FERONIA as an upstream receptor kinase for polar cell growth in plants.

P olarized cell growth is important for morphogenesis and function of specialized cell types in eukaryotes. Typical tip-growing cells in plants are root hairs and pollen tubes (Fig. 1). A root hair is a highly polarized tubular structure emerging from a special type of cell on the root surface, from which water and nutrients are absorbed and supplied to the plant body (Fig. 1A). A root hair also functions in nodule formation during Rhizobium legumes symbiosis. A pollen tube is a male reproductive organ that delivers sperm cells to a female gametophyte (Fig. 1C). Rapid elongation of these cells occurs in response to external cues. It is well established that RAC/ROP (Rho-related small GTPase from plants) signaling, reactive oxygen species (ROS), and calcium homeostasis regulate polarized cell growth in plants. However, the nature of the cell surface receptor transducing external signals for tip growth has remained poorly characterized. In PNAS, Duan et al. (1) report the identification of a receptor-like kinase responsible for ROP signaling and ROS-mediated root hair elongation. Surprisingly, the receptor is FERONIA (FER), a well-known regulator of female fertility (2–4). ROP guanidine exchange factor (ROPGEF) converts ROP from the inactive GDP-bound form to the active GTPbound form. A yeast two-hybrid screen in search of ROPGEF1 interactors in Arabidopsis identified a CrRLK family of receptor-like kinases, including FER. The interaction of FER and ROPGEF1 occurs in plant cells. An active form of ROP is reduced in the loss-of-function fer mutant plants, suggesting that FER is required for ROP activation. Duan et al. (1) further demonstrate that FER, ROPGEF, and ROP form a protein complex in vitro and that association of FER and ROP is enhanced by the presence of GDP. This is consistent with the known activation mechanism of ROPs: ROPGEFs preferentially interact with the GDP-bound inactive form of ROPs, and on nucleotide exchange, the GTP-bound active form of ROPs is released from the ROPGEF complex to transduce downstream signaling (5) (Fig. 1B). fer and its allele sirene (srn) were originally discovered as female gametophytic mutants defective in proper communication betweenmale (pollen tube) and female (synergid cell) gametophytes (2–4). However, FER is expressed widely throughout vegetative tissues, suggesting that FER function is not limited to gametophyte interactions. Indeed, Duan et al. (1) find that the loss-of-function fer alleles conferred short root hairs, which often bulged or ruptured. How does FER promote root hair growth? Root hair growth is stimulated by plant hormone auxin and secondary messenger ROS, both acting via ROP signaling (6). Production of ROS by the NADPH oxidase RHD2 leads to stimulation of Ca channels, elevated cytoplasmic Ca concentrations, and subsequent tip growth (Fig. 1B). The root hair defects in fer closely resemble those of rhd2 (7). Given that FER physically interacts with and acts upstream of ROP, it is predicted to mediate auxinand ROS-mediated root hair growth. Sure enough, Duan et al. (1) demonstrate that accumulation of ROS is negligible in fer root hairs and that fer mutants are not responsive to auxinstimulated ROS accumulation. The root hair and ROS defects were countered by overexpression of ROP2-GFP. Moreover, overexpression of FER, ROPs, or ROPGEF1 resulted in the increased accumulation of ROS in the root. Taken together, Duan et al. (1) establish FER as a cell surface receptor acting upstream of ROP, which targets NADPH oxidase to accumulate ROS for root hair elongation (Fig. 1B). FER