RAPID ALKALINIZATION FACTOR 22 is a key modulator of the proliferation and hyper-elongation responses of root hairs to microbial volatiles in Arabidopsis

RAPID ALKALINIZATION FACTOR (RALF) peptides are important players in regulating cell expansion. In Arabidopsis, volatile compounds (VCs) emitted by the fungal phytopathogen Penicillium aurantiogriseum promote root hair (RH) proliferation and hyper-elongation through ethylene and enhanced photosynthesis signalling actions. A striking alteration in the proteome of fungal VC-treated roots involves up-regulation of RALF22. To test the possible involvement of RALF22 in the fungal VC-promoted RH changes, we characterized RH density and number responses to fungal VCs in ralf22 and fer-4 plants impaired in RALF22 and its receptor FERONIA, respectively. Unlike WT plants, ralf22 and fer-4 RHs responded weakly to fungal VCs, strongly indicating that the RALF22-FERONIA module is a key determinant of the RH response to fungal VCs. To investigate the regulatory mechanisms behind this response, we analysed the RALF22 transcript levels in roots of etr1-3 and eir1 ethylene signalling mutants and those of ethylene-responsive, RH-related RSL4, RHD2, PRX1 and PRX44 transcripts in ralf22 and fer-4 roots. Moreover, we characterized the RH and RALF22 transcript level responses to fungal VCs of the cfbp1 mutant defective in photosynthetic responsiveness to VCs. Unlike in WT roots, fungal VCs weakly enhanced RALF22 expression in etr1-3, eir1 and cfbp1 roots, and RSL4, RHD2, PRX1 and PRX44 expression in ralf22 and fer-4 roots. In addition, fungal VCs weakly promoted RH changes in cfbp1 roots. Collectively, our findings showed that the ethylene and enhanced photosynthesis signalling-mediated RH response to fungal VCs involves RALF22-FERONIA.