HOPping to defense: HOP3 contributes to plant immunity via the regulation of jasmonic acid signaling.

Plants co-exist with a multitude of microorganisms and insects, many of which strongly affect plant growth and survival. Upon a pathogen challenge, plants are able to recognize specific signatures at the pathogen surface, so-called microbe-, herbivore-, or pathogen-associated molecular patterns. In the case of herbivory or necrotrophic pathogen attacks, the recognition of these molecular patterns triggers the biosynthesis of jasmonic acid (JA), which plays a major role in regulating plant immune responses. Bioactive JA is perceived through direct binding to CORONATINE INSENSITIVE 1 (COI1), a member of the (Skip/Cullin/Fbox) E3 ubiquitin ligase complex (SCF) complex. In the presence of JA, SCF physically interacts with the negative regulators of JA signaling JAZMONATE ZIM DOMAIN proteins (JAZ), leading to their degradation via the proteasome (Figure 1; Chini et al., 2007; Wasternack and Hause, 2013). The inhibition of JAZ proteins by SCF therefore allows the activation of downstream JA-mediated defense gene expression. In the absence of COI1, coi1 mutant plants show decreased JA sensitivity and increased susceptibility to the necrotrophic pathogen Botrytis cinerea and the two-spotted spider mite Tetranychus urticae, showing the importance of the proper induction of JA signaling via COI1 to activate plant immunity (Feys et al., 1994; Li et al., 2004; Rowe et al., 2010). In this issue of Plant Physiology, Mu~ noz et al. (2021) demonstrate the role of HEAT-STRESS PROTEIN 70 (HSP70)HEAT-STRESS PROTEIN 90 (HSP90) ORGANIZING PROTEIN 3 (HOP3) in the activation of JA signaling via the regulation of COI1 activity. HOP3 is one of the three members of the HOP family in Arabidopsis (Arabidopsis thaliana) whose role is to assist the chaperone proteins HSP70 and HSP90 in preventing the accumulation of misfolded proteins in the endoplasmic reticulum (ER) and to participate in protein quality control during heat stress (Fernández-Bautista et al., 2017; Fernández-Bautista et al., 2018). Recently, the inhibition of HSP70 and HSP90 was shown to decrease JA sensitivity and down-regulate JA-responsive genes, therefore questioning the role of their interactor HOP3 in the regulation of JA signaling as well (Zhang et al., 2015). To test whether HOP proteins could impact JA-mediated defense in Arabidopsis, Mu~ noz et al. (2021) followed the expression profile of HOP1, HOP2, and HOP3 upon infestation by T. urticae or B. cinerea. Out of the three genes, only HOP3 was induced in response to both organisms. To follow up on the idea that HOP3 is involved in plant immunity, the authors performed bioassays using T. urticae and B. cinerea in hop3 mutant lines. The hop3 mutant displayed increased leaf damage and ion leakage in response to both competitors compared to wild-type plants. In the case of T. urticae, the increased leaf damage was accompanied by enhanced oviposition (egg laying) in hop3 mutant leaves compared to the control. Upon B. cinerea infection, hop3 mutant plants had reduced induction of the JA marker gene PLANT DEFENSIN 1.2 (PDF1.2), pointing toward the involvement of HOP3 in the onset of JA-mediated defense in Arabidopsis. Next, Mu~ noz et al. (2021) performed exogenous methyljasmonate (MeJA) or coronatine treatments to further evaluate the role of HOP3 in the regulation of JA signaling. Using root growth inhibition assays, the authors showed that exogenous MeJA or coronatine could inhibit root growth in wild-type plants while the effect was strongly dampened in hop3 mutants. In addition, the induction of N ew s an d V ie w s