Vegetative axillary bud dormancy induced by shade and defoliation signals in the grasses

Vegetative axillary bud dormancy and outgrowth is regulated by several hormonal and environmental signals. In perennials, the dormancy induced by hormonal and environmental signals has been categorized as eco-, endo- or para-dormancy. Over the past several decades para-dormancy has primarily been investigated in eudicot annuals. Recently, we initiated a study using the monoculm phyB mutant (phyB-1) and the freely branching near isogenic wild type (WT) sorghum (Sorghum bicolor) to identify molecular mechanisms and signaling pathways regulating dormancy and outgrowth of axillary buds in the grasses. In a paper published in the January 2010 issue of Plant Cell and Environment, we reported the role of branching genes in the inhibition of bud outgrowth by phyB, shade and defoliation signals. Here we present a model that depicts the molecular mechanisms and pathways regulating axillary bud dormancy induced by shade and defoliation signals in the grasses.Key words: axillary bud, dormancy, shade, phytochrome, defoliation, shoot branching, teosinte branched1, MAX2, cell cycle, sorghumThe dormancy and outgrowth of axillary buds is regulated by several plant hormones such as auxin, cytokinins, abscisic acid and strigolactones, and by environmental factors such as light quality, quantity and duration as well as water, temperature and nutrient status.^1–3 Since the fate of an axillary bud is regulated by such diverse hormonal and environmental signals and their interactions, the type of dormancy induced varies. In perennials, three types of bud dormancy have been identified.^4,5 Dormancy mediated by factors within the bud is known as endo-dormancy; while dormancy induced by factors within the plant but outside the bud is called paradormancy or correlative inhibition; the best known example being apical dominance. Dormancy induced due to unfavorable environmental conditions is known as eco-dormancy. Although there is an indepth knowledge about para-dormancy in annuals,⁶ few studies have been conducted on eco-dormancy. Similarly, studies of endo-dormancy have largely been restricted to low-temperature mediated growth-cessation of axillary buds of perennial plants.^7,8 To understand the regulation of dormancy and outgrowth of axillary buds in monocots, we initiated a study on the molecular mechanisms inhibiting bud outgrowth by shade and defoliation signals in sorghum. Our results published in the January 2010 issue of Plant, Cell & Environment indicate that different types of dormancy may be induced in axillary buds of annual grasses by various signals and there may be overlapping and independent molecular mechanisms mediating induction of axillary bud dormancy.