miR156/157 targets SPLs to regulate flowering transition, plant architecture and flower organ size in petunia.

miR156/157 plays multiple pivotal roles during plant growth and development. In this study, we identified 11 miR156- and 5 miR157-encoding loci from the genome of P. axillaris and P. inflata, designated as PaMIR0156/157s and PiMIR0156/157s, respectively. qPCR analysis indicated that PhmiR156/157 was expressed predominantly in cotyledons, germinating seeds, flower buds, young fruits, and seedlings. PhmiR156/157 levels declined in shoot apical buds and leaves of petunia before flowering as the plant ages, moreover, the temporal expression patterns of most miR156/157-targeted PhSPLs were complementary to that of PhmiR156/157. Ectopic expression of PhMIR0157a in Arabidopsis and petunia resulted in delayed flowering, dwarf plant stature, increased branches, and reduced organ size. However, PhMIR0156f-overexpressing Arabidopsis and petunia plants showed only delayed flowering. In addition, down-regulation of PhmiR156/157 level by overexpressing STTM156/157 led to higher plant, less branches, longer internodes, and precocious flowering. qPCR analysis indicated that PhmiR156/157 modulates these traits mainly via down-regulating their PhSPL targets and subsequently decreasing the expression of flowering regulatory genes. Our results demonstrated that PhmiR156/157-PhSPL module has conserved but also divergent functions in the growth and development, which will help us deciphering the genetic basis for improvement of flower transition, plant architecture and organ development in petunia.