Tillers are specialized panicle-bearing branches of cereal crops (e.g., rice). The tiller in rice determines plant architecture and grain yield: extreme-spreading plants with prostrate growth occupy too much space, whereas extremely compact rice plants are easily susceptible to diseases. Thus, an optimum tiller angle is a desirable agronomic trait for high-density planting to maximize grain yield (Wang and Li, 2022). Tillers originate from axillary buds that develop at the axil of every leaf. As the seedling develops, the leaf primordium of the mother stem differentiates at the shoot apical meristem, and an axillary bud differentiates opposite to it (at 180°). The axillary bud consists of an axillary meristem, a few leaf primordia, and a prophyll. The axillary bud remains dormant (due to apical bud dominance); however, outgrowth begins once apical dominance is released. The first leaf derived from the axillary bud emerges from the subtending leaf sheath of the mother stem, and the bud develops as a tiller. Primary tillers refer to those that originate directly from the main stem, while secondary tillers arise from the primary tillers, and tertiary tillers develop from the secondary tillers. Tiller formation involves several genes associated with plant growth and development, hormone signaling, phototropism, and gravitropism, which shape the overall shoot architecture. The genetic basis of the tiller angle is diverse between rice cultivars and ancestral species. In different regions of the world, compact plant phenotype was selected during the historical process of crop domestication that led to the development of rice cultivars with smaller tiller angles than their ancestral wild relatives. TAC3, D2, and TAC1 have been subjected to selection during the domestication of Asian cultivated rice, and in African rice, the prostrate-to-erect transition occurred due to the selection of the prog7 allele (Dong et al., 2016). However, the prog1 allele controls prostrate growth in wild rice. Functional genomic or mutant studies have identified several other genes involved in gravitropic response, including LAZY1. The lazy1 mutant shows a wider tiller angle and a prostrate growth phenotype. Using a manual biocuration approach, we have synthesized gene interaction networks associated with shoot gravitropism. The evidence of each reaction and associated information is provided in reaction summations. (Note: Dicots, such as Arabidopsis, do not have tillers; instead, they have rosette branches and cauline branches, which arise from leaf axils of the inflorescence stem. However, some of the genes involved in shoot gravitropism are conserved among higher plants).
