Update on Hormonal Regulation of Branching in Grasses Hormonal Regulation of Branching in Grasses

Axillary meristems, which form in the axils of leaves, play an essential role in plant architecture and reproduction. During vegetative development, axillary meristems give rise to branches, called tillers in grasses, while during reproductive development, axillary meristems give rise to flowering branches or to flowers. The control of branching by axillary meristems is under hormonal, environmental, developmental, and genetic control. In this Update I review the role of hormones in regulation of axillary meristem initiation and outgrowth during both vegetative and inflorescence branching. Hormones play a critical role in regulating branching (McSteen and Leyser, 2005; Beveridge, 2006; Ongaro and Leyser, 2008). Auxin is required for axillary meristem initiation during both vegetative and inflorescence development. In addition, basipetal movement of auxin from the shoot apex suppresses axillary bud outgrowth in the phenomenon known as apical dominance. Cytokinin regulates meristem size and hence indirectly affects branching (Shani et al., 2006; Kyozuka, 2007). In addition, acropetal movement of cytokinin coming from the roots promotes axillary bud outgrowth. It has long been proposed that an additional hormone travels acropetally from the root to inhibit bud outgrowth. The highlight of last year was the identification of this new plant growth hormone, strigolactone (Gomez-Roldan et al., 2008; Umehara et al., 2008). The environment also plays a significant role in regulating branch outgrowth. It is commonly known, especially in grasses, that increased planting density leads to reduced branching (Doust 2007a, 2007b; Kebrom and Brutnell, 2007). This could be due to shading, which is known to inhibit branching, or competition for resources, as fertilizer and nutrients are known to promote branching. Developmental control of axillary meristems is evident from the different fates of axillary meristems during vegetative and reproductive development in different species (Steeves and Sussex, 1989; McSteen and Leyser, 2005). During vegetative development in Arabidopsis (Arabidopsis thaliana), axillary meristems appear late during leaf ontogeny, produce a few leaf primordia, and then arrest until signaled to grow. In maize (Zea mays), axillary buds remain suppressed during development (except for the ear shoot), leading to a single axis of growth (Fig. 1A; Kiesselbach, 1949). In rice (Oryza sativa), axillary meristems grow out to produce a highly tillered plant, though the buds are still under hormonal and environmental control (Fig. 1B; Shimamoto and Kyozuka, 2002). During inflorescence development in Arabidopsis, floral meristems arise in the axils of reduced leaves called bract leaves (Long and Barton, 2000; Grbic, 2005). In grass inflorescence development, axillary meristems give rise to branches and spikelets before they give rise to flowers (McSteen et al., 2000; Bommert et al., 2005). The identity and determinacy of these different meristem types are controlled by transcription factors (Bortiri andHake, 2007;Update by Thompson and Hake, this issue [Thompson and Hake, 2009]). Although hormones have been implicated in regulation of inflorescence branching, the exact mechanism is unknown (Barazesh and McSteen, 2008). The differences in the activity of axillary meristems produced during development imply that all axillary meristems do not respond similarly to the same stimuli. For example, in Arabidopsis, there is acropetal outgrowth of buds during vegetative development and basipetal outgrowth of axillary buds during reproductive development (Hempel and Feldman, 1994; McSteen and Leyser, 2005). In grasses, the basal nodes are the ones from which tillers arise (Fig. 1B). Heterochronic mutations that extend the juvenile phase lead to increased tiller number (Poethig, 1988; Chuck et al., 2007). In foxtail millet (Setaria italica), branches are also produced from the upper nodes of the plant, in addition to tillers from basal nodes, and these are under separable genetic and environmental control (Doust et al., 2004; Doust, 2007a). These studies indicate that genetic, hormonal, and environmental signals intersect with developmental signals. Many genes have been identified that regulate axillary meristem initiation and outgrowth during vegetative and reproductive development (Schmitz and Theres, 2005; Bennett and Leyser, 2006; Doust 2007b). Most of these genes have been identified as mutants 1 This work was supported by the National Science Foundation and the U.S. Department of Agriculture. 2 Note on genetic nomenclature: Arabidopsis and rice use the same nomenclature, but the nomenclature differs for maize. For consistency, the Arabidopsis/rice nomenclature was used throughout this article even for maize and other grasses. * E-mail [email protected]. The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Paula McSteen ([email protected]). [C] Some figures in this article are displayed in color online but in black and white in the print edition. www.plantphysiol.org/cgi/doi/10.1104/pp.108.129056