and Redundantly Shape Rice Tiller Angle by Reducing Expression and Auxin Content.

Tiller angle largely determines plant architecture, which in turn substantially influences crop production by affecting planting density. A recent study revealed that () acts upstream of () to regulate tiller angle establishment in rice (). However, the mechanisms underlying transcriptional regulation of remain unknown. In this study, two class II homeodomain-Leu zipper genes, and , were identified as positive regulators of tiller angle by affecting shoot gravitropism. and showed strong transcriptional suppressive activity in rice protoplasts and formed intricate self- and mutual-transcriptional negative feedback loops. Moreover, OsHOX1 and OsHOX28 bound to the pseudopalindromic sequence CAAT(C/G)ATTG within the promoter of , thus suppressing its expression. In contrast to and , and attenuated lateral auxin transport, thus repressing the expression of () and in the lower side of the shoot base of plants subjected to gravistimulation. Genetic analysis further confirmed that and act upstream of Additionally, both and inhibit the expression of multiple genes and decrease auxin biosynthesis. Taken together, these results demonstrated that and regulate the local distribution of auxin, and thus tiller angle establishment, through suppression of the - pathway and reduction of endogenous auxin content. Our finding increases the knowledge concerning fine tuning of tiller angles to optimize plant architecture in rice.