Basal branching in grasses, or tillering, is an important trait determining both form and function of crops. Although similarities exist between eudicot and grass branching programs, one notable difference is that the tiller buds of grasses are covered by the subtending leaf, whereas eudicot buds are typically unconstrained. The current study shows that contact with the leaf sheath represses sorghum bud growth by providing a mechanical signal that cues the bud to refrain from rapid growth. Leaf removal resulted in massive reprogramming of the bud transcriptome that included signatures of epigenetic modifications and also implicated several hormones in the response. Bud abscisic acid transiently increased, then decreased following leaf removal relative to controls, and abscisic acid was necessary to repress bud growth in the presence of the leaf. Jasmonic acid (JA) levels and signalling increased in buds following leaf removal. Remarkably, application of JA to buds in situ promoted growth. The repression of bud growth by leaf contact shares characteristics of thigmomorphogenic responses in other systems, including the involvement of JA, though the JA effect is opposite. The repression of bud growth by leaf contact may represent a mechanism to time tillering to an appropriate developmental stage of the plant.
