A methylation that offers plants protection

Gasotransmitters (GTs) are signaling molecules that regulate plant responses to various developmental processes and stress responses, including salt, drought, low temperature, and heavy metals. GTs are endogenously synthesized in plant cells and include nitric oxide (NO), carbon monoxide (CO), methane (CH₄), and hydrogen sulfide (H₂S) (Fang et al., 2021). H₂S, long considered as solely a toxic gas, has been shown in the last decade to act as a protective GT, promoting plant resistance to biotic and abiotic stresses (Hancock and Whiteman, 2014; Kabil et al., 2014). For example, H₂S pretreatments in cucumber (Cucumis sativus) or Chinese cabbage (Brassica rapa) enhance plant tolerance toward heavy metal stress induced by cadmium (Cd²⁺) exposure through increasing photosynthesis and the activity of antioxidant enzymes (Zhang et al., 2015; Luo et al., 2020; Wang et al., 2021).

In plants, one of the metabolic pathways for H₂S synthesis involve the degradation of cysteine via L-CYSTEINE DESULFHYDRASE (LCD) and D-CYSTEINE DESULFHYDRASE (DCD) into H₂S, NH₃, and pyruvate (Papenbrock et al., 2007). LCD and DCD gene expressions are upregulated under Cd²⁺-induced stress, and in Arabidopsis (Arabidopsis thaliana) AtLCD is transcriptionally regulated by the stress-responsive TGACG-BINDING FACTOR 3 (TGA3) and WRKY DNA-BINDING PROTEIN 13 (WRKY13) transcription factors, which can bind directly to the promoter of AtLCD, activating its expression (Zhang et al., 2015, 2020). There is a direct association between LCD expression and plant resistance to Cd²⁺, where the null mutant atlcd is more sensitive and an AtLCD overexpressing line is more resistant to Cd²⁺ compared to wild-type (WT) plants (Qiao et al., 2015). Yet, evidence for regulating the endogenous production of H₂S is scarce, and knowledge of any post-translational regulation of LCD activity has been lacking.

A similar stress-sensitive phenotype as seen in atlcd has been reported in a null mutant line for PROTEIN ARGININE METHYLTRANSFERASE 5 in Arabidopsis, atprmt5 (Zhang et al., 2011). AtPRMT5 acts on the symmetric dimethylation of Arginine 3 on histone 4 (H4R3sme2) and is a crucial component in plant perception of its environment, raising the possibility that AtLCD could be regulated by AtPRMT5.

In this issue of Plant Physiology, Cao and colleagues (Cao et al., 2022) report a recently discovered mechanism in which H₂S production is enhanced through post-translational modification in AtLCDs by AtPRMT5, increasing Arabidopsis resistance toward Cd²⁺-induced stress (Figure 1). Such interaction represents an unexpected target of AtPRMT5, as AtLCD is a nonhistone protein, and opens another dimension into endogenous GT signaling.

Figure 1.

AtPRMT5-dependent regulation of AtLCD. A, H₂S production is mediated, among other pathways, by LCD in Arabidopsis thaliana (AtLCD), and its production is enhanced under cadmium (Cd²⁺) treatment. B, PROTEIN ARGININE METHYLTRANSFERASE5 (AtPRMT5) interacts with AtLCD and is sufficient to methylate its Arg83 (R83), increasing AtLCD activity and promoting H₂S production under Cd²⁺-induced stress. C, Null mutants for AtLCD (atlcd) or AtPRMT5 (atprmt5) are more susceptible to Cd²⁺-induced stress, yet the double mutant does not have an additive effect, indicating that the function of AtLCD is upstream of AtPRMT5.

The authors confirmed previous results that both AtLCD and AtPRMT5 transcripts are induced under Cd²⁺-induced stress and that the null mutants atlcd, atprmt5-1, and atprmt5-2 are more sensitive than WT to Cd²⁺ (Figure 1). After treatment, intracellular Cd²⁺ levels were lower in WT plants compared to the single mutants, which corresponds with the degree of root growth inhibition. Overexpression of either AtLCD or AtPRMT5 rendered plants more resistant to Cd²⁺-induced stress. To assess if AtPRMT5 has a regulatory effect over AtLCD, the authors measured AtLCD activity in plant extracts and confirmed that AtLCD activity decreased in atprmt5. Lack of AtPRMT5 also limited H₂S production; however, transcript and protein levels of the H₂S-producing AtLCD1 were not affected in the atprmt5 mutants, suggesting that AtPRMT5 regulation over AtLCD activity must be post-translational. Moreover, by mutating the predicted methylation target, they demonstrated that AtPRMT5 directly methylates Arg83 of AtLCD.

To establish the hierarchy of this pathway, they generated a double mutant (atprmt5-2 atlcd) that, upon Cd²⁺ treatments, was as susceptible as the single mutant atlcd (Figure 1). The nonadditive susceptibility phenotype in the double mutant indicated that the function of AtLCD must be downstream of AtPRMT5.

Since high expression levels of AtPRMT5 increased plant resistance to Cd²⁺, the authors tested if overexpression of AtPRMT5 could rescue the Cd²⁺-susceptible phenotype of the atlcd mutant, overriding its low H₂S levels. AtPRMT5-OE/atlcd lines showed a partial amelioration of the sensitivity toward Cd²⁺ compared to atlcd, indicating that AtPRMT5-dependent regulation of Cd²⁺ tolerance partially depends on H₂S. Pharmacological complementation of H₂S by fumigation alleviated the Cd²⁺-induced stress in both WT and the atprmt5 mutant.

This work demonstrates that a single post-translational modification can activate LCD activity. Next step could focus on testing if other proteins related to H₂S synthesis might be regulated similarly to LCD. As AtPRMT5 plays multiple roles in plant development and stress responses, future work may identify additional targets post-translationally controlled by AtPRMT5.

Conflict of interest statement. The author declare that there is no conflict of interest.