Amino acids were required for bud outgrowth in nodal segments of rose in vitro Le Moigne et al. Whether amino acids act as signaling entities in bud outgrowth remains to be investigated. Besides its role as an energy source for photosynthesis, light is also a powerful environmental signal that controls many developmental processes de Wit et al.
In particular, it is involved in the shade avoidance syndrome SAS , characterized by typical morphological changes such as leaf hyponasty, an increase in hypocotyl and internode elongation, and extended petioles, which aim to maximize light interception by the plant for photosynthesis Franklin, In bud outgrowth regulation, light also acts as a signal that may prevent a new branch from developing in low light conditions. In accordance with the signaling role of light, a very low light intensity on the bud was sufficient to trigger bud outgrowth in decapitated rose Girault et al.
Tillering can cease in grasses before the occurrence of a significant reduction in PAR intensity due to canopy closure, but concomitantly with a reduction of the R:FR ratio Ballare et al. Simulation studies support a role of light in shaping plant branching architecture in different species. In trees, the global branching structure can be explained qualitatively by space colonization algorithms, which consider competition for space as the key factor determining the branching structure of the tree Runions et al.
In herbaceous species, the inhibiting effect of shading or high plant densities can be simulated by regulating bud outgrowth by the local light environment on the apical meristem at the time of bud formation Gautier et al. Remarkably, light, hormones, and nutrients seem to converge to the same regulating hubs Quail, ; Moore et al.
Compared to the endogenous network responsible for apical dominance, relatively few studies have focused on the interaction of light with hormones and nutrients in the control of axillary bud outgrowth. Most studies have focused on the effect of the R:FR ratio, which is a signal of canopy closure. More recently, the effect of light intensity was also investigated. Studies were made by directly manipulating light quality or by using phyB Arabidopsis mutants, which are deficient in phytochrome B-mediated red light perception and display a low branching level as compared to the wild-type Kebrom et al.
Those studies highlight that enhanced ABA biosynthesis in the bud has a main role in the effect of the R:FR ratio on bud outgrowth. Furthermore, Arabidopsis mutants deficient in ABA biosynthesis nced and aba exhibited lower suppression of bud outgrowth by low R:FR than the wild type Reddy et al. Low R:FR-induced ABA biosynthesis may repress bud outgrowth partly by reducing bud auxin biosynthesis, since both phyB Arabidopsis mutants and exogenous ABA supply to wild-type plants reduced the expression of an auxin biosynthesis gene within the bud Finlayson et al.
Auxin plays a key role in the shade-avoidance syndrome, including the promotion of hypocotyl and petiole growth, leaf hyponasty, and phototropism Iglesias et al. In seedlings, low R:FR increases auxin level in the foliage by stimulating its biosynthesis; auxin then moves to the stem where it reaches epidermal tissues through lateral orientation of PIN proteins to drive the auxin flux to the epidermis to promote growth Iglesias et al.
Similarly, relationships have been observed between auxin and bud outgrowth inhibition in Arabidopsis phyB mutants, which cannot perceive red light. The branching inhibition reported in phyB Arabidopsis mutants was alleviated by disrupting auxin signaling Finlayson et al. In this case, branching inhibition was related to elevated auxin sensitivity and signaling in the shoot segments proximal to axillary buds Reddy and Finlayson, Low auxin level supply to isolated stem segments inhibited phyB buds more than wild-type, and phyB shoots displayed elevated auxin-responsive genes expression compared to the wild-type.
This obviously raises the question of how auxin- and ABA-mediated pathways interact to regulate bud outgrowth in response to R:FR.
ABA pathway may be responsible for a rapid response of the bud to R:FR, while auxin signaling in the stem may sustain this rapid response. Low auxin transport rate was also observed in the shoots of phyB mutants but its role in inhibiting bud outgrowth was not demonstrated Reddy and Finlayson, Besides auxin, SL biosynthesis- and signaling-related genes were also found to be up-regulated by low R:FR or by phyB mutation in chrysanthemum, sorghum, or petunia buds Kebrom et al.
Furthermore, bud outgrowth inhibition by phyB mutation was impaired in SL biosynthesis max4 or signaling max2 mutants as compared to wild-type Arabidopsis Finlayson et al.
This is in accordance with the main role of the SL signaling-related gene MAX2 in light-regulated hypocotyl elongation in Arabidopsis seedlings Shen et al. The interaction between light intensity and hormonal regulation of bud outgrowth has mainly been investigated in rose. First data indicate that GAs are not sufficient to mimic the promotive effect of light in dark-placed buds Choubane et al.
For decapitated plants, dark-repressed bud outgrowth correlated with a down-regulation of two GA biosynthesis genes, and light-induced bud outgrowth was inhibited by GA biosynthesis inhibitors, but GA supply to plants in the dark could not rescue bud outgrowth. Recent experimental studies on rose support a model in which light intensity stimulates CK biosynthesis in the stem, which in turn stimulates bud outgrowth. As compared to darkness or low light intensity, a higher light intensity rapidly and significantly increased the CK content in the nodal segment bearing the light-stimulated bud Roman et al.
This was correlated with rapid up-regulation of genes encoding CK synthesis, transport and signaling, and down-regulation of genes encoding CK degradation RhCKX1 Roman et al. This is in line with the known effect of light on CK biosynthesis, metabolism, and transport in other biological processes Zubo et al. In addition, local exogenous CK application restored the bud outgrowth ability under non-permissive light conditions Roman et al. Interestingly, studies on the shoot apical meristem in tomato and Arabidopsis also demonstrated the involvement of CKs in the light-induced activity of the apical meristem Yoshida et al.
Light-induced bud outgrowth may involve the two CK-related processes controlling bud outgrowth: BRC1 repression and PIN up-regulation which would increase auxin canalization capacity Dun et al. In addition, both light and CKs supply to rose decapitated plants decreased the expression of the SL signaling-related gene MAX2 and up-regulated sugar metabolism-related genes Djennane et al.
For rose intact plants, high light intensity also decreased ABA level in the node adjacent to the bud compared to low light intensity, and ABA exogenous supply to the node could antagonize the promoting effect of CK supply under low light intensity Corot et al.
All these changes underline the complexity of the regulation, and further research is required to understand the basic mechanism behind the light effect on bud outgrowth. Besides CKs located in bud vicinity, it is likely that root-derived CKs contribute to bud outgrowth stimulation in response to light intensity. Strong evidence is given about a main role of competition for carbohydrates, indicated by the source-sink ratio, in bud outgrowth regulation in garden pea and grasses Kebrom et al.
The carbohydrate source-sink ratio may be affected by the plant light environment: a low R:FR ratio enhances stem growth Demotes-Mainard et al. As proposed in some tillering models Luquet et al.
This is supported by studies reporting a negative impact of a low R:FR ratio on the sugar content or on genes related to sugar metabolism and signaling in the bud Kebrom and Mullet, ; Yuan et al. However, the involvement of sugar in the effect of light has not been proved by physiological experiments yet. Experimental data rather indicate that local sugar availability in the stem or in the bud may not be limiting for bud outgrowth in case of low PAR intensity.
In decapitated and defoliated rose plants under white light, preventing light perception by the bud by masking it while leaving the photosynthetic stem under white light maintained the bud inhibited, while applying a photosynthesis inhibitor on the bud did not prevent its outgrowth Girault et al.
In addition, local exogenous sugar supply to decapitated shoot stumps under darkness, to the petioles of intact plants under low PAR intensity, or to rose nodal segments cultivated in vitro in darkness did not induce bud outgrowth Henry et al.
The activity of isolated apical meristems of Arabidopsis was also prevented by darkness and was not restored by exogenous sugar supply Yoshida et al. For both apical meristem and axillary buds under limiting light conditions, CKs may be a limiting factor explaining the inability of sugars to promote bud outgrowth locally.
CKs could act by limiting the bud sink strength for sugars Albacete et al. Experimental studies have revealed an interaction between light and hormonal regulators at the scale of the nodal stem segment and its bud. As illustrated in Figure 2 , an increase in light intensity stimulates CK level in the stem, which promotes bud outgrowth, while a low R:FR ratio stimulates ABA synthesis in the bud, leading in turn to rapid bud inhibition, a process that could be reinforced by auxin signaling increase in stem.
Besides these main pathways, several other endogenous regulators are impacted by light, such as the SL signaling-related genes or sugars, but their exact role has still to be understood. Evidence coming from rose under darkness or low PAR intensity indicates that stem sugars in the vicinity of the bud are not a locally limiting factor of bud outgrowth in these particular light conditions. Literature data on nodal stem segments in vitro rather indicate that light intensity and sugars may have a synergetic effect on bud outgrowth Henry et al.
This leads to the idea that the light lock should be lifted for a high sugar status of the shoot to stimulate bud outgrowth. Additional studies are also required to understand the role of elevated sugar levels in other plant parts than the stem segment bearing the bud in bud outgrowth.
For example, sugars regulate nitrogen uptake by the roots Lejay et al. Figure 2 Interaction of light intensity and the R:FR ratio with the endogenous regulators of bud outgrowth.
A low R:FR ratio stimulates ABA production in the bud, which inhibits bud outgrowth, a phenomenon that is reinforced later on by auxin signaling stimulation through an unknown mechanism solid dark orange arrows and text ; low R:FR also up-regulates the SL signaling-related gene MAX2 dotted dark orange arrows , but the contribution of these changes to bud outgrowth regulation by the R:FR ratio is not known yet. Low light intensity reduces CK contents in the nodal stem by reducing the expression of CK synthesis genes and increasing that of CK degradation genes, which inhibits bud outgrowth solid light orange arrows and text and up-regulates MAX2 but the contribution of this change to bud inhibition by low light intensity is not known yet dotted light orange arrows and text ; low light intensity also decreases the sugar content, but this is not a main limiting factor in the undertaken studies.
For color and arrow significations, see also Figure 1. Light signaling modulates plant involvement in lateral branching by controlling the release of axillary buds from apical dominance. So far, studies mainly conducted on annuals have provided an almost complete picture of the intricate hormonal regulatory network involved in apical dominance, regardless of environmental factors Figure 1.
In particular, great progress has been made since the s with the discovery of SL mutants and of the role of PIN proteins. The development of simulation tools made it easier to investigate complex regulations like those related to the canalization theory or to the SL molecular network, both involving feedbacks Domagalska and Leyser, The demonstration that the degree of competition for sugars within the plant regulates bud outgrowth is more recent Mason et al.
Recent evidences of interplays between sugar and hormones further complicate bud outgrowth regulating network. In addition, the main branching-related hormones display dose-dependent effects on bud outgrowth Chatfield et al.
For instance, H 2 0 2 -dependent bud outgrowth inhibition may be linked to promotion of auxin biosynthesis in the apex which inhibits CK biosynthesis in the stem in tomato Chen et al. The presence of different regulators quantitatively regulating bud outgrowth raises the question of their integration within the bud. Recently, studies indicate that integration could be done in the regulation of carbon metabolism of the bud Tarancon et al. Although the major role of light intensity and quality in branching regulation has been known for decades, knowledge about the interaction between light and the endogenous regulators of bud outgrowth emerged only recently.
The current knowledge Figure 2 indicates that i light intensity stimulates production of CKs inducer of bud outgrowth in the nodal stem segment and ii a low R:FR ratio stimulates production of ABA inhibitor of bud outgrowth in the bud, and this process seems to be later reinforced by an increase in auxin signaling in the stem.
This knowledge remains however very fragmented and does not provide a comprehensive understanding of bud outgrowth regulation at the scale of the plant, as discussed below. First, knowledge is missing about light interaction with other endogenous regulators close to the bud. Indeed, light impacts sugar level and SL signaling Finlayson et al. Second, no study has addressed the question of the role of light effects on organs located at distance from buds.
Light induces changes in plant growth Granier and Tardieu, ; Nagel et al. Light modulation of plant growth may also induce changes in hormone metabolism, signaling, and transport, and thereby hormone distribution and quantities.
Understanding all these changes is necessary for building a comprehensive picture of light effect on bud outgrowth. Third, light regulation of bud outgrowth pattern at the scale of an axis is unknown. Light was reported to influence the number of outgrowing buds and the time between successive outgrowths Demotes-Mainard et al.
Future tasks would be to investigate whether light effect could result from heterogeneous distribution of the different regulators along the axis and from a temporal feedback loop by which outgrowing buds modify the regulator levels in the vicinity of the remainder buds, maintaining them dormant. However, different sensitivities of the buds to their local regulators, due to bud age, light history for example, may obviously complicate bud outgrowth regulation at axis level.
All these elements highlight the complexity of light-mediated bud outgrowth regulation at the plant scale. In recent years, the use of modeling has become prevalent to gain insight into the complex regulation of developmental processes by both endogenous and exogenous processes. These models, combining biological process description with an explicit computational description of the plant biological structure, called functional—structural plant models FSPM , have proved meaningful to address the complexity of developmental systems as a collection of interacting constituents at molecular or cellular level for example.
FSPMs make it possible to identify and test various hypotheses on the local interaction rules and to compare qualitatively and quantitatively, with the experiments, the result emerging from these simulated interactions at an integrated level. This approach has been successfully used in the last decade to study various aspects of plant development such as flowering and inflorescence architecture development Prusinkiewicz et al. In the study of branching regulation as well, these models have been used to help deciphering the complexity of associated regulation networks and branching processes Evers and Vos, —for example, in the analysis of the competition for sugars Luquet et al.
Likewise, approaches combining quantitative experimental observations and computer simulations in FSPMs are thus expected to be instrumental in providing new insights into light interplay with sugar and hormones network in bud outgrowth regulation at the plant scale. The authors declare that the research was conducted in the absence of any personal, professional, or financial relationships that could potentially be construed as a conflict of interest.
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Also known as apical buds, terminal buds are able to hinder the growth of other buds known as axillary buds. Axillary bud also known as lateral bud is located where the leaf petiole is attached to the stem. When the apical bud is removed, the hormone signal stops and the axillary buds can grow more vigorously. Upvote 4. Riya Kumai replied Jul 15, Latent buds are inactive buds found at nodes, while adventitious buds are less visible types of buds.
Apical buds are the dominant buds. These can cause all other lateral buds below them to remain dormant. Apical buds also have special types of tissues known as apical meristem. These cells can divide indefinitely and produce all kinds of plant growth, including reproductive organs and vegetative growth. In most plants, the apical bud or terminal bud is the main area of growth.
Around the apical bud are complex arrangements of internodes, nodes and maturing leaves. Apical buds are responsible for the primary growth of plants and are dominant, while axillary buds are dormant when apical dominance exists. While apical buds are responsible primarily for the growth of the plant in terms of height, axillary buds are responsible for producing clusters of flowers or branches. Apical buds can occur both in monocot and dicot plants, while axillary buds only exist in dicots.
When you prune an apical bud, it stops the hormone signaling process and causes the axillary buds to grow more vigorously.
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