Broad-Spectrum Disease Resistance Conferred by the Overexpression of Rice RLCK BSR1 Results from an Enhanced Immune Response to Multiple MAMPs

Abstract

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.

Overexpressed BSR1-Mediated Enhancement of Disease Resistance Depends on the MAMP-Recognition System

“…Transcriptional activation of defense-related genes was assessed by quantitative reverse-transcription (RT-q)PCR. PROBENAZOLE-INDUCIBLE PROTEIN 1 ( PBZ1 ) and KAURENE SYNTHASE-LIKE 4 ( KSL4 ), the expression of which is upregulated in response to chitin [ 12 ], were analyzed. Chitin treatment remarkably increased PBZ1 and KSL4 transcript levels in BSR1 ox6 leaf strips, but not in OsCERK1 ko: BSR1 ox#4-4 and #10-2 leaf strips ( Figure 2 b).…”

“…Overexpression of BSR1 enhances disease resistance [ 6 , 7 ]. In a previously suggested model, the underlying mechanism was as follows: intracellular signaling following MAMP-recognition would be amplified via overaccumulation of BSR1, resulting in enhanced broad-spectrum immunity [ 12 ]. The simultaneous overexpression and knockout experiment in our study revealed that the enhancement of resistance depends on OsCERK1 ( Figure S4 ).…”

“…Accumulating evidence indicates that RLCKs act as phosphorylation signaling factors that are crucial for the activation of PTI [ 10 ]. Knockout of BSR1 in rice strongly suppresses immune responses triggered by MAMPs, such as chitin, peptidoglycan, and lipopolysaccharide (LPS) [ 11 , 12 ]. The BSR1 protein possesses protein kinase activity [ 13 ]; this indicates that, in wild-type (WT) rice, it may mediate the intracellular phosphorylation signaling downstream of pattern recognition receptors.…”

“…The BSR1 protein possesses protein kinase activity [ 13 ]; this indicates that, in wild-type (WT) rice, it may mediate the intracellular phosphorylation signaling downstream of pattern recognition receptors. In BSR1 -overexpressing rice, MAMP-triggered immune responses are hyperactivated, resulting in enhanced disease resistance [ 12 ]. The broad-spectrum disease resistance induced by BSR1 overexpression has been attributed to the hyperactivation of PTI; however, this remained to be experimentally validated.…”

Recent Advances in Molecular Research in Rice: Agronomically Important Traits

“…Furthermore, there are many studies investigating the molecular mechanisms of rice immune responses, such as receptor-mediated pathogen recognition, host immune signaling, and pathogen effector-mediated susceptibility. In this Special Issue, Kanda et al focuses on the BSR1 gene, which encodes a receptor-like cytoplasmic kinase subfamily VII protein and concludes that the hyperactivation of microbe-associated molecular pattern (MAMP)-triggered immune responses conferred robust BSR1 -mediated broad spectrum resistance to fungal and bacterial pathogens [ 17 ]. Liu et al reports that the OsAAA-ATPase1 gene enhanced the expression of pathogenesis-related genes, salicylic acid (SA)-mediated defense responses, and resistance to rice blast fungus [ 18 ].…”