Cytokinin-inducible response regulator SlRR6 regulates plant height through gibberellin and auxin pathways in tomato.

Plant height is a key agronomic trait regulated by several phytohormones like gibberellins (GAs) and auxin. However, little is known about how cytokinin (CK) participates in this process. Here, we report that SlRR6, a type-A response regulator in CK signaling pathway, positively regulates plant height in tomato. SlRR6 was induced by exogenous kinetin and GA3, but inhibited by indole-3-acetic acid (IAA). Knockout of SlRR6 reduced tomato plant height through shortening internode length, while overexpression of SlRR6 caused higher plant due to increased internode number. Cytological observation of longitudinal stems showed that both knockout and overexpression of SlRR6 generated larger cells, but significantly reduced cell numbers in each internode. Further studies demonstrated that overexpression of SlRR6 enhanced GA accumulation and lowered IAA content, along with expression changes in GA- and IAA-related genes. Exogenous paclobutrazol and IAA treatments restored the increased plant height phenotype in SlRR6-overexpressing lines. Yeast two-hybrid, bimolecular fluorescence complementation and co-immunoprecipitation assays showed that SlRR6 interacts with a small auxin up RNA protein SlSAUR58. Moreover, SlSAUR58-overexpressing plants were dwarf with decreased internode length. Overall, our findings establish SlRR6 as a vital component in the CK signaling, GA, and IAA regulatory network that controls plant height.