SlMYB21 and SlMYB24 cooperatively regulate petal coloration and movement in Solanum lycopersicum
In seed plants, reproductive success depends on the precise coordination of floral organ development. Here, we identify and characterize the Solanum lycopersicum transcription factor (TF) SlMYB24 as a member of the R2R3-MYB subgroup 19. While Slmyb24 mutants exhibited no obvious phenotypic alterations, the previously characterized Slmyb21 mutant displayed defects in floral development that were further exacerbated in the Slmyb21 Slmyb24 double mutant, resulting in a severe phenotype characterized by female sterility and whitish petals that failed to open. Petals of Slmyb21 Slmyb24 exhibited reduced xanthophyll content and elevated abscisic acid (ABA) levels, accompanied by reduced water content and decreased levels of primary sugars. Integration of transcriptome (RNA-sequencing) and cistrome (DNA affinity purification sequencing, DAP-seq) data identified genes involved in carotenoid and ABA biosynthesis and water transport as candidate regulatory targets of SlMYB21 and SlMYB24. These TFs activated the promoters of PHYTOENE SYNTHASE2 (PSY2), LYCOPENE {beta}-CYCLASE (LCY-B) and the petal-expressed aquaporin TONOPLAST INTRINSIC PROTEIN1.1 (TIP1.1), whereas they repressed 9-cis-EPOXYCAROTENOID DIOXYGENASE1 (NCED1) promoter activity. In addition, SlMYB21 bound to the SlMYB24 promoter and activated its expression. Promoter studies and electrophoretic mobility shift assays revealed that SlMYB21 and SlMYB24 bind the AC-rich DNA-binding motif GTTAGGT, which is to be found in the respective promoters and is consistent with the extensive overlap in their binding profiles. Together, these results reveal that the functions of SlMYB21 and SlMYB24 extend beyond fertility to coordinate petal pigmentation and opening.