Molecular Mechanisms of Host Resistance to Biotrophs

Abstract

Molecular mechanisms of crucifers host resistance against biotrophs have been revealed in different Brassica species during host–pathogen interactions at different stages of host growth and disease development. The R-genes and QTLs have been identified, functionally characterized, and molecularly mapped on the chromosomes of Brassica species expressed during pathogens’ interactions. Timing of the expression of defense-related genes plays a crucial role during pathogenesis and host resistance for mounting a successful defense response. A single gene (Acr) responsible for conferring resistance to Albugo was mapped on a densely populated Brassica juncea RFLP map. A single dominant RAC1 gene in accession Ksk 1 of Arabidopsis thaliana confers resistance to Albugo candida isolate ACeM1. Two accessions of A. thaliana (Ksk1 and Ksk2) were used to identify and map three loci (RAC1, RAC2, and RAC3) of genes that confer resistance to A. candida. In Brassica rapa, the ACA1 locus was mapped to linkage group 4 and was flanked by RFLP marker loci. WRR 4 genes encode a cytoplasmic TIR-NBL-LRR receptor-like protein in Columbia A. thaliana and confer broad-spectrum white rust resistance to four races of A. candida. Three WRR (WRR4BCol-0, WRR8Sf-2, and WRR9Hi-0) genes against Ac2V and a gene WRR12 (SOC3) conferring NHR to AiBoT have been identified. Analysis of allelic variants of BjuA046215 in Indian, east Europe, and Chinese gene pool suggested the presence of three types of alleles. Allele 1 is present in the Chinese Tumida, Allele 2 in Indian, and Allele 3 in east European gene pools. Host resistance in Brassica species to powdery mildew is multilayered and multicomponent at both pre- and postpenetration stages. Host resistance is activated either through SA signaling or simultaneous perception of ethylene and jasmonic acid. MLO genes encoding seven transmembrane calmodulin-binding proteins confer broad-spectrum resistance to Arabidopsis powdery mildew. The Pmr mutants confer resistance to powdery mildew through altered cell wall composition of host. Salicylic acid enhances the expression of RPW 8.1 and RPW 8.2, leading to HR or SHL and resistance. BjNPR1 gene activates SAR to confer broad-spectrum resistance to powdery mildew of B. juncea. There is role of WRKY transcription factors and overexpression of R-genes like PMR, MLO, PEN, EDR, MAPK, MAPK 65–3, NPR1, PAD3, PAD4, ED5, SNARE, RLKs, and KDL (AtCEP1) to confer resistance to powdery mildew of crucifers. Powdery mildew resistance genes of Arabidopsis have been mapped on chromosomes II (RPW 1), III (RPW 2, 3, 7, 8), IV (RPW 4), and V (RPW 5, 6). The papain-type KDEL Cys EP CEP1 fulfills function in plant defense during late development of Erysiphe cruciferarum in close spatial association with the fungal haustorium and haustorial callose encasement.