CRISPR/Cas9 mutants of tomato MICRORNA164 genes uncover their functional specialization in development

Plant MICRORNA164 (miR164) plays diverse regulatory functions by post-transcriptional repression of certain NAM/ATAF/CUC-domain transcription factors. However, the involvement of miR164 in fleshy fruit development and ripening remains poorly understood. Here, de novo prediction of tomato (Solanum lycopersicum) MIR164 genes identified four genes (SlMIR164a-d), of which SlMIR164d has an atypically long pre-miRNA. The roles of the fruit expressed SlMIR164a, b, and d were studied by analysis of their Clustered Regularly Interspaced Short Palindromic Repeats mutants. The slmir164bCR mutant plants exhibited shoot and flower abnormalities characteristic of ectopic boundary specification, whereas the shoot and flower development of slmir164aCR and slmir164dCR mutants were indistinguishable from wild-type. Strikingly, the knockout of SlMIR164a practically eliminated sly-miR164 from the developing and ripening fruit pericarp. The sly-miR164-deficient slmir164aCR fruits were smaller than the wild-type, due to reduced pericarp cell division and expansion, and displayed intense red color and matte, instead of glossy appearance, upon ripening. We found that the fruit skin phenotypes were associated with morphologically abnormal outer epidermis and thicker cuticle. Quantitation of sly-miR164 target transcripts in slmir164aCR ripening fruits demonstrated the upregulation of SlNAM3 and SlNAM2. Specific expression of their miR164-resistant versions in the pericarp resulted in the formation of extremely small fruits with abnormal epidermis, highlighting the importance of their negative regulation by sly-miR164a. Taken together, our results demonstrate that SlMIR164a and SlMIR164b play specialized roles in development: SlMIR164b is required for shoot and flower boundary specification, and SlMIR164a is required for fruit growth including the expansion of its outer epidermis, which determines the properties of the fruit skin.