Brassinosteroids as Metahormones: Evidence for their Specific Influence during the Critical Period in Sorghum Development

Abstract: In Sorghum bicolor, the effect of brassinosteroid (BR) treatments on blade elongation depends both on concentration and on the stage of development. A specific period of increase in sensitivity to BR is reported during early vegetative development. It coincides with emergence of a critical period during which the between-organs relationship is readjusted as a function of new developmental events or new environmental conditions. Accordingly, the influence of BR cannot be completely understood without separating its effects during stable phases (phenophases) and during critical periods. A high level of redundancy exists in networks of regulation, so that modifications due to BR treatments generally remain cryptic. Nevertheless, it is shown that BR affects the pattern of relationships between organs, confirming its involvement in emergence of a new network of regulation. It is suggested that, during critical periods, brassinosteroids act as "metahormones" integrating the new emerging regulation network by triggering changes in cellular sensitivity to PGRs.     

Developmental Changes in Effect of Cytokinin and Gibberellin on Shoot K+ and Na+ Accumulation in Salt-Treated Sorghum Plants

Abstract: The effect of cytokinin (CK) and/or gibberellin (GA) treatments on shoot accumulation of Na⁺ and K⁺ was investigated in Sorghum bicolor exposed to 150 mM NaCl. These hormonal treatments modified the shoot content of Na⁺ and K⁺, but the effect varied throughout development. Comparison of ion concentration versus ion content in shoots indicates that regulation of shoot concentration of K⁺ is modified during a transition period of development. This change is concomitant with reorganization of the regulation network for meristem activity, an event also involving changes in sensitivity to CK and GA. This evidence suggests a strong interdependency between dynamic changes in a between-organ network of relations and control of accumulation of monovalent ions in the shoot. Moreover, a new pattern of regulation of shoot Na⁺ concentration emerges during the transition period. During this process GA appears progressively involved in regulation of Na retranslocation, while CK is rather controlling the root uptake of Na⁺. Accordingly, the spontaneous emergence of Na-includer and Na-excluder individuals observed from an initially homogeneous population is interpreted as related to variations in sensitivity to GA and CK during differentiation of this newly emerging pathway of regulation.     

Opposing effects of plant growth regulators via clonal integration on apical and basal performance in alligator weed

植物生长调节剂通过克隆整合对空心莲子草顶端和基部生长的不同作用 入侵植物不仅对全球生物多样性造成了巨大的威胁,同时也严重影响了农业生产与粮食安全。克隆整合使得相连植株进行资源共享,能促进入侵植物的生长从而获得优势。然而,入侵杂草 在植物调节剂(plant growth regulators, PGRs)影响下的克隆整合作用则很少有报道。PGRs被广泛应用于 农作物生产上,并能通过土壤淋溶、侵蚀和径流作用,影响分布在作物附近的农田杂草的生长。本 研究采用两种PGRs赤霉素(gibberellins, GA)和多效唑(paclobutrazol,PAC)处理恶性入侵杂草空心莲子草 (Alternanthera philoxeroides)基端,并保持或者通过剪切达到控制基端与顶端的连通,从而探究克隆整合作用在空心莲子草响应两种农业常用PGRs中的作用。研究结果表明,GA和PAC对空心莲子草生长的作用相反。GA通过克隆整合作用显著促进顶端植株的地上生长。相反地,PAC显著抑制基端和顶端的地 上生长,但是能够通过克隆整合作用显著促进基端和顶端的地下生长。这些研究结果解释了克隆整合作用能促进PGRs对空心莲子草生长的促进作用,这很可能是外来杂草能够成功入侵人为干扰较多的农业生态系统的重要原因之一。     

Interaction Between Mineral Nutrients, Cytokinin and Gibberellic Acid During Growth of Sorghum at High NaCI Salinity

Sorghum bicolor (L.) Moench, cv. 610, adapted to high salinitywas able to grow at 300 mol m^–3 NaCl only when half-strengthHoagland's solution was enriched with mineral nutrients. Theoptimal growth rate was observed in full strength Hoagland'ssolution; at higher or lower concentrations growth rates werelower. In contrast, growth rate of plants exposed to 150 molm^–3 NaCl was not affected by similar modification of theHoagland solution concentration. At high salinity, additionof cytokinin (CK) or gibberellic acid (GA), or a mixture ofboth, can induce the same effect on growth as the increasedmineral nutrient concentration. Phytohormones and increasedmineral concentration have similar effects, possibly becausean imbalance in phytohormones, rather than a mineral deficiency,limits growth at 300 mol m^–3 NaCl in the presence of half-strengthHoagland solution. The change in mineral concentration in thenutrient medium, in addition to its nutritional effect, alsoapparently acts as a signal involved in hormonal balance whichallows growth at high salinity. Exposure of Sorghum to 300 molm^–3 NaCl causes a decrease in the range of nutrient concentrationswhich can sustain growth. Adjustment of the nutrient concentrationmay induce the synthesis of endogenous CK and GA concentrationsrequired for growth. In contrast, addition of CK or GA at similarconcentrations during the adaptation (pretreatment) period inhibitsgrowth and prevents the adaptation process. The response tothe exogenous phytohormone treatments depends on the time elapsedfrom the beginning of salinization. Key words: Adaptation to salinity, cytokinin, gibberellic acid, mineral nutrition, growth, Sorghum, NaCl     

GA signaling and <Emphasis Type="Italic">CO/FT</Emphasis> regulatory module mediate salt-induced late flowering in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis>

Flowering timing is very important for the reproductive success of higher plants. However, effects of salt on plant flowering and the underlying molecular mechanisms are largely unknown. Here, we show that salt stress delays flowering in Arabidopsis in a dose-dependent manner. Mild salt stress (≤50 mM NaCl) promoted and prolonged the vegetative growth, whereas high salinity (≥100 mM NaCl) largely delayed or inhibited the transition from vegetative growth to reproductive development. The gibberellin (GA)-pathway plays an important role in this phenotype, and application of exogenous GA could restore late flowering induced by salt. In addition, the CONSTANS (CO)/FLOWERING LOCUS T (FT) module may also play a critical role in mediating the effects of salt on flowering. The mRNA abundance of CO was significantly reduced by salt stress in a dose-dependent manner. The constans (co-2) mutants did not respond to moderate salt stress, whereas over-expressing CO manifested no delay in flowering time in response to salinity. Expression of FT, SOC1 and LFY in the downstream of the pathways was also reduced by salt according to dose. Moreover, salt-sensitive mutant salt overly sensitive3 (sos3) exhibited greater sensitivity in flowering, further suggesting that ion disequilibrium mediates salt-induced late flowering. Kexue Li and Youning Wang contributed equally to this report.     

Growth and Dormancy Cycles in Citrus Bud Cultures and Their Hormonal Control

An in vitro bud culture method was devised in order to better understand the control mechanism of Citrus bud development. This technique offers a new approach to the study of hormonal control of growth, dormancy and flowering cycles in perennial plants. Buds were excised from orchard trees throughout the year, cultured on defined media for prolonged periods, and their vegetative growth responses to various growth hormones were determined. The buds proceeded with their vegetative development in vitro and achieved sprouting on a basal medium. The various growth regulators affected both the time required for sprouting (TRS) and the type of growth. In summer buds, IAA delayed sprouting, while GA enhanced it and caused shoot elongation. Cytokinins specifically induced the formation of numerous adventitious buds, whereas ABA completely inhibited sprouting; this inhibition, however, was reversible. A marked decrease in total protein and in the rate of its synthesis was evident during the first 20 days of sprouting induction and early bud growth. The annual growth rhythm was determined in spring buds sampled and cultured throughout the year, and an innate dormancy of citrus buds was revealed. Both the dormancy and the sprouting periods of buds in vitro corresponded to the natural periods occurring under field conditions. The effect of exogenous IAA, GA and cytokinins on the TRS varied at different periods along the season, suggesting the concept of “critical levels” in the endogenous balance of hormones.     

Changes in carbohydrate contents of <Emphasis Type="Italic">Zantedeschia</Emphasis> leaves under gibberellin-stimulated flowering

The effect of gibberellic acid (GA₃) on the carbohydrate accumulation in relation to vegetative growth of Zantedeschia ‘Black Magic’ plants undergoing transition to flowering was investigated. In response to GA treatment the carbohydrate level increased independently of earlier stimulation of shoot emergence. Under vegetative growth stage the content of reducing sugars of leaf blades was 2.5-fold higher than in control plants, and suggests the stimulation of photosynthetic activity. The changes observed during the flowering, in principle noted in petiole tissues, support the GA-effect on assimilate transport to the sink organs. Moreover, the high level of non-structural carbohydrates in petiole tissues, in particular reducing sugars, can be an effect of photosynthetic activity of these organs and/or essential for osmoregulation and high turgor pressure. The results indicate that apart from the influence on the shoot emergence, the GAs may stimulate the photosynthetic activity from the beginning of shoot growth and are thus responsible for the enhancement of callas flower yield.     

Perturbation in leaves of salt-treated Sorghum: elements for interpretation of the normal development as an adaptive response

Developmental windows are specific periods of sensitivity in which a perturbation may be adaptively integrated. In Sorghum bicolor , two developmental windows which enable adaptive adjustment to salinity (increase in tolerance) have been described during vegetative development. A third developmental window is open during the transition between vegetative and reproductive development. This third developmental window was analysed using morphological markers (specific malformations on leaves), and their relationship with vegetative and reproductive events. A positive link was observed between fertility and malformations on the last leaf. We concluded that this late window enables an adaptive adjustment of reproductive development, counteracting the negative effect of salt adaptation on fertility. Developmental windows open following rapid changes in growth of the different organs. They permit adaptive adjustments to emergence or senescence of various organs. This phenomenon is integrated within normal development, but developmental windows are enlarged for plants exposed to perturbation and for their progeny.     

Salt sensitivity of the vegetative and reproductive stages in chickpea (Cicer arietinum L.): Podding is a particularly sensitive stage

Soil salinity is an increasing problem, including in regions of the world where chickpea is cultivated. Salt sensitivity of chickpea was evaluated at both the vegetative and reproductive phase. Root-zone salinity treatments of 0, 20, 40 and 60 mM NaCl in aerated nutrient solution were applied to seedlings or to older plants at the time of flower bud initiation. Even the reputedly tolerant cultivar JG11 was sensitive to salinity. Plants exposed to 60 mM NaCl since seedlings, died by 52 d without producing any pods; at 40 mM NaCl plants died by 75 d with few pods formed; and at 20 mM NaCl plants had 78-82% dry mass of controls, with slightly higher flower numbers but 33% less pods. Shoot Cl exceeded shoot Na by 2-5 times in both the vegetative and reproductive phase, and these ions also entered the flowers. Conversion of flowers into pods was sensitive to NaCl. Pollen from salinized plants was viable, but addition of 40 mM NaCl to an in vitro medium severely reduced pollen germination and tube growth. Plants recovered when NaCl was removed at flower bud initiation, adding new vegetative growth and forming flowers, pods and seeds. Our results demonstrate that chickpea is sensitive to salinity at both the vegetative and reproductive phase, with pod formation being particularly sensitive. Thus, future evaluations of salt tolerance in chickpea need to be conducted at both the vegetative and reproductive stages.     

Regulation of growth: the meristem network approach

Gibberellin (GA) and/or cytokinin (CK) supplied to the root medium modified the rate of growth of the shoot, the adventitious and seminal roots in young seedlings of Sorghum bicolor (L.) Moench. For each population submitted to a hormonal treatment, the link between the weight of these three organs was analysed. All these links build a network of relationships, also termed a meristem network. Treatments with 1–100 μ M CK modified the relative growth of these three organs, as well as the meristem network. Treatments with 1–100 μ M GA slightly modified the growth, but the meristem network was considerably changed. When GA and CK were applied simultaneously, the growth was similar to that of CK-treated plants, and the meristem network was similar to that of GA-treated plants. Similar experiments were performed on plants exposed to 150 m M NaCl. They showed that both growth and meristem networks are modified by salinity. Growth was unrelated to the network variations in non-salinized plants, while an optimal level of relationship for growth was observed in salinized plants. It is concluded that stress generates an imbalance between the self-stimulation of meristem activity and its regulation by the other organs. A definition of stress, independent from the rate of growth, is proposed.     

Nitrate or NaCl regulates floral induction in Arabidopsis thaliana

Flowering, the transition from the vegetative to reproductive phase in plants, is regulated by both endogenous and environmental signals. Exposure to an extended period of stress (such as low nitrate or NaCl) can also promote flowering in many species, but little is known about how these forms of stress regulate floral induction. In this study, we found that stress induced by low concentrations of nitrate or NaCl activated the biosynthesis of gibberellin (GA) as evidenced by increased expression of the GA biosynthetic enzyme GA1. Expression of CO and SOC1 were also enhanced, leading to an acceleration of flowering. The effects of nitrate and NaCl on the photoperiod pathway were distinct, however. Two genes related to the photoperiod pathway, CCA1 and LHY, were repressed only under low NaCl treatment, while expression was unaltered by nitrate. Therefore, we suggest that the biosynthesis of gibberellin (GA) may play an important role in integrating signals induced by exogenous stress to regulate flowering in Arabidopsis.     

Phenological growth stages of shea tree (Vitellaria paradoxa subsp. paradoxa) according to the BBCH scale

The shea tree is among the socio-economically and environmentally most important plants in the Sudano-Sahelian region of Africa. Shea butter is internationally valued for use in pharmaceutical, cosmetic and confectionary industries. Scaling and describing phenological growth stages is of great importance in crops management. However, such phenological scale is still lacking for shea tree. To fill this gap, we documented the different growth stages of shea tree by referencing to the Biologische Bundesantalt, Bundessortenamt und Chemische Industrie (BBCH) scale. Eight of the ten principal growth stages (0–9) and 43 secondary growth stages (0–9) were described in shea tree. Among the eight principal stages, four were related to reproductive phenophases: inflorescence emergence stage (5), flowering stage (6), fruit development stage (7), and fruit maturity stage (8). The remaining four principal stages concerned vegetative phenophases and contained: bud development stage (0), leaf development stage (1), shoot development stage (3), and senescence stage (9). Observations of shea tree phenophases depicted asynchronous growth patterns with overlapping secondary growth stages of different principal stages within an individual tree canopy at the same time. The proposed phenological growth scale specific to shea tree but compatible with other crops is a valuable contribution for the implementation of crops management protocols and the standardisation of research between different laboratories.     

Effects of auxins and cytokinins on epigenetic instability of callus-propagated Kalanchoe blossfeldiana Poelln.

A main problem in the vegetative propagation of ornamental plants in vitro is the epigenetic instability of cells removed from their organized environment. With calluses of leaf explants of Kalanchoe blossfeldiana Poelln., cv. Yucatan, the role of plant growth regulators (PGRs) in the occurrence of fasciation was studied.In various combinations of auxins and cytokinins, the auxin 2,4-D (2,4-dichlorophenoxyacetic acid) gave only deformed, inseparable shoot primordia. The most rapid callus induction with regeneration of well-developed sprouts was obtained with the natural IAA (indoleacetic acid) and Z (zeatin).As a first symptom of fasciation, aberration in decussate phyllotaxis can be observed. At increasing concentrations of IAA + Z, this symptom gradually decreased but fasciation proper increased. The optimum concentration was at 1 M for both PGRs. Reduction of exposure to the PGRs from six to three weeks reduced the epigenetic instability.     

Variable increases in cold hardiness induced in winter rape by plant growth regulators

Triazole and conventional growth regulators were tested for their ability to extend cold hardiness and improve the winter survival of winter rape (Brassica napus L.). Winter rape plants were grown in the field (Ottawa 45°23 N) and in growth cabinets. Plant growth regulators (PGRs) were applied during the early vegetative stage and the plants were allowed to cold harden. Cold-hardened plants from the field and cabinet were subjected to freezing and ice encasement tests in the laboratory. Some of the triazole PGRs reduced plant size by limiting cell expansion and increasing cell numbers. While cold hardiness and ice encasement tolerance were increased by some growth regulators, these effects were not consistent with time nor were they reflected in increased winter survival. Natural cold hardening may have eclipsed the PGR-induced hardening.PGR contribution number 1381.     

Fish emulsion as a food base for rhizobacteria promoting growth of radish (Raphanus sativus L. var. sativus) in a sandy soil

Commercial fish emulsion was evaluated as a plant growth medium and as a nutrient base to enhance radish (Raphanus sativus L. var. sativus) growth by bacterial and actinomycete isolates. Six bacterial isolates including three actinomycetes were selected from a screening of 54 bacteria (including 23 actinomycetes) based on their ability to produce plant growth regulators (PGRs) and to colonize radish roots. These isolates were tested in the presence and absence of autoclaved or non-autoclaved fish emulsion or inorganic fertilizers. The nutrient contents and types and levels of PGRs in tissues of treated plants were assayed to determine the basis of growth promotion. Fish emulsion was found to support plant growth in a sandy soil as effectively as an applied inorganic fertilizer. The plant growth promotion by bacterial and actinomycete isolates was most pronounced in the presence of autoclaved or non-autoclaved fish emulsion than in the presence of the inorganic fertilizers. The bacterial and actinomycete isolates were capable of producing auxins, gibberellins and cytokinins and appeared to use fish emulsion as a source of nutrients and precursors for PGRs. PGR levels in planta following combined treatments of the bacterial and actinomycete isolates and fish emulsion were found to be significantly enhanced over other treatments. The effect of fish emulsion appears to be more related to its role as a nutrient base for the bacterial and actinomycete isolates rather than to the increased activity of the general microflora of treated soil. This is the first report of fish emulsion as a nutrient base for plant growth promoting rhizobacteria. These results also indicate that the successful treatment can be effective and economical for horticultural production in sandy soils such as those found in the United Arab Emirates where fish emulsion is already in use as a substitute or supplement for inorganic fertilizer.     

Morphogenetic responses of tomato plants to combined and individual applications of gibberellic acid,Phosfon and B-nine

Summary Two growth retardants, B-nine (N-dimethylamino succinamic acid) and Phosfon (2,4-dichlorobenzyl-tributyl phosphonium chloride) were applied to tomato plants, either singly or in combination with gibberellic acid (GA), in order to determine various morphogenetic responses. GA (5 ml of 10^– M ⁴ per plant) and B-nine (5 ml of 1.56×10^–2 M per plant) were applied as foliar spray whereas Phosfon (1.5×10^–3 M in 10 ml of water per plant) was applied as soil amendment. Growth retardation by Phosfon persisted through the time of harvest and was somewhat neutralized by GA. Fruit set and extent of seediness of fruits were the maximum in Phosfon-treated plants compared to others. Plants receiving B-nine, however, recovered from the initial growth retardation and indicated no residual action at harvest. GA in combination with B-nine produced significantly greater vegetative growth and dry weight accumulation than did GA alone. This indicated that applied and endogenous GA responded differently to different growth retardants. None of the treatments had any noticeable effect on the time of flowering.     

Gibberellins and bud break,vegetative shoot growth and flowering in Metrosideros collina cv. Tahiti

Applications of the growth promotive gibberellins (GAs) GA₄ and 2,2-dimethyl GA₄, and of C-16,17 endo-dihydro GA₅, which is known to promote flowering while inhibiting stem growth in the long-day grass Lolium temulentum, were made to micropropagated plants of Metrosideros collina cv. Tahiti, a highly ornamental cultivar with an intermittent flowering pattern. Gibberellin A₄ and 2,2-dimethyl GA₄ stimulated vegetative growth both in elongating shoots, and internodes of shoots developing from buds that were quiescent at the time of GA application. Abscission of the apices of expanding shoots, a feature of mature Metrosideros plants, was inhibited by these GAs, the rejuvenation of micropropagated plantlets being enhanced. However, C-16,17 endo-dihydro GA₅ differed from GA₄ and 2,2-dimethyl GA₄ by having no promotive effects on vegetative growth, and no inhibition of apical abscission. Notwithstanding this contrasting effect on vegetative growth, high doses of GA₄ or C-16,17 endo-dihydro GA₅ similarly reduced flowering on shoots to which either GA was applied. Reduced flowering in response to applied GAs is common in many woody angiosperms, and in this instance was probably the combined result of abortion of developing floral structures in quiescent buds, and a preferential inhibition of bud break for floral buds relative to vegetative buds, particularly by GA₄. Finally, both C-16,17 endo-dihydro GA₅ and GA₄ strongly inhibited bud break in this woody angiosperm, although GA₄ could initially stimulate bud break when applied to vegetative buds close to the expansion stage. The above findings, in toto, highlight the sensitivity of Metrosideros to both classes of GA in a variety of growth and development processes.     

Growth Responses and Allocation of Assimilates of Rice Seedlings by Paclobutrazol and Gibberellin Treatment

Paclobutrazol [(2RS,3RS)-1-(4-chlorophenyl)methyl-4,4-dimethyl-2-(1h-1,2,4-trizol-1-yl)penten-3-ol] effectively decreased vegetative growth of rice (Oryza sativa L.) seedlings and increased the chlorophyll content. The number of veins in a leaf, the calculated number of stomata per leaf, and the length of guard cells were not altered by the paclobutrazol treatment, suggesting an effect on cell elongation. The allocation pattern of carbohydrates was changed by either gibberellin (GA) or paclobutrazol treatment. GA₃ induced more shoot growth and less accumulation of starch than the control and paclobutrazol-treated seedlings. Photosynthetic ability was not affected by either paclobutrazol or GA₃ treatment. Paclobutrazol-treated plants allocated a smaller amount of photosynthates for vegetative shoot growth and stored more as starch in the crowns than the control and GA₃-treated plants. The same starch degrading activity in the crown tissue of paclobutrazol-treated seedlings as in control plants suggests that the accumulated starch is utilized in a normal activity for growth including leaf emergence, tiller formation, and root production, resulting in improved seedling quality. Received May 30, 1996; accepted December 10, 1996