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The photoperiodic response is critical for plants to adjust their reproductive phase to the most favorable season. Wheat heads earlier under long days (LD) than under short days (SD) and this difference is mainly regulated by thePHOTOPERIOD1(PPD1) gene. Tetraploid wheat plants carrying thePpd-A1aallele with a large deletion in the promoter head earlier under SD than plants carrying the wildtypePpd-A1ballele with an intact promoter. PhytochromesPHYBandPHYCare necessary for the light activation ofPPD1, and mutations in either of these genes result in the downregulation ofPPD1and very late heading time. We show here that both effects are reverted when thephyBmutant is combined with loss-of-function mutations inEARLY FLOWERING 3(ELF3), a component of the Evening Complex (EC) in the circadian clock. We also show that the wheat ELF3 protein interacts with PHYB and PHYC, is rapidly modified by light, and binds to thePPD1promoterin planta(likely as part of the EC). Deletion of the ELF3 binding region in thePpd-A1apromoter results in its upregulation at dawn, similar toPPD1alleles with intact promoters in theelf3mutant background. The upregulation ofPPD1is correlated with the upregulation of the florigen geneFLOWERING LOCUS T1(FT1) and early heading time. Loss-of-function mutations inppd1result in the downregulation ofFT1and delayed heading, even when combined with theelf3mutation. Taken together, these results indicate that ELF3 operates downstream ofPHYBas a direct transcriptional repressor ofPPD1, and that this repression is relaxed both by light and by the deletion of the ELF3 binding region in thePpd-A1apromoter. In summary, the regulation of the light mediated activation ofPPD1by ELF3 is critical for the photoperiodic regulation of wheat heading time.Author SummaryThe coordination of reproductive development with the optimal season for seed production is critical to maximize grain yield in crop species. Plants can perceive the length of the day or night (photoperiod) and use this information to anticipate seasonal changes. In most eudicot plants,CONSTANSplays a central role in the perception of photoperiod, but in wheat the main photoperiod gene isPHOROPERIOD1(PPD1). In this study, we show that the clock geneEARLY FLOWERING 3(ELF3) regulates the phytochrome-mediated light activation ofPPD1. Loss-of-function mutations inELF3result in the upregulation ofPPD1at and dawn, and in early heading under both long and short days, even in the absence ofPHYB. A deletion in thePPD1promoter including an ELF3 binding region also results in earlier heading under short days, indicating that ELF3 acts as a direct transcriptional repressor ofPPD1. This study shows thatELF3plays a critical role in the wheat photoperiod pathway by regulating the light signal between the phytochromes andPPD1. ELF3 provides an additional entry point to engineer heading time in wheat, an important trait for the development of better adapted varieties to a changing environment.

The rice immune receptor XA21 is activated by the sulfated microbial peptide RaxX (required foractivation ofXA21-mediated immunityX) produced byXanthomonas oryzaepv.oryzae(Xoo). Mutational studies and targeted proteomics revealed that RaxX is processed and secreted by the protease/transporter RaxB, whose function can be partially fulfilled by a noncognatepeptidase-containing transporterB(PctB). RaxX is cleaved at a Gly-Gly motif, yielding a mature peptide that retains the necessary elements for RaxX function as an immunogen and host peptide hormone mimic. These results indicate that RaxX is a founding member of a previously unclassified and understudied group of tyrosine sulfated RiPPs (ribosomally synthesized,post-translationally modifiedpeptides). We further demonstrate that sulfated RaxX directly binds XA21 with high affinity. This work reveals a complete, previously uncharacterized biological process: bacterial RiPP biosynthesis, secretion, binding to a eukaryotic receptor and triggering of a robust host immune response.

SummaryThe biotrophic pathogenXanthomonas oryzaepv.oryzae(Xoo) produces a sulfated peptide named RaxX, which shares similarity to peptides in the PSY (plant peptide containingsulfatedtyrosine) family. We hypothesize that RaxX functionally mimics the growth stimulating activity of PSY peptides.Root length was measured in Arabidopsis and rice treated with synthetic RaxX peptides. We also used comparative genomic analysis and Reactive Oxygen Species (ROS) burst assay to evaluate the activity of RaxX and PSY peptides.Here we found that a synthetic sulfated RaxX derivative comprising 13 residues (RaxX13-sY), highly conserved between RaxX and PSY, induces root growth in Arabidopsis and rice in a manner similar to that triggered by PSY. We identified residues that are required for activation of immunity mediated by the rice XA21 receptor but that are not essential for root growth induced by PSY. Finally, we showed that aXanthomonasstrain lackingraxXis impaired in virulence.These findings suggest that RaxX serves as a molecular mimic of PSY peptides to facilitateXooinfection and that XA21 has evolved the ability to recognize and respond specifically to the microbial form of the peptide.