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

Abstract

ABSTRACTRAPID ALKALINIZATION FACTOR (RALF) peptides are important players in regulating cell expansion. In Arabidopsis, volatile compounds (VCs) emitted by the fungal phytopathogenPenicillium aurantiogriseumpromote 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 inralf22andfer-4plants impaired in RALF22 and its receptor FERONIA, respectively. Unlike WT plants,ralf22andfer-4RHs 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 theRALF22transcript levels in roots ofetr1-3andeir1ethylene signalling mutants and those of ethylene-responsive, RH-relatedRSL4, RHD2, PRX1andPRX44transcripts inralf22andfer-4roots. Moreover, we characterized the RH andRALF22transcript level responses to fungal VCs of thecfbp1mutant defective in photosynthetic responsiveness to VCs. Unlike in WT roots, fungal VCs weakly enhancedRALF22expression inetr1-3, eir1andcfbp1roots, andRSL4, RHD2, PRX1andPRX44expression inralf22andfer-4roots. In addition, fungal VCs weakly promoted RH changes incfbp1roots. Collectively, our findings showed that the ethylene and enhanced photosynthesis signalling-mediated RH response to fungal VCs involves RALF22-FERONIA.