Scuffi, D. | bioRxiv Science

bioRxiv2026

Alarmone ((p)ppGpp) signalling tunes Arabidopsis thaliana nuclear gene expression to shape the balance of plant immune outcomes

RSH enzymes (RelA/SpoT homologs) synthesize ppGpp (guanosine tetra-/pentaphosphate) in plastids, regulating organelle function. More importantly, whether ppGpp acts as a simple rheostat on immune output or determines which mode of defence a plant deploys remains poorly understood. Using Arabidopsis thaliana lines with high (RSH3OX) or null (rshq) ppGpp levels, we show that ppGpp controls nuclear-encoded defence genes in salicylic acid (SA) and jasmonic acid (JA) metabolism, including hormone-inactivating enzymes in RSH3OX and MeSA-to-SA conversion genes in rshq. Conversely, pathogen infection induces the ppGpp synthases RSH2 and RSH3, with induction preceding defence gene activation and requiring SA biosynthesis and a functional Type III secretion system. RSH3OX plants are hypersusceptible to Pseudomonas syringae pv. tomato DC3000, show impaired pattern-triggered immunity, and exhibit an accelerated decline in photosynthetic efficiency, while rshq plants show enhanced resistance to P. syringae but increased susceptibility to the necrotrophic fungus Botrytis cinerea. Using an inducible line to acutely elevate ppGpp, we confirm this susceptibility is directly caused by ppGpp levels, not a chronic developmental consequence of RSH3OX. Together, these results indicate that ppGpp governs the mode of immune execution rather than simply scaling its magnitude, acting as a chloroplast-based checkpoint linking organellar status to the nuclear defence transcriptome.

plant biology↗

bioRxiv2025

Hydrogen Sulfide modulates Flagellin-Induced Stomatal Immunity

Stomata are natural pores through which plants exchange gases with the environment, mainly carbon dioxide and oxygen required for photosynthesis and respiration, as well as water vapor through evapotranspiration. However, they also serve as entry points for microbial pathogens such as Pseudomonas syringae pv. tomato (Pst) bacteria. To prevent microbe invasion, guard cells detect pathogens-associated molecular patterns (PAMPs), including the bacterial peptide flagellin (flg22), triggering stomatal closure. This study identifies hydrogen sulfide (H2S) and its cytosolic source L-CYSTEINE DESULFHIDRASE 1 (DES1), as key players in stomatal immunity. We demonstrate that H2S and DES1 are involved in flg22- and bacterial-induced responses, including stomatal closure and modulation of reactive oxygen species (ROS) production. We have found that knock out mutants in DES1 gene exhibits reduced susceptibility to Pst spray-inoculation and lower apoplastic and cytosolic H2O2 levels in response to flg22. Additionally, H2S independently induces cytosolic H2O2 levels in guard cells without requiring RBOHD activity. All together, these findings establish H2S and its source, DES1, as critical components of the stomatal immune response. One Sentence SummaryH2S and DES1 actively participate in flg22-induced stomatal closure modulating apoplastic and cytosolic ROS production.

plant biology↗