Control of Outgrowth and Dormancy in Axillary Buds

The shoot system has an important role in generating a large variety of diverse plant forms (Steeves and Sussex, 1989). The overall architecture of the shoot system is derived from the activity of the primary shoot apical meristem (SAM), arising during embryogenesis, together with the activity of the additional meristems subsequently formed after seed germination. The primary SAM provides the main axis of the plant body. Plant architecture is further modified by shoot branching that results from the activity of the additional meristems. The complexity of the branching pattern depends on the temporal and spatial development of these branches. These characteristics, although they are plastic in their response to environmental cues, are genetically determined. The developmental program that specifies branching patterns in different plant species is fundamentally important for generating species-specific plant forms. The shoot branching process generally involves two developmental stages: the formation of axillary meristems in the leaf axils and the growth of axillary buds. In many plant species, the growth of axillary meristems is inhibited by the primary shoot or primary inflorescence. This phenomenon is generally known as apical dominance. The plant hormones auxin and cytokinin are thought to have a major role in controlling this process (Phillips, 1975; Cline, 1994; Tamas, 1995; Napoli et al., 1999). Auxin has an inhibitory effect on the growth of axillary buds, whereas cytokinin promotes axillary bud outgrowth. The mechanisms of axillary bud outgrowth depend on the ratio of these two hormones rather than the absolute levels of either hormone. A variety of experimental approaches have been used to examine the mechanisms controlling dormancy and outgrowth of axillary buds. These range from physiological studies, such as measurement and exogenous application of plant hormones, to analyses of transgenic plants overexpressing hormone biosynthetic genes to alter endogenous hormone levels. Isolation and characterization of mutations that cause alterations in shoot branching patterns are powerful approaches. These molecular genetic approaches combined with the conventional physiological studies, such as grafting experiments, revealed that not only do auxin and cytokinin function to control the growth of axillary buds, but other factors and/or signals also have important roles. More recently, the genes expressed in dormant axillary buds were isolated and characterized. This review focuses on recent findings uncovered by physiological, genetic, and molecular studies and approaches to investigate the control of shoot branching, apical dominance, and dormancy in plants. Regulation of the initiation of axillary meristems is important for controlling the overall plant form (Kerstetter and Hake, 1997;Schmitz and Theres, 1999; Sussex and Kerk, 2001). Axillary meristems are typically located on the leaf axils. In some plants like tomato, groups of meristematic cells appear to be derived directly from the SAM of the main shoot. The cells can be recognized in early developmental stages of leaf primordia in the axils. These observations suggest that axillary meristems are formed from detached parts of the primary SAM. In other plants, including Arabidopsis, axillary meristems cannot be detected in the axils during the vegetative growth phase of the primary SAM. After the primary SAM is transformed into the reproductive phase, that is, an inflorescence meristem, differentiated cells in the leaf axils undergo dedifferentiation and regain meristematic potential. Thus, axillary meristems are formed in the leaf axils. Extrapolated developmental stages of outgrowth and dormancy in axillary buds. Axillary meristems are initiated in the leaf axil and form axillary buds (1). The axillary buds enter transition stages (2). Transition stages indicate an intermediate between dormancy and growth. Newly formed axillary buds in transition stages become dormant, dependent on their developmental program (5). Alternatively, some axillary buds in transition stages undergo growth (6). Usually dormant axillary buds enter transition stages through (5) and (3) in response to their developmental program or environmental cue, and then undergo growth (6). In some cases, axillary buds in temporary growth at transition stages might re-enter dormancy through (4) and (5). Dormancy, broadly defined, is “the temporary suspension of visible growth of any plant structure containing a meristem” (Lang, 1987). Apical dominance is defined broadly as “the inhibitory control of the shoot apex over the outgrowth of lateral buds” (Cline, 1997;Napoli et al., 1999). Apical dominance acts as a plant survival mechanism by providing a reservoir of meristems that can replace the damaged primary shoot. This mechanism works when the primary shoot is damaged or removed through disease, herbivore grazing, or pruning. Apical dominance can also be released, depending not only on environmental cues but also on developmental programs. In some plants, dormant axillary buds start their outgrowth after the primary SAM differentiates into the determinate organ, such as a flower or an inflorescence meristem. These supplemental additional shoots are important for increasing the total number of leaves or flowers to be more fruitful. Apical dominance controls bud growth in the vegetative developmental stages of many herbaceous plants and the juvenile stages of some trees (Cline, 2000). In woody plants, seasonal cycles of growth and dormancy occur in over-wintering buds (Powell, 1988). For example, at the end of the growing season, perennial plants cease development and assume a dormant and freezing-tolerant state, even when temperatures still favor growth. This strategy protects against the sudden arrival of winter. Later in winter, they might anticipate spring by breaking dormancy while the freezing tolerance remains high (Weiser, 1970). It is not known whether the mechanism underlying the seasonal cycle of growth and dormancy in perennial plants is the same or similar to that in axillary buds of herbaceous plants. Therefore, our discussion on dormancy and growth of axillary buds is limited to herbaceous plants. Apical dominance was one of the first developmental phenomena shown to be regulated by plant hormones (Thimann and Skoog, 1934;Thimann, 1937). Auxin, derived from the terminal bud, inhibits the growth of axillary buds, whereas cytokinin derived mainly from the roots, promotes the growth of axillary buds. The role of auxin in vivo is supported by the following observations. Decapitation ofVicia spp. plants causes the outgrowth of axillary buds, but application of auxin to the stump prevents the outgrowth of axillary buds. These observations have been confirmed in many plant species. Furthermore, application of the auxin-transport inhibitor, 2,3,5-triiodobenzoic acid, in lanolin to the stems of intact plants can reduce or abolish apical dominance (Snyder, 1949; Panigrahi and Audus, 1966). These data strongly support the hypothesis that apically derived auxin is transported basipetally and inhibits outgrowth of the axillary buds. In addition, direct application of auxin to axillary buds cannot prevent bud growth. Radiolabeled auxin applied to the stump is not translocated into the axillary buds. The indoleacetic acid (IAA) level of dormant axillary buds is low and that of the axillary buds after decapitation of the terminal shoot actually increases (Gocal et al., 1991). On the other hand, direct application of cytokinin to axillary buds promotes the outgrowth of axillary buds. As with auxin, these observations have been confirmed in many plant species. Outgrowth of axillary buds is well correlated with the cytokinin level in the buds. It is thought that auxin acts to control the concentration of cytokinin derived from the roots (Bangerth, 1994). Cytokinins are produced not only in the root, however, but also in the shoot (Chen et al., 1985). Cytokinin concentrations in bean xylem exudate of bean after decapitation increase within 16 h and gradually return to basal levels (Bangerth, 1994), and those in chickpea axillary buds after decapitation increase 7-fold by 6 h and 25-fold by 24 h (Turnbull et al., 1997), suggesting that cytokinins are necessary to initiate outgrowth of axillary buds. Several reports indicate that the abscisic acid (ABA) content of axillary buds is closely correlated with bud dormancy. The decline of the ABA level after decapitation precedes the onset of bud outgrowth, whereas application of auxin to the stump recovers the ABA level to that before decapitation (Knox and Wareing, 1984). Moreover, theera1 mutant, which is hypersensitive to ABA, has reduced branching (Pei et al., 1998), suggesting that ABA inhibits bud outgrowth. Little is known, however, about the relationship between ABA and auxin in growth inhibition. A physiological approach was recently taken using a new assay system in Arabidopsis (Chatfield et al., 2000). In this assay system, plant hormones are applied to both ends of excised nodal sections including axillary buds. It was reported that cytokinin acts independently to regulate the growth of axillary buds, rather than as a second messenger for auxin. This type of experiment with hormone-signaling or -synthesizing mutants might provide new findings regarding hormonal regulation of axillary bud growth. There is an interesting study of apical dominance using Lupinus angustifolius. The vegetative shoot of this plant produces approximately 20 nodes. The axillary buds at nodes 1 through 5 (counting acropetally) and at nodes 13 through 20 grow rapidly, whereas axillary buds at nodes 6 through 12 grow slowly in intact plants. Decapitation of the main shoot above node 12 promotes accelerated growth of the axillary buds at nodes 8 through 12 (Miguel et al., 1998). Thus, the axillary buds in the mid-region of this plant have the potential to grow, however they cannot grow on intact plants. The concentrations of IAA, cytokinins, and ABA were measured in the apical meristems of axillary buds at different stages of development (Emery et al., 1998). The rate of bud growth did not correlate with the absolute concentrations of IAA or cytokinins. The ratio of these two hormones, however, correlated with the rate of axillary bud growth in the early developmental stages. The ratio of cytokinins to IAA was high in the rapidly growing axillary buds at the basal and upper nodes, whereas the ratio was low in the slow growing axillary buds at the middle node. In early developmental stages, ABA concentrations did not correlate with the rate of axillary bud growth. In later developmental stages, ABA had a strong negative correlation with the rate of axillary bud growth when the cytokinins to IAA ratio did not correlate with the rate of axillary bud growth. Thus, the potential of axillary bud outgrowth, which is related to position on the main axis, appears to be determined by a balance among several hormones. This balance can be changed during developmental stages. It is possible that outgrowth and dormancy of axillary buds is regulated by the hormonal status of particular plant organs, depending on their developmental program. Analyses of transgenic plants have also contributed to a better understanding of the role of plant hormones in the control of apical dominance. Transgenic cytokinin-overproducing plants, e.g.ipt (isopentenyl transferase from Agrobacterium tumefaciens), and transgenic plants with reduced levels of IAA, e.g. iaaL (indole-3-acetic acid Lys synthetase fromPseudomonas savastanoi), have been produced and reported to exhibit reduced apical dominance (Medford et al., 1989; Romano et al., 1991). On the other hand, transgenic plants with elevated levels of IAA, e.g. iaaH and iaaM (indoleacetamide hydrolase and Trp monooxygenase from A. tumefaciens), had increased apical dominance (Sitbon et al., 1992; Romano et al., 1993). In addition, transgenic plants that contain both IAA- and cytokinin-overproducing genes have an intermediate phenotype (Klee and Estelle, 1991). These results support the theory that the ratio of auxin to cytokinin might be the central factor controlling the growth of axillary buds. It is unclear, however, whether auxin is a primary signal to induce changes in growth and development of axillary buds. Auxin alone increases ethylene levels. Therefore, to uncouple auxin and ethylene effects, transgenic plants with increased levels of auxin were crossed with transgenic plants defective in ethylene biosynthesis (Romano et al., 1993). The results indicate that auxin increases apical dominance independently of the ethylene concentration. It seems, however, that the ratio of auxin to cytokinin is not the only regulatory factor that influences apical dominance. In both transgenic cytokinin-overproducing plants and transgenic reduced-auxin plants, the axillary buds remained dormant in the vegetative stage even though the ratio of auxin to cytokinin was altered. In the reproductive stage of these transgenic plants, the axillary buds grow more rapidly than do the wild-type plants. It is possible that the axillary buds of juvenile transgenic plants are insensitive to a favorable ratio of hormones for release of apical dominance, or alternatively, some other factors are involved in these mechanisms. More recently, it was proposed that indole-3-acetaldoxime is the metabolic branch between IAA and indole glucosinolate biosynthesis in Arabidopsis (Bak et al., 2001). Loss-of-function cytochrome P450, CYP83B1, mutants exhibit increased apical dominance, whereas gain-of-function CYP83B1 mutants have decreased apical dominance. In addition, CYP83B1 catalyzes the first step in indole glucosinolate biosynthesis by metabolizing indole-3-acetaldoxime. These results suggest that the level of IAA is regulated by the flux of indole-3-acetaldoxime to the glucosinolate pathway byCYP83B1 gene activity. Thus, the hormone levels regulated by the P450 gene family might control the growth of axillary buds. To analyze the mechanisms controlling apical dominance and shoot branching, mutants were isolated and characterized from Arabidopsis, maize, tomato, petunia, and pea. These mutants were divided into three classes based on their phenotype. In the first class, the mutants had increased branching. This phenotype is caused by the release of dormant axillary buds, that is, release of apical dominance. In this category, mutations do not affect early developmental stages of the axillary meristem. In other words, the timing and the number of formed axillary meristems are normal. teosinte branced 1, decreased apical dominance, ramosus, auxin resistant, and iaa28 mutants belong to this category. Leyser's group recently isolated novel mutants max 1–4 (more axillary branching 1–4). At the 17th International Conference on Plant Growth Substances, Leyser reported thatMAX2 acts in the bud and encodes an F-box protein, which is expected to be involved in auxin signal transduction. MAX3 acts outside the bud. MAX4 might act outside the bud and encodes an NCED (nine-cis-epoxycarotenoid dioxygenase)-like protein, which is thought to be the rate-limiting enzyme in ABA biosynthesis. Because the data are not published, however, the details are not known. In the second class, there is an increased number of formed axillary meristems. These mutants are affected in early developmental stages of the axillary meristem. The supershoot/bushymutant belongs to this category. In the third class, the mutants have decreased branching. Mutations in this class usually affect the axillary meristem initiation. The lateral suppressor andtorosa-2 mutants belong to this category. Some reported mutants characterized so far are described as follows. Teosinte is a wild ancestor of maize and has elongated axillary shoots with terminal male inflorescences (tassels) at most nodes and short secondary axillary shoots that bear female inflorescences (ears;Doebley et al., 1997). It seems that TB1 acts as a repressor of axillary shoot growth and regulates the sex of the inflorescences terminating the shoot. The TB1 gene encodes a transcriptional regulator that might function in proliferating tissues to influence growth and cell division. Based on the patterns of TB1 in the TB1 gene be or expressed at low levels in the primordia of primary to On the other hand, in maize, the TB1 gene be in lateral shoot their outgrowth. The has a growth pattern from a of studies indicate that a when wild-type is to a phenotype similar to the wild Furthermore, a wild-type between a and a is to the to wild Thus, is in that branching. It is possible that the wild-type gene act as of axillary To there is molecular on the of this In several mutants have been isolated based on the phenotype of branching patterns et al., and mutants have increased branching at basal and nodes, reduced and elevated levels of indicate that apical dominance is in both mutants by grafting a wild-type et al., et al., 2001). analyses revealed that and control a different pathway in the regulation of the branching of endogenous hormone levels in these mutants suggest that the endogenous levels of IAA and cytokinin are not correlated with the of apical dominance, and cytokinin is not the only signal branching in plants. It is possible that the additional novel signal regulated by the gene in shoots and acts as a branching The was on the of to auxin in of have reduced auxin The phenotype is such as and increased shoot branching at These observations suggest that the gene is for auxin The gene encodes a related to enzyme et al., 1993). The development of lateral shoots in plants was with that in wild-type plants et al., 1999). not affect the timing of axillary meristem The mutant, however, has more growth of the lateral These findings suggest that auxin in the stages of lateral shoot growth following formation of the axillary meristem. These findings are with the analyses of transgenic plants. The Arabidopsis has defective lateral plant and decreased apical dominance et al., 2001). The gene encodes a of the Analyses of the function suggest the of genes that induce lateral initiation in response to auxin The isolated from Arabidopsis or more inflorescences in a plant et al., et al., 2001). of the plants that the primary is an increase in the number of meristems formed in leaf together with release of bud in branch formation from and The gene encodes a cytochrome P450, In plants, the levels of cytokinin are increased to with wild-type plants. These findings suggest that gene to hormone levels in plants. The gene is strongly expressed at the leaf axils. It is possible that the in cytokinin levels at of bud initiation control both initiation of the axillary meristem and growth of axillary buds. IAA content and however, are also increased in Therefore, the that IAA to these cannot be Transgenic Arabidopsis with of gene has a of apical dominance et al., 2001). In tomato, several mutations defective in axillary meristem initiation have been isolated The prevents the initiation of axillary meristems during the vegetative phase and although axillary buds form after In addition, have a in development to the of the second of flower The results of a different that the endogenous of acid auxin, and ABA in the shoot are whereas cytokinin levels are The belongs to a family of of et al., 1999). This family the Arabidopsis et al., and et al., 1998). genes act as negative of the signal This to the hypothesis that also as a negative regulator in and also has a role in controlling the formation of axillary meristems. the limited of and however, more experimental is to support this The in the axillary meristem in many leaf axils. This is correlated with reduced levels of cytokinin in the plants with the wild type and In both and the primary SAM is than normal. It is possible that the in the primary SAM the initiation of axillary meristems. In tomato, axillary meristems are directly derived from the primary SAM. The primary SAM in these mutants might not the axillary meristems of the Several molecular approaches have been used to the with the outgrowth and dormancy of axillary buds. The of axillary buds before and after decapitation was using and Sussex, 1988). of are in the stages. In addition, the dormant axillary buds at a rate similar to that of growing buds. This that dormant axillary buds are as as growing buds. Based on these of genes expressed in dormant axillary buds might have a role in the of dormant stages in axillary buds. Therefore, several such as and were isolated and characterized from plants et al., and The acid is similar to that of an whereas that of has to that of and from On the other hand, a of the to any are well known that were similar to and and are in although the acid are not mainly in dormant axillary buds on intact plants. The of both rapidly after decapitation of the terminal buds, whereas it rapidly when axillary buds become dormant to The temporal and spatial patterns of were using analyses and in the leaf and apical meristem in dormant axillary buds. After in the axillary buds. These changes occur in a similar to the in which the differentiates into these observations suggest that has a role in the of growth and or in the of the dormant stages in axillary buds. The of axillary buds following of the terminal bud was to the release from apical dominance. Analyses of the that of the terminal bud rapidly promotes cell in axillary buds 1987). In axillary buds, the proliferating and of the cell cycle in which the dormant bud cells are cell cycles are characterized by patterns of several cell genes and 1995; and These gene in a cell For example, the of and cell during the phase, the and the phase, and and The levels of the genes were low in the dormant axillary buds on intact plants. the axillary buds were to grow by levels increased Moreover, the patterns of of the genes were and then These results suggest that most cells in dormant axillary buds are at the phase in the cell In the and of the cell cycle are during the phase in response to the developmental program and environmental and negative and the controls the and of the cell cycle and the of cell cycle was detected in dormant axillary buds of plants. The was in axillary buds after decapitation and In one of the most important for the is the of the suppressor gene and The of are regulated by in a cell The and characterization of some from plants were recently reported et al., 2000). with an against an of plants, the of the was in axillary buds during the After decapitation of the terminal bud, the plant was and these results suggest that the transition in axillary buds is by mechanisms similar to those the cell cycle in of control mechanism of apical dominance by plant hormones. This review focuses on recent in understanding the control of axillary bud outgrowth and dormancy. approaches have been used to analyze the molecular mechanisms of the regulation of the growth of axillary buds. The conventional plant physiological such as exogenous application of plant hormones, that plant hormones have an important role in axillary bud growth. Analyses of transgenic plants with endogenous hormone levels confirmed the of plant hormones. The and characterization of mutations that cause alterations in shoot branching patterns have become powerful approaches. In addition, molecular genetic approaches combined with grafting have new Several plant hormones control outgrowth and dormancy of axillary buds. factors also have a role in axillary bud outgrowth and dormancy. This phenomenon of apical dominance be better by the findings from approaches.