Fig. 2.

Roles for ER family receptors and EPFL peptides in inflorescence development. (A) The phenotype of the er mutant on stem elongation. Inflorescence architectures and vertical stem sections are illustrated. The mutant shows short internodes and pedicels with a decreased number of cortex cells (cyan). Cortex cells in the mutant also show abnormal enlargement. (B) The phenotype of the er erl1 mutant on procambial maintenance. The mutant has fewer procambial cells than the wild type. Occasionally, some phloem cells directly contact xylem cells without intervening procambial cells. (C) Current model for action of the ER receptor and EPFL peptides in stem elongation. EPFL4 and EPFL6 peptides secreted from the endodermis of stems are perceived by ER in phloem, which activates stem elongation accompanied by cortex cell proliferation through unknown secondary cell–cell communication. In the schematic illustration of a vascular bundle, epidermis, cortex, endodermis, phloem, procambium (or cambium), xylem, and pith tissues are arranged from the outside to the inside. Sieve elements and companion cells are formed in phloem. Vessels are produced in the xylem. (D) Current model for action of ER-family (ERf) receptors and EPFL peptides in procambial maintenance. Endodermis-derived EPFL4 and EPFL6 are perceived either by only ER or by both ER and ERL1 in the phloem. Another unknown ligand may also activate both ER and ERL1 or it may activate only ERL1. The molecular nature of phloem-derived secondary signals downstream of ERf signaling is unknown. TDIF peptides secreted from phloem are perceived by TDR/PXY in the procambium. ER and TDR signaling pathways act in parallel for procambial maintenance.