TILLER ANGLE CONTROL 1 modulates plant architecture in response to photosynthetic signals.

Light serves as an important environmental cue in regulating plant architecture. Previous work had demonstrated that both photoreceptor-mediated signaling and photosynthesis play a role in determining the orientation of plant organs. TILLER ANGLE CONTROL 1 (TAC1) was recently shown to function in setting the orientation of lateral branches in diverse plant species, but the degree to which it plays a role in light-mediated phenotypes is unknown. Here, we demonstrated that TAC1 expression was light dependent, as expression was lost under continuous dark or far-red growth conditions, but did not drop to these low levels during a diurnal time course. Loss of TAC1 in the dark was gradual, and experiments with photoreceptor mutants indicated this was not dependent upon red/far-red or blue light signaling, but partially required the signaling integrator CONSTITUTIVE PHOTOMORPHOGENESIS 1 (COP1). Overexpression of TAC1 partially prevented the narrowing of branch angles in the dark or under far-red light. Treatment with the carotenoid biosynthesis inhibitor norflurazon or the PSII inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) led to loss of TAC1 expression similar to dark or far-red conditions, but expression increased in response to the PSI inhibitor paraquat. Treatment of adult plants with norflurazon resulted in upward growth angle of branch tips. Our results indicate that TAC1 plays an important role in modulating plant architecture in response to photosynthetic signals.