Photosynthetic leaf area modulates tiller bud outgrowth in sorghum

Shoot branches or tillers develop from axillary buds. The dormancy versus outgrowth fates of buds depends on genetic, environmental and hormonal signals. Defoliation inhibits bud outgrowth indicating the role of leaf-derived metabolic factors such as sucrose in bud outgrowth. In this study, the sensitivity of bud outgrowth to selective defoliation was investigated. At 6d after planting (6 DAP), the first two leaves of sorghum were fully expanded and the third was partially emerged. Therefore, the leaves were selectively defoliated at 6 DAP and the length of the bud in the first leaf axil was measured at 8 DAP. Bud outgrowth was inhibited by defoliation of only 2cm from the tip of the second leaf blade. The expression of dormancy and sucrose-starvation marker genes was up-regulated and cell cycle and sucrose-inducible genes was down-regulated during the first 24h post-defoliation of the second leaf. At 48h, the expression of these genes was similar to controls as the defoliated plant recovers. Our results demonstrate that small changes in photosynthetic leaf area affect the propensity of tiller buds for outgrowth. Therefore, variation in leaf area and photosynthetic activity should be included when integrating sucrose into models of shoot branching. The importance of sucrose as a primary regulator of shoot branching or tillering has been reported recently (Kebrom etal., 2012; Mason etal., 2014). Sucrose levels in tiller buds reflect the relative balance between sucrose supply from photosynthetic leaves, and sucrose utilization for growth, metabolism, and/or storage. Our paper demonstrates that bud outgrowth can be inhibited by a reduction in photosynthetic leaf area providing the basis for integrating sucrose, in addition to hormonal and environmental signals, into models of shoot branch regulation. Commentary: