Alberto P. Macho

Pathogens target and degrade the extracellular matrix surrounding plant cells. A central question is how cell wall–derived damage-associated molecular patterns (DAMPs) are recognized and integrated to trigger immune responses. We address this question by determining the structure of the extracellular multidomain of the impaired in glycan perception 1 (IGP1) receptor in both apo form and bound to the cellulose-derived DAMP cellotriose. Structural analyses reveal that constitutive interactions of leucine-rich repeat (LRR)–malectin domains preconfigure IGP1 for ligand recognition and that the receptor features a highly specific sugar-binding pocket in the LRR domain capable of distinguishing fine variations in glycan structures. By directly sensing cello-oligomers, IGP1 operates as a cell wall surveillance receptor that links pathogen-induced wall degradation to immune alerting, equipping plants to mount rapid and robust defense responses.

Cellulose, a central structural component of plant cell walls, is produced by cellulose synthase complexes (CSCs) at the plasma membrane. Salinity stress is particularly damaging to cellulose biosynthesis, and therefore, plants have developed adaptive mechanisms to cope with these conditions. TETRATRICOPEPTIDE THIOREDOXIN-LIKE (TTL) proteins are essential for growth under salt stress and show a salt-dependent association with CSCs through an as-yet unknown mechanism. Here, we identify a phosphorylation-dependent regulatory mechanism linking salt stress signaling to cellulose biosynthesis through the coordinated action of TTL3 and the receptor-like cytoplasmic kinase BOTRYTIS-INDUCED KINASE 1 (BIK1). Phosphorylation of Serine 93 in the N-terminal intrinsically disordered region of TTL3 controls its localization, retaining it in the cytosol, while dephosphorylation promotes association with CSCs at the plasma membrane. Biochemical and genetic analysis identified BIK1 as the kinase responsible for TTL3-S93 phosphorylation, with bik1 mutants phenocopying the phosphoablative TTL3S93A in vivo. Transcriptomic analyses reveal a strong overlap of differentially expressed genes between bik1 and a cellulose-deficient mutant, supporting a broader role for BIK1 in cell wall regulation. Notably, TTL proteins do not appear to be involved in the assayed canonical immune responses, suggesting pathway specificity downstream of BIK1. Together, these findings define a signaling module that connects salt stress perception to CSCs regulation and establish BIK1-dependent TTL3 phosphorylation as a molecular switch for maintaining cell wall integrity under abiotic stress.

Pathogens target and degrade the extracellular matrix surrounding plant cells. A central question is how cell wall–derived damage-associated molecular patterns (DAMPs) are recognized and integrated to trigger immune responses. We address this question by determining the structure of the multidomain receptor IGP1 in both apo form and bound to the cellulose-derived DAMP cellotriose. Structural analyses reveal that constitutive Leucine Rich Repeat–malectin interactions preconfigure IGP1 for ligand recognition and that the receptor features a highly specific sugar-binding pocket in the LRR domain capable of distinguishing fine variations in glycan structures. By directly sensing cello-oligomers, IGP1 acts as a cell wall sentinel that links pathogen-induced wall degradation to immune alerting, equipping plants to mount rapid and robust defense responses.

Summary: Plants have developed a complex immune system to detect and respond to invading pathogens. A critical aspect of this defense relies on regulatory mechanisms that control the activation of immune responses, ensuring these are efficient yet do not compromise overall plant performance. Ralstonia solanacearum is a soil‐borne bacterial pathogen that causes bacterial wilt disease in many plant species. Its virulence depends on the secretion of type III effector proteins that suppress plant immune responses. In this study, we investigated the function of one such effector, RipAV, in Arabidopsis thaliana, focusing on its role in modulating pattern‐triggered immunity. We show that RipAV targets members of the plant‐specific ubiquitin ligase (PUB) family and calcium‐dependent protein kinase 28 (CPK28), which has been shown to phosphorylate a set of PUBs to enhance their activity and regulate the stability of the key immune regulator BIK1. RipAV association enhances the CPK28‐mediated phosphorylation of PUBs, inducing the proteasome‐mediated degradation of BIK1 and the suppression of immunity. Importantly, we found that RipAV is required for maintaining a low accumulation of BIK1 during a R. solanacearum infection. These findings provide new insights into the sophisticated strategies employed by pathogens to subvert plant immunity.

Summary: Gram‐negative bacterial pathogens inject effector proteins inside plant cells using specialized secretion systems. These effectors manipulate plant cellular functions and suppress the plant immune system in order to promote bacterial proliferation. Although bacterial effectors are exogenous proteins that could be targeted by protein degradation systems within plant cells, they are able to perform their virulence functions. In this study, we found that RipBM, an effector protein secreted by the bacterial wilt pathogen Ralstonia solanacearum, undergoes phosphorylation at specific residues within plant cells, which enhances its stability. This phosphorylation promotes the interaction of RipBM with plant 14‐3‐3 proteins, which contributes to its deubiquitination and stabilization. Furthermore, we demonstrated that RipBM contributes to Ralstonia virulence in tomato plants and that its phosphorylated residues, required for 14‐3‐3 interaction, are essential for this virulence function. These findings suggest that RipBM hijacks plant 14‐3‐3 proteins to promote its own stability and virulence activity. This highlights effector stability and degradation in plant cells as a critical molecular battleground in the coevolutionary arms race between plants and pathogens.

Summary: The association between Arabidopsis FLS2 and BAK1/SERK3 triggered by bacterial flagellin is the best‐studied example of plant PRR receptor/co‐receptor complex. Although most plants are able to perceive the flagellin epitope flg22 from most bacterial species, certain pathogens, such as Agrobacterium tumefaciens or Ralstonia solanacearum , have evolved polymorphisms in their flg22 sequence, avoiding perception by most plants tested so far. Specific plant species, such as Vitis riparia or Glycine max , respectively, have subsequently evolved FLS2 versions with the ability to recognize these polymorphic flg22 peptides. The model plants Arabidopsis or N. benthamiana are not able to perceive the flg22 peptide from R. solanacearum (flg22 ^Rso ). However, in this work, we found that the expression of AtFLS2 in N. benthamiana confers responsiveness to flg22 ^Rso , leading to the activation of specific downstream responses. AtFLS2 interacts with the BAK1 orthologs in N. benthamiana (NbSERKs), which contribute to flg22 ^Rso recognition by AtFLS2 and subsequent immune responses. Finally, we found that this recognition activates anti‐bacterial immunity that restricts Ralstonia growth, suggesting that cross‐species PRR‐co‐receptor compatibility could be exploited for the development of tailored immunity against pathogens with polymorphic PAMPs.

Gram-negative bacterial pathogens inject effector proteins inside plant cells using a type III secretion system. These effectors manipulate plant cellular functions and suppress the plant immune system in order to promote bacterial proliferation. Despite the fact that bacterial effectors are exogenous threatening proteins potentially exposed to the protein degradation systems inside plant cells, effectors are relative stable and able to perform their virulence functions. In this work, we found that RipE1, an effector protein secreted by the bacterial wilt pathogen, Ralstonia solanacearum, undergoes phosphorylation of specific residues inside plant cells, and this promotes its stability. Moreover, RipE1 associates with plant ubiquitin proteases, which contribute to RipE1 deubiquitination and stabilization. The absence of those specific phosphorylation sites or specific host ubiquitin proteases leads to a substantial decrease in RipE1 protein accumulation, indicating that RipE1 hijacks plant post-translational modification regulators in order to promote its own stability. These results suggest that effector stability or degradation in plant cells constitute another molecular event subject to co-evolution between plants and pathogens.

Summary: Plants have developed a complex immune system to detect and respond to invading pathogens. A critical aspect of this defence relies on regulatory mechanisms that control the activation of immune responses, ensuring these are efficient yet do not compromise overall plant performance. Many pathogens secrete proteins called effectors to manipulate plant cellular functions, suppressing plant immunity and promoting infection. Here, we show that the bacterial effector RipAV, secreted by the soil-borne bacterial pathogen Ralstonia solanacearum , hijacks a major immune signalling hub to suppress immune responses. RipAV targets members of the plant-specific ubiquitin ligase (PUB) family and calcium-dependent protein kinase 28 (CPK28), which has been shown to phosphorylate a set of PUBs to enhance their activity and regulate the stability of the key immune regulator BIK1. RipAV association enhances the CPK28-mediated phosphorylation of PUBs, inducing the proteasome-mediated degradation of BIK1 and the suppression of immunity. Importantly, we found that RipAV is required for maintaining a low accumulation of BIK1 during a R. solanacearum infection. These findings provide new insights into the sophisticated strategies employed by pathogens to subvert plant immunity.

Microbial pathogens and other parasites can modify the development of their hosts, either as a target or a side effect of their virulence activities. The plant‐pathogenic bacterium Ralstonia solanacearum , causal agent of the devastating bacterial wilt disease, is a soilborne microbe that invades host plants through their roots and later proliferates in xylem vessels. In this work, we studied the early stages of R. solanacearum infection in the model plant Arabidopsis thaliana , using an in vitro infection system. In addition to the previously reported inhibition of primary root length and increase in root hair formation at the root tip, we observed an earlier xylem differentiation during R. solanacearum infection that occurs in a HrpG‐dependent manner, suggesting that the pathogen actively promotes the development of the vascular system upon invasion of the root. Moreover, we found that the phytohormone auxin, of which the accumulation is promoted by the bacterial infection, is required for the R. solanacearum ‐triggered induction of root hair formation but not earlier xylem differentiation. Altogether, our results shed light on the capacity of R. solanacearum to induce alterations of root developmental pathways and on the role of auxin in this process.