Spontaneous loss of a conditionally dispensable chromosome from the Alternaria alternata apple pathotype leads to loss of toxin production and pathogenicity

Abstract

The Alternaria alternata apple pathotype causes Alternaria blotch of susceptible apple cultivars through the production of a cyclic peptide, host-specific toxin, AM-toxin. We recently cloned a cyclic peptide synthetase gene, AMT, whose product catalyzes the production of AM-toxin and showed that it resides on chromosomes of 1.8 Mb or less, depending on the A. alternata apple pathotype strain. Reverse transcriptase (RT)-PCR, using primers specific to AMT, on laboratory sub-cultured strains previously shown to produce AM-toxin, identified one isolate that did not express the gene. A leaf necrosis bioassay confirmed an AM-toxin-minus phenotype. However, an original isolate of this strain which had not undergone sub-culture gave a positive result by both RTPCR and bioassay. Contour-clamped homogeneous electric field electrophoresis and Southern hybridization demonstrated the loss of a 1.1-Mb chromosome in the non-toxin-producing isolate. Since this chromosome can be entirely lost without affecting growth, but is necessary for pathogenicity, we propose it is a conditionally dispensable chromosome.

Contribution of Peroxisomes to Secondary Metabolism and Pathogenicity in the Fungal Plant Pathogen Alternaria alternata

“…Host-selective toxin biosynthetic genes of A. alternata pathotypes appeared to be clustered on small chromosomes of Ͻ2.0 Mb in most of the strains tested (2,3,13,15,20,21,33,35). We demonstrated that small chromosomes that contain toxin biosynthetic genes from the strawberry, apple, and tomato pathotypes are conditionally dispensable (CD) chromosomes (2,15,21).…”

“…We demonstrated that small chromosomes that contain toxin biosynthetic genes from the strawberry, apple, and tomato pathotypes are conditionally dispensable (CD) chromosomes (2,15,21). We are now focusing on the structural and functional analysis of the 1.05-Mb CD chromosome that contains the AFT genes of the strawberry pathotype to identify the entire gene cluster and to understand the evolution of toxin biosynthesis and the origin of CD chromosomes.…”

Horizontal Chromosome Transfer, a Mechanism for the Evolution and Differentiation of a Plant-Pathogenic Fungus

“…In Alternaria alternata plant pathogens (37), we have shown that all strains of the A. alternata pathotypes harbor small extra chromosomes of less than 1.7 Mb, whereas nonpathogenic isolates do not have these small chromosomes (5). A cyclic peptide synthetase gene, AMT, which is involved in host-specific AM toxin biosynthesis of the apple pathotype of A. alternata, was located on a small chromosome of 1.1 to 1.7 Mb, depending on the strain (22,23). The AF toxin biosynthesis gene cluster was also present on a single small chromosome of 1.05 Mb in the strawberry pathotype of A. alternata (18).…”

“…The AF toxin biosynthesis gene cluster was also present on a single small chromosome of 1.05 Mb in the strawberry pathotype of A. alternata (18). Based on biological and pathological observations, those small chromosomes were regarded as supernumerary chromosomes, or conditionally dispensable chromosomes (CDCs) (10,18,22). Fungal supernumerary chromosomes, which are not important for normal growth but confer advantages for colonizing an ecological niche, such as infecting host plants, are regarded as CDCs (21).…”

Magnaporthe oryzae Genetic Diversity and Its Outcomes on the Search for Durable Resistance

“…These chromosomes are known to be easily lost from the www.intechopen.com genome, probably because they are unnecessary (Johnson et al, 2001). This property could be adaptatively advantageous because pathogen population can adapt quickly to cultivars carrying correspondent R genes by losing supernumerary chromosomes.…”