Plant‐parasitic nematode secreted peptides hijack a plant secretory pathway
Résumé
Small plant signalling peptides play key roles in plant development and in plant–microbe interactions. The plant CLAVATA3 (CLV3)/ENDOSPERM SURROUNDING REGION (ESR) (CLE) peptides have been shown to orchestrate shoot meristem differentiation and to be involved in root growth and vascular development (Yamaguchi et al., 2016). CLE peptides have also been implicated in symbiosis and parasitism. Intriguingly, plant-parasitic nematodes (PPNs) (Mitchum et al., 2012; Gheysen & Mitchum, 2019), endomycorrhizal fungi (Le Marquer et al., 2019), and other fungi and bacteria (Ronald & Joe, 2018) have been shown to secrete molecules mimicking plant peptides within plant roots, to promote infection. The soybean cyst nematode (CN) Heterodera glycines produces CLE propeptides that are delivered to the cytoplasm of plant cells, but ultimately function in the apoplast, consistent with their proposed role as ligand mimics of plant CLE peptides (Wang et al., 2010). A cryptic endoplasmic reticulum (ER) translocation signal has been identified in the variable domain (VD) of HgCLE2, but the precise translocation motif responsible for directing the CLE peptide to the apoplast has not been deciphered (Wang et al., 2010). In this issue of New Phytologist, Wang et al. (2021; pp. 563–574) present very interesting new data explaining how PPN peptide effectors exploit a conserved host post-translational mechanism of trafficking through the ER to the apoplast, using a specific translocation signal: the variable Domain I translocation signal (VDIT) (Fig. 1a). This discovery reminds us that studies of the molecular dialogue between plants and micro- or macro-organisms may improve our understanding of fundamental plant cellular processes, in this case, the protein secretion system. Wang et al. (2021) obtained functional evidence, by assessing the secretion of CLV3 or avirulence AVR9 peptides fused to different full-length or truncated versions of nematode CLEs, to identify the minimal motif required for the ER translocation and secretion of the injected propeptide. They then used an elegant split-GFP approach to validate the passage of the effector from the cytoplasm to the ER, Golgi apparatus and, finally, the extracellular space.