Calcium and autophagy connect photorespiratory oxidative signals with salicylic acid–mediated defense

In plants, photorespiration occurs in photosynthetically active tissues—such as mesophyll cells and guard cells—and is initiated in chloroplasts when Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) reacts with oxygen (O₂), producing a 3-carbon 3-PGA (3-phosphoglycerate) and a 2-carbon 2-PG (2-phosphoglycolate). 2-PG is subsequently converted to glycolate and transported to peroxisomes, where glycolate-oxidase (GOX) oxidizes it to glyoxylate, producing hydrogen peroxide (H₂O₂) (Foyer et al 2009) (Fig. 1). H₂O₂, a relatively stable form of reactive oxygen species (ROS), acts as an intracellular signaling molecule but can cause oxidative damage when excessively produced (Waszczak et al 2018). Under normal conditions, H₂O₂ from photorespiration is efficiently removed by peroxisomal catalases, restricting oxidative stress (Foyer et al 2009) (Fig. 1).

In Arabidopsis thaliana, CATALASE 2 (CAT2) is the major catalase in leaves, and cat2 mutants only retain ∼10% to 15% of wild-type leaf catalase activity (Queval et al 2007). Under ambient CO₂ and long-day growth conditions, cat2 displays pronounced leaf lesions and induces the expression of ROS-responsive marker genes (eg, OXI1 [oxidative signal-inducible 1]), suggesting that impaired photorespiratory H₂O₂ scavenging causes strong redox perturbation (Queval et al 2007). Loss of CAT2 also constitutively activates the salicylic acid (SA) pathway, resulting in increased SA biosynthesis and induction of SA-marker genes—PR1 (pathogen-related 1), PR2, and ICS1 (isochorismate synthase 1). These constitutive SA responses have been associated with enhanced S-nitrosoglutathione reductase 1 (GSNOR1) activity in cat2 (Zhang et al 2020) (Fig. 1). GSNOR metabolizes S-nitrosoglutathione (GSNO) (a reservoir of nitric oxide [NO]) to oxidized glutathione and ammonia (NH₃), thereby regulating cellular S-nitrosothiol homeostasis; its activity is sensitive to the cellular redox state (eg, H₂O₂) (Li et al 2021). Disruption of GSNOR1 partially suppressed cat2 phenotypes, greatly abolishing leaf lesion, SA accumulation, and SA-marker gene induction. These findings place GSNOR1 downstream of photorespiratory H₂O₂ signaling (Zhang et al 2020). Moreover, GOX activity was similarly reduced in both cat2 and cat2 gsnor1 double mutants, indicating that suppression of SA signaling in cat2 gsnor1 was not caused by a further reduced H₂O₂-generating capacity (Zhang et al 2020). By contrast, loss of GOX1 in cat2 partially attenuated lesion formation and SA-signaling activation, confirming that GOX1-mediated H₂O₂ formation also contributes to the cat2 phenotype (Yang et al 2025). Together, these findings demonstrate GSNOR1-modulated protein S-nitrosation status as a key mechanism linking photorespiratory H₂O₂ to SA signaling (Zhang et al 2020; Yang et al 2025), although how impaired photorespiratory H₂O₂ scavenging enhances GSNOR1 activity remains unknown.

In a recent study published in Plant Physiology, Luo et al (2026) addressed this gap by showing that H₂O₂-induced calcium (Ca²⁺) signaling links photorespiratory H₂O₂ to GSNOR1-dependent SA signaling and disease resistance through regulation of GSNOR1 stability. High-throughput yeast two-hybrid sequencing identified several hundred putative GSNOR1-interacting proteins, among which the Ca²⁺ sensor calmodulin 3 (CaM3) showed the highest read count and was selected for further analysis. Interestingly, the CaM3-GSNOR1 interaction was enhanced by pretreatment with either H₂O₂ or catalase inhibitors, supporting that oxidative stress promotes the association between CaM3 and GSNOR1 (Luo et al 2026). The cat2 cam3 double mutants had reduced GSNOR1 protein abundance and activity, and cam3 partially restored cat2 growth. cat2 cam3 mutants also accumulated less SA; showed weaker induction of PR1, PR2, and ICS1; and reduced resistance to bacterial pathogen Pseudomonas syringae (Luo et al 2026). These findings identify CaM3 as a positive regulator of GSNOR1-dependent SA signaling (Fig. 1).

The involvement of CaM3 implicates that a Ca²⁺ signaling pathway may operate between H₂O₂ and CaM3-GSNOR1 module (Luo et al 2026). HPCA1 (hydrogen peroxide induced Ca²⁺ increases 1), a plasma-membrane leucine-rich repeat receptor kinase, senses apoplastic H₂O₂ and subsequently facilitates Ca²⁺ influx (Wu et al 2020) (Fig. 1). To determine the contribution of HPCA1 to photorespiratory H₂O₂-linked SA signaling, Luo et al (2026) generated cat2 hpca1 double mutants, which largely phenocopied cat2 cam3 double mutants. Overexpression of CaM3 in cat2 (cat2 CAM3-OE) further enhanced GSNOR1 abundance and activity, whereas these effects were attenuated by loss of HPCA1 in cat2 hpca1 CAM3-OE. Together with the established role of HPCA1 as an apoplastic H₂O₂ sensor, these genetic data place CaM3 downstream of HPCA1, suggesting that HPCA1-mediated H₂O₂ perception is coupled with intracellular Ca²⁺-dependent CaM3 signaling (Wu et al 2020; Luo et al 2026) (Fig. 1).

A previous study showed that inhibition of autophagy increases GSNOR1 abundance through stabilizing the GSNOR1 protein (Li et al 2021) (Fig. 1). Consistently, Luo et al (2026) found that treatment with autophagy inhibitors restored GSNOR1 protein levels in cat2 cam3 and cat2 hpca1 to that in cat2. Similarly, introduction of the autophagy-related 18a (atg18a) into cat2 cam3 and cat2 hpca1, generating cat2 cam3 atg18a and cat2 hpca1 atg18a triple mutants, reversed GSNOR1 protein abundance and activity, SA signaling, and P. syringae resistance toward cat2 levels. As such, autophagic degradation mediates the effects of H₂O₂-triggered Ca²⁺ signaling on GSNOR1 protein stability (Fig. 1).

Taken together, this research indicates that HPCA1/CaM3 signaling sustains GSNOR1 abundance and activity by restricting autophagic degradation, providing a mechanistic link between H₂O₂-triggered Ca²⁺ signaling and downstream SA signaling (Luo et al 2026). Nevertheless, how impaired scavenging of photorespiratory H₂O₂ in peroxisomes is communicated with the apoplastic H₂O₂ and perceived by HPCA1 remains unresolved. Luo et al (2026) proposed that RBOHF (respiratory burst oxidase homolog F)-dependent apoplastic ROS production may contribute to this process. A complementary mechanism was suggested by Zhang et al (2023), who identified that AT1 (alkaline tolerance 1), encoding an atypical G protein γ subunit. AT1 regulates the phosphorylation of plasma-membrane PIP2 aquaporins and modulates H₂O₂ movement between the cytosol and apoplast. Enhanced PIP2 phosphorylation promoted H₂O₂ efflux and apoplastic ROS accumulation (Zhang et al 2023). This raises the possibility that excessive intracellular H₂O₂ generated by impaired peroxisomal photorespiratory H₂O₂ scavenging may be similarly exported through aquaporins to apoplast for activating HPCA1 (Zhang et al 2023; Luo et al 2026); however, this remains to be experimentally tested (Fig. 1). Notably, guard cell–specific manipulation of photorespiration was recently shown to alter guard cell H₂O₂ accumulation and stomatal behavior (Sun et al 2026). This raises another question of whether a similar HPCA1-CaM3-GSNOR1 signaling circuit could be reciprocal in guard cells to couple photorespiratory redox status with stomatal regulation.