Broad-Spectrum Resistance 1 (BSR1) encodes a rice receptor-like cytoplasmic kinase, and enhances disease resistance when overexpressed. Rice plants overexpressing BSR1 are highly resistant to diverse pathogens, including rice blast fungus. However, the mechanism responsible for this resistance has not been fully characterized. To analyze the BSR1 function, BSR1-knockout (BSR1-KO) plants were generated using a clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated protein 9 (Cas9) system. Experiments using suspension-cultured cells revealed that defense responses including HO production (i.e. oxidative burst) and expression of defense-related genes induced by autoclaved conidia of the rice blast fungus significantly decreased in BSR1-KO cells. Furthermore, a treatment with chitin oligomers which function as microbe-associated molecular patterns (MAMPs) of the rice blast fungus resulted in considerably suppressed defense responses in BSR1-KO cells. These results suggest that BSR1 is important for the rice innate immunity triggered by the perception of chitin.

Plants activate their immune system through intracellular signaling pathways after perceiving microbe-associated molecular patterns (MAMPs). Receptor-like cytoplasmic kinases mediate the intracellular signaling downstream of pattern-recognition receptors. BROAD-SPECTRUM RESISTANCE 1 (BSR1), a rice (Oryza sativa) receptor-like cytoplasmic kinase subfamily-VII protein, contributes to chitin-triggered immune responses. It is valuable for agriculture because its overexpression confers strong disease resistance to fungal and bacterial pathogens. However, it remains unclear how overexpressed BSR1 reinforces plant immunity. Here we analyzed immune responses using rice suspension-cultured cells and sliced leaf blades overexpressing BSR1. BSR1 overexpression enhances MAMP-triggered production of hydrogen peroxide (H2O2) and transcriptional activation of the defense-related gene in cultured cells and leaf strips. Furthermore, the co-cultivation of leaves with conidia of the blast fungus revealed that BSR1 overexpression allowed host plants to produce detectable oxidative bursts against compatible pathogens. BSR1 was also involved in the immune responses triggered by peptidoglycan and lipopolysaccharide. Thus, we concluded that the hyperactivation of MAMP-triggered immune responses confers BSR1-mediated robust resistance to broad-spectrum pathogens.

Plant pathogens evade basal defense systems and attack different organs and tissues of plants. Genetic engineering of plants with genes that confer resistance against pathogens is very effective in pathogen control. Conventional breeding for disease resistance in ornamental crops is difficult and lagging relative to that in non-ornamental crops due to an inadequate number of disease-resistant genes. Therefore, genetic engineering of these plants with defense-conferring genes is a practical approach. We used rice BSR2 encoding CYP78A15 for developing transgenic Torenia fournieri Lind. lines. The overexpression of BSR2 conferred resistance against two devastating fungal pathogens, Rhizoctonia solani and Botrytis cinerea. In addition, BSR2 overexpression resulted in enlarged flowers with enlarged floral organs. Histological observation of the petal cells suggested that the enlargement in the floral organs could be due to the elongation and expansion of the cells. Therefore, the overexpression of BSR2 confers broad-spectrum disease resistance and induces the production of enlarged flowers simultaneously. Therefore, this could be an effective strategy for developing ornamental crops that are disease-resistant and economically more valuable.