Control of Outgrowth and Dormancy in Axillary Buds

Abstract

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