for a putative auxin efflux carrier) and LE (a gibberellin biosynthesis gene, GA3ox), and also examined mutual hormonal regulation of these genes, in pea shoot tips, including a number of mutants. The Uni promoter possessed putative auxin and gibberellin response elements. The PsPIN1 mRNA levels were increased in afila, which replaces leaflets with branched tendrils; and reduced in tendrilless, which replaces tendrils with leaflets, compared with the wild type (WT). In contrast, mRNA levels of LE were increased in uni and tendrilless and decreased in afila compared with the WT. Uni, PsPK2 and PsPIN1 are positively regulated by gibberellin and auxin, and were induced to higher levels by simultaneous application of auxin and gibberellin. Auxin induction of Uni, PsPK2 and PsPIN1 did not require de novo protein synthesis. LE was positively regulated by auxin and cytokinin. In conclusion, these results support the hypothesis that auxin and gibberellin positively regulate Uni, which controls pea compound leaf development. Also, Uni, PsPIN1, PsPK2 and LE are expressed differentially in the leaf mutants, suggesting that mutual regulation by auxin and gibberellin promotes compound leaf development.     

Factors affecting the cold hardiness of the peach-potato aphid Myzus persicae

Supercooling point studies were used to investigate the factors influencing the cold hardiness of the peach-potato aphid Myzus persicae, a freezing-susceptible insect. Overwintering adults lost cold hardiness as winter progressed, with a variable proportion showing a marked reduction in supercooling ability. Cold hardiness increased in spring so that all individuals demonstrated extensive supercooling ability typical of aphids reared in the laboratory at 20°C with a long photoperiod; these levels of cold hardiness were maintained in the field during summer and early autumn. First instar nymphs demonstrated considerable cold hardiness all year. Surface moisture caused inoculative freezing in some first instar nymphs and adults when supercooled, but the majority were unaffected. In the laboratory, adults starved for 7 days at 5°C showed distinct losses of supercooling potential equivalent to those observed in the field during mid to late winter. No loss of cold hardiness was found in first instar nymphs starved under the same conditions. The results demonstrate that the cold hardiness characteristics of M. persicae are atypical of those observed in other freezing-susceptible insects and it is suggested that continued feeding during mild winter conditions allows maintenance of cold hardiness particularly in adult aphids, and provides a possible explanation for the successful anholocyclic overwintering of M. persicae during such winters.     

Opposite patterns in the annual distribution and time-course of endogenous abscisic acid and indole-3-acetic acid in relation to the periodicity of cambial activity in Eucommia ulmoides Oliv

The seasonal change of free abscisic acid (ABA) and indole-3-acetic acid (IAA) and their relationship with the cambial activity in Eucommia ulmoides trees were investigated by ABA and IAA immunolocalization using primary polyclonal and rhodamine-red fluorescing secondary antibodies, ABA and IAA quantification using high performance liquid chromatography (HPLC), and systematic monitoring of vascular cell layers production. ABA and IAA clearly displayed opposite annual distribution patterns. In the active period (AP), both immunolocalization and HPLC detected an abrupt decrease of ABA, reaching its lowest level in the summer. During dormancy, ABA started increasing in the first quiescence (Q1) (autumn), peaked in the rest (winter), and gradually decreased from the onset of the second quiescence (Q2) (the end of winter). IAA showed a reverse pattern to that of ABA: it sharply increased in AP, but noticeably decreased from the commencement of Q1. Longitudinally, the ABA distribution increased apico-basally, contrasting with IAA. Laterally, most of the ABA was located in mature vascular tissues, whereas the IAA essentially occurred in the cambial region. The concomitant IAA-ABA distribution and seasonal changes in vascular tissues greatly correlated with xylem and phloem cell production, and late wood differentiation and maturation. Interestingly, the application of exogenous ABA to quiescent E. ulmoides branches, in a water-culture system, inhibited external IAA action on cambial activity reactivation. These results suggest that, in E. ulmoides, ABA and IAA might probably interact in the cambial region. The annual cambial activity could be influenced by an IAA:ABA ratio; and ABA might play a key role in vascular cambium dormancy in higher plants. The relationship between hormonal changes and the (particular) annual life cycle of E. ulmoides is also discussed.     

Ecotype-dependent control of growth,dormancy and freezing tolerance under seasonal changes in <Emphasis Type="Italic">Betula pendula</Emphasis> Roth

Woody plants in the temperate and boreal zone undergo annual cycle of growth and dormancy under seasonal changes. Growth cessation and dormancy induction in autumn are prerequisites for the development of substantial cold hardiness in winter. During evolution, woody plants have developed different ecotypes that are closely adapted to the local climatic conditions. In this study, we employed distinct photoperiodic ecotypes of silver birch (Betula pendula Roth) to elucidate differences in these adaptive responses under seasonal changes. In all ecotypes, short day photoperiod (SD) initiated growth cessation and dormancy development, and induced cold acclimation. Subsequent low temperature (LT) exposure significantly enhanced freezing tolerance but removed bud dormancy. Our results suggested that dormancy and freezing tolerance might partially overlap under SD, but these two processes were regulated by different mechanisms and pathways under LT. Endogenous abscisic acid (ABA) levels were also altered under seasonal changes; the ABA level was low during the growing season, then increased in autumn, and decreased in winter. Compared with the southern ecotype, the northern ecotype was more responsive to seasonal changes, resulting in earlier growth cessation, cold acclimation and dormancy development in autumn, higher freezing tolerance and faster dormancy release in winter, and earlier bud flush and growth initiation in spring. In addition, although there was no significant ecotypic difference in ABA level during growing season, the rates and degrees of ABA alterations were different between the ecotypes in autumn and winter, and could be related to ecotypic differences in dormancy and freezing tolerance.     

Plasma corticosterone concentrations in European robins during spring and autumn migration

To estimate differences in hormonal mechanisms of regulation of spring and autumn migration in European robins Erithacus rubecula, the plasma corticosterone (CORT) concentrations were compared in birds caught during both migratory seasons. A total of 414 blood samples were analyzed. It was found that the baseline and stress-induced CORT concentrations in free-living robins during spring migration were practically twice as high as during autumn passage. Our results demonstrate that autumn and spring migrations are independent stages of the avian annual cycle. Probably, the increase in the CORT concentrations in spring can be considered to be physiological preparation for the breeding season.