A prophage tail-like protein facilitates the endophytic growth of Burkholderia gladioli and mounting immunity in tomato

A prophage tail-like protein (Bg_9562) of Burkholderia gladioli strain NGJ1 possesses broad-spectrum antifungal activity, and it is required for the bacterial ability to forage over fungi. Here, we analyzed whether heterologous overexpression of Bg_9562 or exogenous treatment with purified protein can impart disease tolerance in tomato. The physiological relevance of Bg_9562 during endophytic growth of NGJ1 was also investigated. Bg_9562 overexpressing lines demonstrate fungal and bacterial disease tolerance. They exhibit enhanced expression of defense genes and activation of mitogen-activated protein kinases. Treatment with Bg_9562 protein induces defense responses and imparts immunity in wild-type tomato. The defense-inducing ability lies within 18-51 aa region of Bg_9562 and is due to sequence homology with the bacterial flagellin epitope. Interaction studies suggest that Bg_9562 is perceived by FLAGELLIN-SENSING 2 homologs in tomato. The silencing of SlSERK3s (BAK1 homologs) prevents Bg_9562-triggered immunity. Moreover, type III secretion system-dependent translocation of Bg_9562 into host apoplast is important for elicitation of immune responses during colonization of NGJ1. Our study emphasizes that Bg_9562 is important for the endophytic growth of B. gladioli, while the plant perceives it as an indirect indicator of the presence of bacteria to mount immune responses. The findings have practical implications for controlling plant diseases.

RAV1 family members function as transcriptional regulators and play a positive role in plant disease resistance

Phytopathogens pose a severe threat to agriculture and strengthening the plant defense response is an important strategy for disease control. Here, we report that AtRAV1, an AP2 and B3 domain-containing transcription factor, is required for basal plant defense in Arabidopsis thaliana. The atrav1 mutant lines demonstrate hyper-susceptibility against fungal pathogens (Rhizoctonia solani and Botrytis cinerea), whereas AtRAV1 overexpressing lines exhibit disease resistance against them. Enhanced expression of various defense genes and activation of mitogen-activated protein kinases (AtMPK3 and AtMPK6) are observed in the R. solani infected overexpressing lines, but not in the atrav1 mutant plants. An in vitro phosphorylation assay suggests AtRAV1 to be a novel phosphorylation target of AtMPK3. Bimolecular fluorescence complementation and yeast two-hybrid assays support physical interactions between AtRAV1 and AtMPK3. Overexpression of the native as well as phospho-mimic but not the phospho-defective variant of AtRAV1 imparts disease resistance in the atrav1 mutant A. thaliana lines. On the other hand, overexpression of AtRAV1 fails to impart disease resistance in the atmpk3 mutant. These analyses emphasize that AtMPK3-mediated phosphorylation of AtRAV1 is important for the elaboration of the defense response in A. thaliana. Considering that RAV1 homologs are conserved in diverse plant species, we propose that they can be gainfully deployed to impart disease resistance in agriculturally important crop plants. Indeed, overexpression of SlRAV1 (a member of the RAV1 family) imparts disease tolerance against not only fungal (R. solani and B. cinerea), but also against bacterial (Ralstonia solanacearum) pathogens in tomato, whereas silencing of the gene enhances disease susceptibility.

Heterologous overexpression of PDH45 gene of pea provides tolerance against sheath blight disease and drought stress in rice

Biotic and abiotic stress tolerant crops are required for sustainable agriculture as well as ensuring global food security. In a previous study, we have reported that heterologous overexpression of pea DNA helicase (PDH45), a DEAD-box family member protein, provides salinity stress tolerance in rice. The improved management of photosynthetic machinery and scavenging of reactive oxygen species (ROS) are associated with PDH45 mediated salinity stress tolerance. However, the role of PDH45 in biotic and other abiotic stress (drought) tolerance remains unexplored. In the present study, we have generated marker-free transgenic IR64 rice lines that overexpress PDH45 under the CaMV35S promoter. The transgenic rice lines exhibited a significant level of tolerance against sheath blight disease, caused by Rhizoctonia solani, a polyphagous necrotrophic fungal pathogen. The defense as well as antioxidant responsive marker genes were significantly upregulated in the PDH45 overexpressing (OE) rice lines, upon pathogen infection. Moreover, the OE lines exhibited tolerance to drought stress and various antioxidant as well as drought responsive marker genes were significantly upregulated in them, upon drought stress. Overall, the current study emphasizes that heterologous overexpression of PDH45 provides abiotic as well as biotic stress tolerance in rice. Tolerance against drought as well as sheath blight disease by overexpression of a single gene (PDH45) signifies the practical implication of the present study. Moreover, considering the conserved nature of the gene in different plant species, we anticipate that PDH45 can be gainfully deployed to impart tolerance against multiple stresses in agriculturally important crops.

Concurrent overexpression of rice G-protein β and γ subunits provide enhanced tolerance to sheath blight disease and abiotic stress in rice

Our study demonstrates that simultaneous overexpression of RGB1 and RGG1 genes provides multiple stress tolerance in rice by inducing stress responsive genes and better management of ROS scavenging/photosynthetic machineries. The heterotrimeric G-proteins act as signalling molecules and modulate various cellular responses including stress tolerance in eukaryotes. The gamma (γ) subunit of rice G-protein (RGG1) was earlier reported to promote salinity stress tolerance in rice. In the present study, we report that a rice gene-encoding beta (β) subunit of G-protein (RGB1) gets upregulated during both biotic (upon a necrotrophic fungal pathogen, Rhizoctonia solani infection) and drought stresses. Marker-free transgenic IR64 rice lines that simultaneously overexpress both RGB1 and RGG1 genes under CaMV35S promoter were raised. The overexpressing (OE) lines showed enhanced tolerance to R. solani infection and salinity/drought stresses. Several defense marker genes including OsMPK3 were significantly upregulated in the R. solani-infected OE lines. We also found the antioxidant machineries to be upregulated during salinity as well as drought stress in the OE lines. Overall, the present study provides evidence that concurrent overexpression of G-protein subunits (RGG1 and RGB1) impart multiple (both biotic and abiotic) stress tolerance in rice which could be due to the enhanced expression of stress-marker genes and better management of reactive oxygen species (ROS)-scavenging/photosynthetic machinery. The current study suggests an improved approach for simultaneous improvement of biotic and abiotic stress tolerance in rice which remains a major challenge for its sustainable cultivation.