EPFL Signals in the Boundary Region of the SAM Restrict Its Size and Promote Leaf Initiation.

The shoot apical meristem (SAM) enables the formation of new organs throughout the life of a plant. ERECTA family (ERf) receptors restrict SAM size and promote initiation of leaves while simultaneously supporting establishment of correct phyllotaxy. In the epidermis and during organ elongation ERf activity is regulated by a family of Epidermal Patterning Factor-Like (EPFL) secreted Cys-rich small proteins. Here we show that ERfs play a critical role in communication between the SAM leaf boundary and the central zone in Arabidopsis (Arabidopsis thaliana). Ectopic expression of ERECTA in the central zone using the CLAVATA3 promoter is sufficient to restrict meristem size and promote leaf initiation. Genetic analysis demonstrated that four putative ligands: EPFL1, EPFL2, EPFL4, and EPFL6 function redundantly in the SAM. These genes are expressed at the SAM-leaf boundary and in the peripheral zone. Previously EPFL4 and EPFL6 have been linked with elongation of aboveground organs. Here we demonstrate that EPFL1 and EPFL2 promote organ elongation as well. In addition, we show that expression of ERECTA in the central zone of the SAM has a strong impact on elongation of internodes and pedicels and growth of leaves. These results suggest that ERfs can stimulate organ growth cell nonautonomously.