Magnaporthe griseaavirulence geneACE1belongs to an infection‐specific gene cluster involved in secondary metabolism

Abstract

Summary• The avirulence gene ACE1 from the rice blast fungus Magnaporthe grisea encodes a polyketide synthase (PKS) fused to a nonribosomal peptide synthetase (NRPS) probably involved in the biosynthesis of a secondary metabolite recognized by Pi33 resistant rice (Oryza sativa) cultivars.• Analysis of the M. grisea genome revealed that ACE1 is located in a cluster of 15 genes, of which 14 are potentially involved in secondary metabolism as they encode enzymes such as a second PKS-NRPS (SYN2), two enoyl reductases (RAP1 and RAP2) and a putative Zn(II) 2 Cys 6 transcription factor (BC2).• These 15 genes are specifically expressed during penetration into the host plant, defining an infection-specific gene cluster. A pORF3-GFP transcriptional fusion showed that the highly expressed ORF3 gene from the ACE1 cluster is only expressed in appressoria, as is ACE1. Phenotypic analysis of deletion or disruption mutants of SYN2 and RAP2 showed that they are not required for avirulence in Pi33 rice cultivars, unlike ACE1. Inactivation of other genes was unsuccessful because targeted gene replacement and disruption were inefficient at this locus.• Overall, the ACE1 gene cluster displays an infection-specific expression pattern restricted to the penetration stage which is probably controlled at the transcriptional level and reflects regulatory networks specific to early stages of infection.

Evolutionary Mechanisms Involved in Development of Fungal Secondary Metabolite Gene Clusters

“…13 Both clusters occur in different classes of fungi, and a heterogeneous collection of gene clusters in related and more distant fungal taxa can help explain the evolutionary processes that led to their formation, maintenance, and inactivation/loss. Together, they illustrate the interplay of recombination and HGT, as well as other genomic modifications that can impact SM biosynthetic pathways in fungi.…”

“…13 In this fungus, the ACE1 hybrid polyketide synthasenonribosomal peptide synthetase (PKS-NRPS) is involved in the production of a polyketide of unknown structure that is recognized by the rice resistance protein Pi33. 26 The ACE1 gene cluster is also present in many other ascomycete species, 27 including Aspergillus clavatus in which it is involved in the production of cytochalasins (Figure 18.1).…”

Genomic organization and sequence dynamics of the AvrPiz-t locus in Magnaporthe oryzae

“…The repetitive sequences including REP1, RETRO5, and MGR691/ MGR508 were identified at the 5′ terminus of the avr1-CO39 locus in virulent isolates (Farman et al, 2002). Another Avr gene, ACE1, which encodes a polyketide synthase fused to a nonribosomal peptide synthetase (PKS-NRPS), is located in a gene cluster encoding different enzymes potentially involved in secondary metabolism (Böhnert et al, 2004;Collemare et al, 2008).…”

Functional Identification of Genes Encoding Effector Proteins in Magnaporthe oryzae

“…Functional identification of M. oryzae effectors can elucidate some pathogenicity mechanisms of the blast fungus, providing a clue to better manage blast disease. Several Avr genes have been cloned and characterized from M.oryzae, such as Avr-Pita (Orbach et al, 2000;Valent et al, 1991), Avr1-CO39 (Farman and Leong, 1998), Ace1 (Bohnert et al, 2004;Collemare et al, 2008) and the Pwl effectors Sweigard et al, 1995). The Avr-Pita effector appears homologous to fungal zinc-dependent metalloproteases and is dispensable for virulence on rice (Jia et al, 2001;Orbach et al, 2000).…”

“…Ace1 effector is a putative cytoplasmic fusion polypeptide containing a polyketide synthase (PKS) and a nonribosomal peptide synthetase (NRPS), two distinct classes of enzymes that are involved in the production of microbial secondary metabolites (Bohnert et al, 2004;Collemare et al, 2008). Ace1 is thought to function as avirulence indirectly by producing a secondary metabolite that activates Pi33.…”