Extracellular proteolytic cascade in tomato activates immune protease Rcr3.

Proteolytic cascades regulate immunity and development in animals, but these cascades in plants have not yet been reported. Here we report that the extracellular immune protease Rcr3 of tomato is activated by P69B and other subtilases (SBTs), revealing a proteolytic cascade regulating extracellular immunity in solanaceous plants. Rcr3 is a secreted papain-like Cys protease (PLCP) of tomato that acts both in basal resistance against late blight disease (Phytophthora infestans) and in gene-for-gene resistance against the fungal pathogen Cladosporium fulvum (syn. Passalora fulva) Despite the prevalent model that Rcr3-like proteases can activate themselves at low pH, we found that catalytically inactive proRcr3 mutant precursors are still processed into mature mRcr3 isoforms. ProRcr3 is processed by secreted P69B and other Asp-selective SBTs in solanaceous plants, providing robust immunity through SBT redundancy. The apoplastic effector EPI1 of P. infestans can block Rcr3 activation by inhibiting SBTs, suggesting that this effector promotes virulence indirectly by preventing the activation of Rcr3(-like) immune proteases. Rcr3 activation in Nicotiana benthamiana requires a SBT from a different subfamily, indicating that extracellular proteolytic cascades have evolved convergently in solanaceous plants or are very ancient in the plant kingdom. The frequent incidence of Asp residues in the cleavage region of Rcr3-like proteases in solanaceous plants indicates that activation of immune proteases by SBTs is a general mechanism, illuminating a proteolytic cascade that provides robust apoplastic immunity.

The chemical compound 'Heatin' stimulates hypocotyl elongation and interferes with the Arabidopsis NIT1-subfamily of nitrilases.

Temperature passively affects biological processes involved in plant growth. Therefore, it is challenging to study the dedicated temperature signalling pathways that orchestrate thermomorphogenesis, a suite of elongation growth-based adaptations that enhance leaf-cooling capacity. We screened a chemical library for compounds that restored hypocotyl elongation in the pif4-2-deficient mutant background at warm temperature conditions in Arabidopsis thaliana to identify modulators of thermomorphogenesis. The small aromatic compound 'Heatin', containing 1-iminomethyl-2-naphthol as a pharmacophore, was selected as an enhancer of elongation growth. We show that ARABIDOPSIS ALDEHYDE OXIDASES redundantly contribute to Heatin-mediated hypocotyl elongation. Following a chemical proteomics approach, the members of the NITRILASE1-subfamily of auxin biosynthesis enzymes were identified among the molecular targets of Heatin. Our data reveal that nitrilases are involved in promotion of hypocotyl elongation in response to high temperature and Heatin-mediated hypocotyl elongation requires the NITRILASE1-subfamily members, NIT1 and NIT2. Heatin inhibits NIT1-subfamily enzymatic activity in vitro and the application of Heatin accordingly results in the accumulation of NIT1-subfamily substrate indole-3-acetonitrile in vivo. However, levels of the NIT1-subfamily product, bioactive auxin (indole-3-acetic acid), were also significantly increased. It is likely that the stimulation of hypocotyl elongation by Heatin might be independent of its observed interaction with NITRILASE1-subfamily members. However, nitrilases may contribute to the Heatin response by stimulating indole-3-acetic acid biosynthesis in an indirect way. Heatin and its functional analogues present novel chemical entities for studying auxin biology.

Cleavage of a pathogen apoplastic protein by plant subtilases activates host immunity.

The plant apoplast is a harsh environment in which hydrolytic enzymes, especially proteases, accumulate during pathogen infection. However, the defense functions of most apoplastic proteases remain largely elusive. We show here that a newly identified small cysteine-rich secreted protein PC2 from the potato late blight pathogen Phytophthora infestans induces immunity in Solanum plants only after cleavage by plant apoplastic subtilisin-like proteases, such as tomato P69B. A minimal 61 amino acid core peptide carrying two key cysteines, conserved widely in most oomycete species, is sufficient for PC2-induced cell death. Furthermore, we showed that Kazal-like protease inhibitors, such as EPI1, produced by P. infestans prevent PC2 cleavage and dampen PC2 elicited host immunity. This study reveals that cleavage of pathogen proteins to release immunogenic peptides is an important function of plant apoplastic proteases.

Purification of His-Tagged Proteases from the Apoplast of Agroinfiltrated N. benthamiana.

Protein expression in plants by agroinfiltration and subsequent purification is increasingly used for the biochemical characterization of plant proteins. In this chapter we describe the purification of secreted, His-tagged proteases from the apoplast of agroinfiltrated Nicotiana benthamiana using immobilized metal affinity chromatography (IMAC). We show quality checks for the purified protease and discuss potential problems and ways to circumvent them. As a proof of concept, we produce and purify tomato immune protease Pip1 and demonstrate that the protein is active after purification.

Evolution of a guarded decoy protease and its receptor in solanaceous plants.

Rcr3 is a secreted protease of tomato that is targeted by fungal effector Avr2, a secreted protease inhibitor of the fungal pathogen Cladosporium fulvum. The Avr2-Rcr3 complex is recognized by receptor-like protein Cf-2, triggering hypersensitive cell death (HR) and disease resistance. Avr2 also targets Rcr3 paralog Pip1, which is not required for Avr2 recognition but contributes to basal resistance. Thus, Rcr3 acts as a guarded decoy in this interaction, trapping the fungus into a recognition event. Here we show that Rcr3 evolved > 50 million years ago (Mya), whereas Cf-2 evolved <6Mya by co-opting the pre-existing Rcr3 in the Solanum genus. Ancient Rcr3 homologs present in tomato, potato, eggplants, pepper, petunia and tobacco can be inhibited by Avr2 with the exception of tobacco Rcr3. Four variant residues in Rcr3 promote Avr2 inhibition, but the Rcr3 that co-evolved with Cf-2 lacks three of these residues, indicating that the Rcr3 co-receptor is suboptimal for Avr2 binding. Pepper Rcr3 triggers HR with Cf-2 and Avr2 when engineered for enhanced inhibition by Avr2. Nicotiana benthamiana (Nb) is a natural null mutant carrying Rcr3 and Pip1 alleles with deleterious frame-shift mutations. Resurrected NbRcr3 and NbPip1 alleles were active proteases and further NbRcr3 engineering facilitated Avr2 inhibition, uncoupled from HR signalling. The evolution of a receptor co-opting a conserved pathogen target contrasts with other indirect pathogen recognition mechanisms.

Bodyguards: Pathogen-Derived Decoys That Protect Virulence Factors.

Recent studies on plant-pathogen interactions have exposed a new strategy used by plant pathogens: decoy effectors that protect virulence factors. Examples of these "bodyguards" include the recently discovered PsXLP1 from Phytophthora sojae and truncated TALEs from Xanthomonas oryzae. These examples suggest important roles for seemingly non-functional effector proteins in distracting the host.