The apical control of lateral bud development in excised shoot tips of Cuscuta reflexa cultured in vitro

Excised shoot tips of Cuscuta reflexa Roxb. (dodder), a rootless and leafless angiospermic plant parasite, were cultured in vitro for the study of the control of lateral bud development by the apex. In a chemically defined medium lacking hormones, the basal bud alone developed into a shoot. The addition of coconut milk to the growth medium induced the activation of multiple lateral buds, but only a single bud developed further into a shoot. The decapitation of this shoot induced the development of another shoot and the process could be repeated. This showed the controlling effect of the apex in correlative control of bud development. Application of indole-3-acetic acid to the shoot tip explant delayed the development of the lateral bud. Gibberellic acid A₃ induced a marked elongation growth of the explant and reinforced apical dominance. The direct application of cytokinin to an inhibited bud relieved it from apical dominance. A basipetally decreasing concentration gradient of auxin may prevail at the nodes. Bud outgrowth is probably stimulated by cytokinin produced locally in the bud.     

The role of hormones in apical dominance. New approaches to an old problem in plant development

The role of hormones in apical dominance has been under investigation with traditional 'spray and weigh' methods for nearly 5 decades. Even though the precision of hormone content analyses in tissue has greatly improved in recent years, there have been no significant breakthroughs in our understanding of the action mechanism of this classical developmental response. Auxin appears to inhibit axillary bud outgrowth whereas cytokinins will often promote it. Conclusive evidence for a direct role of these or other hormones in apical dominance has not been forthcoming. However, promising new tools and approaches recently have begun to be utilized. The manipulation of endogenous hormone levels via the use of transgenic plants transformed with bacterial genes ( iaaM and ipt from Agrobacterium tumefaciens and iaaL from Pseudomonas syringae pv. savastanoi ) has demonstrated powerful effects of auxin and cytokinin on axillary bud outgrowth. Also, possible auxin and cytokinin involvement of rolB and C genes from Agrobacterium rhizogenes whose activity is associated with reduced apical dominance in dicotyledons has received considerable attention. The characterization of unique mRNAs and proteins in non-growing and growing lateral buds before and after apical dominance release is helping to lay the groundwork for the elucidation of signal transduction and cell cycle regulation in this response. The use of auxin-deficient, and auxin/ethylene-resistant mutants has provided another approach for analyzing the role of these hormones. The presumed eventual employment of molecular assay systems (SAUR/GH3 promoters fused with GUS reporter gene) which are presently being developed for analyzing auxin localized in lateral buds will hopefully provide a critical test for the direct auxin inhibition hypothesis.     

Axillary bud growth in relation to adventitious root formation in cuttings

Single-node leaf-bud cuttings of Schefflera arboricola Hayata and Stephanotis floribunda Brongn. were set and root formation, onset of axillary bud growth and plant height were measured. An increase in the number of roots in Schefflera, which was achieved with increasing cutting position on the stock plant (measured from top to base) or with increasing stem length below the node, accelerated the onset of axillary bud growth and resulted in an increase in plant height. Increasing the number of roots per cutting in Stephanotis through an increase in basal temperature also accelerated bud and shoot growth. Positional effects on root formation in Stephanotis showed no relationship with axillary bud growth and plant height. A positive relationship between number of roots per cutting and axillary bud growth was found among clones of Stephanotis . In general the results suggest that, with some exceptions, the onset of axillary bud growth is accelerated in cuttings as a result of accelerated root formation and a higher number of roots per cutting.     

Distribution of labelled auxin and derivatives in stem tissues of intact and decapitated broad-bean plants in relation to apical dominance

[³H]-auxin (0.13 to 0.18 nmol) was applied to the apical bud of broadbean plants (Vicia faba L. cv. Aguadulce). After 24 h, the exportation from the donor organ was ended. After 48 h, i.e. 10–15 h after the passage of the [³H]-auxin pulse into the root system, the distribution and the nature of labelled molecules located in the basal part of the stem and in the axillary buds were investigated. Chromatographic analyses concerned both intact plants and plants decapitated 12 h, 24 h or 42 h after the [³H]-auxin application. In intact plants, there was no significant amount of [³H]-auxin in the axillary buds, whose radioactivity was very low compared to the stem tissues. The labelled molecules with the Rf of auxin represented 50% or more of the whole radioactivity of the stem tissues. The distribution of [³H]-auxin was not uniform along the stem. In particular, the cotyledonary node zone, bearing the most inhibited buds, which is known to be an important centre of label retention, contained the highest amounts of labelled auxin both in intact and decapitated plants. The decapitation was quickly followed by a decrease of the [³H]-auxin amount in the stem base more than 15 cm away from the wound, particularly in the scale leaf nodes, whose axillary buds were mainly the ones to grow after relief from apical dominance. The induction of this early decrease was clearly distinct in plants decapitated when auxin exportation from the donor organ was ended.     

Distribution of free and bound forms of cis-trans and trans-trans abscisic acid in broad-bean plants in relation to apical dominance

Levels of endogenous abscisic acid (ABA; free and bound forms) have been determined by gas chromatography in stems and buds of broad-bean plants ( Vicia faba L. cv. Aguadulce) in relation to apical dominance. A downward gradient of free cis-trans ABA occurred along the stem, from the apical bud to the roots. Except for the actively growing apical bud the levels of free cis-trans ABA were higher in the buds than in the corresponding nodes. An inverse correlation can be set up between levels of free cis-trans ABA and growth of buds, except for the cotyledonary ones. High levels of bound ABA ( cis-trans form) are correlated with the growth of the apical bud and that of the axillary bud ax1. The hormonal regulation of the growth of the cotyledonary buds, which contained high levels of trans-trans ABA in bound forms, is apparently different from that of the other buds.     

The semidian rhythm in flowering response of Pharbitis nil in relation to dark period time measurement and to a circadian rhythm

When seedlings of Pharbitis nil Choisy, cv. Violet, are exposed to a single inductive dark period at 27°C, brief interruptions with red light (R) can be promotive after 2–3 h of darkness but increasingly inhibitory to flowering up to the 8–9th h of darkness. This rhythmic response to R interruptions can be advanced in phase by > 1 h when the preceding light period is interrupted with far-red (FR) 2 h before darkness (FR -2 h) or with FR – 15 h, whereas FR –8 h or FR–22 h retard the rhythm. These shifts in the R interruption rhythm are paralleled by equal shifts in the length of the dark period required for flowering. Brief FR interruptions of darkness displayed a similar rhythm which was also advanced by FR –2 h and retarded by FR –8 h. We conclude therefore that the semidian rhythm in the light, which we have previously described, continues through at least the first 12 h of darkness, is manifested in the R interruption rhythm, and determines the critical night length. A circadian rhythm with a marked effect on flowering was also identified, but several lines of evidence suggest that the circadian and semidian rhythms have independent additive effects on flowering and do not appear to show phase interaction.     

Changes in the carbohydrate content of the leaf and the apical bud of Sinapis during transition to flowering

Vegetative plants of Sinapis alba L. were induced to flower by a single long day of 20 h or by a single short day of 8 h starting at an unusual time of the 24-h cycle (displaced short day). The soluble sugar and starch contents of the just-expanded leaf and the apical bud were measured at various times after the start of each of these two photoinductive treatments. Associated with the induction of flowering there were temporary increases in the soluble sugar and starch contents of the leaf and of the bud. These increases were apparent 14 h after the start of the long day and 12 h after the start of the displaced short day. The starch content of the bud increased later. These results indicate that an increase of the soluble sugar content of the bud is required for its transition from the vegetative to the reproductive condition.     

Response of cytokinin concentration in the xylem exudate of bean (Phaseolus vulgaris L.) plants to decapitation and auxin treatment,and relationship to apical dominance

When xylem exudate of previously untreated Phaseolus vulgaris plants was analysed for cytokinins by radioimmunoassay, a low concentration (about 5 ng · ml^–1) was found. However, when the plants were decapitated about 16 h before the xylem exudate was collected, an almost 25-fold increase in cytokinin concentration was observed. Twenty-four hours after decapitation this increase even reached 4000 compared to control plants. Applying naphthaleneacetic acid (NAA) to the shoot of decapitated plants almost eliminated the effect of shoot tip removal on cytokinin concentration, suggesting that cytokinins in the xylem exudate of intact plants are under the control of the polar auxin transport system. Other xylem constituents, such as potassium or free amino acids did not show this strong increase after decapitation and did not respond to NAA application. It is concluded that the observed auxin/cytokinin interaction has an important regulatory role to play, not only in apical dominance but in many other correlative events as well.Abbreviations AD apical dominance - CKs cytokinin(s) - iAde/iAdo isopentenyladenine/iospentenyladenosine - NAA naphthaleneacetic acid - Z/ZR zeatin/zeatin riboside