Interplay of auxin and jasmonates in tomato gynoecia development and fruit set

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Successful plant reproduction depends on tight coordination of flower-organ development, fertilization and fruit set. The plant hormones auxin and jasmonic acid (JA) play important roles in the timing and coordination of these processes. Our preliminary work suggested that auxin and JA interact in the regulation of gynoecia development and of the initiation of fruit set in response to fertilization. It also identified the transcription factors SlMYB21 and SlMYB24 as regulated by both auxin and JA. In the first period of the project, we identified the auxin-response factors SlARF8A and SlARF8B as central regulators of gynoecia development and fruit set. We found that Slarf8a Slarf8b double mutants are impaired in style and stigma development and produce seedless fruits independently of fertilization. Furthermore, gynoecia of Slarf8a Slarf8b mutant plants have reduced JA levels as well as SlMYB21 and SlMYB24 expression. We have generated mutants harboring a knockout of SlMYB24 and Slmyb21 Slmyb24 double mutant plants and found that Slmyb24 substantially enhances the Slmyb21 phenotype with respect to gynoecia development and fruit set. These results suggest that SlARF8, SlMYB21/24 and JA interact to regulate flower and fruit development. In the renewal of the proposal, we aim to address the following hypotheses: (i) SlARF8 promotes JA production, possibly by activating SlMYB21, and SlMYB21 inhibits SlARF8 expression; (ii) SlMYB21 and SlMYB24 act together to regulate flower development and the transition to fruit set; and (iii) SlARF8, SlMYB21/24 and JA are central regulators of stigma development. We will test these hypotheses by examining the genetic and molecular interactions among SlARF8, SlMYB21/24 and JA in gynoecia development and particularly stigma development, and in fruit set. We will: 1. Elucidate the regulatory interaction between SlMYB21 and SlARFs by generation and characterization of double and triple mutants. 2. Characterize the effect of simultaneous loss of function of both SlMYB21 and SlMYB24. 3. Characterize stigma development and gene expression in wild type and mutant plants. The research is expected to provide insights on the coordinated function of MYBs and ARFs in tomato reproduction and will shed light on the crosstalk between auxin and JA in the regulation of reproduction in tomato.