Genetic analysis of Theobroma cacao defense response against Moniliophthora perniciosa, the causal agent of witches broom

Abstract

Witches broom, caused by the basidiomycete Moniliophthora perniciosa, is an important disease of cacao (Theobroma cacao). The characterized sources of resistance contain distinct genomic regions associated with, and display different kinetics of defense gene expression. To dissect the defense response using resistant or susceptible cacao genotypes, inoculated or not with isolates from various biotypes, the expression of genes encoding LRR-kinase receptor-like and components of the subsequent cascade response; enzymes involved in jasmonate, ethylene and salicylate biosynthesis; enzymes involved in flavonoid biosynthesis and pathogenesis-related proteins will be evaluated based on sequences available in interaction libraries or by amplification using conserved sequence regions. The pattern of gene expression and single nucleotide polymorphism (SNPs) in individuals from a segregating population for resistance would be used to locate these genes associated with defense in relation to known QTLs. The promoter region of genes induced by infection will be characterized and functionally analysed, after optimization of cacao genetic transformation. The tomato Micro-Tom cultivar, a miniature genetic model susceptible to the M. perniciosa biotype-S will be employed to investigate the role of hormones in pathogenesis using available mutants, and also for functional analysis of candidate genes associated with defense by transgenesis. Moniliophthora perniciosa isolates from the Brazilian Amazon will be categorized based on microsatellites and characterized according to aggressiveness to differential cacao genotypes, and their putative virulence and/or pathogenicity genes analysed for polymorphism and in vitro expression under stress condition. Fungal cell wall proteins already identified will be expressed heterologously and their effects on defense response activation will be evalauted. Therefore, the project intends to demonstrate that the defense response of T. cacao to M. perniciosa would be regulated by the jasmonate/ethylene pathway, triggered by LRR-kinase like receptors; with some of the investigated genes would be co-localized with identified resistance QTLs; the defense response would be activated by fungal cell wall components; and that the differential aggressiveness of isolates would associated with genes expressed under stress conditions. (AU)