Post‐transcriptional control of GRF transcription factors by microRNA miR396 and GIF co‐activator affects leaf size and longevity

Summary The growth‐regulating factors ( GRF s) are plant‐specific transcription factors. They form complexes with GRF ‐interacting factors ( GIF s), a small family of transcriptional co‐activators. In Arabidopsis thaliana, seven out of the nine GRF s are controlled by micro RNA miR396. Analysis of Arabidopsis plants carrying a GRF 3 allele insensitive to miR396 revealed a strong boost in the number of cells in leaves, which was further enhanced synergistically by an additional increase of GIF 1 levels. Genetic experiments revealed that GRF 3 can still increase cell number in gif1 mutants, albeit to a much lesser extent. Genome‐wide transcript profiling indicated that the simultaneous increase of GRF 3 and GIF 1 levels causes additional effects in gene expression compared to either of the transgenes alone. We observed that GIF 1 interacts in vivo with GRF 3, as well as with chromatin‐remodeling complexes, providing a mechanistic explanation for the synergistic activities of a GRF 3– GIF 1 complex. Interestingly, we found that, in addition to the leaf size, the GRF system also affects the organ longevity. Genetic and molecular analysis revealed that the functions of GRF s in leaf growth and senescence can be uncoupled, demonstrating that the miR396‐ GRF ‐ GIF network impinges on different stages of leaf development. Our results integrate the post‐transcriptional control of the GRF transcription factors with the progression of leaf development.