Three Genes for Metabolism of the Phytoalexin Maackiain in the Plant Pathogen Nectria haematococca : Meiotic Instability and Relationship to a New Gene for Pisatin Demethylase

Abstract

Some isolates of the plant-pathogenic fungus Nectria haematococca mating population (MP) VI metabolize maackiain and medicarpin, two antimicrobial compounds (phytoalexins) synthesized by chickpea (Cicer arietinum L.). The enzymatic modifications by the fungus convert the phytoalexins to less toxic derivatives, and this detoxification has been proposed to be important for pathogenesis on chickpea. In the present study, loci controlling maackiain metabolism (Mak genes) were identified by crosses among isolates of N. haematococca MP VI that differed in their ability to metabolize the phytoalexin. Strains carrying Maki or Mak2 converted maackiain to la-hydroxymaackiain, while those with Mak3 converted it to 6a-hydroxymaackiain. Maki and Mak2 were unusual in that they often failed to be inherited by progeny. Maki was closely linked to Pda6, a new member in a family of genes in N. haematococca MP VI that encode enzymes for detoxification of pisatin, the phytoalexin synthesized by garden pea. Like Maki, Pda6 was also transmitted irregularly to progeny. Although the unusual meiotic behaviors of some Mak genes complicate genetic analysis, identification of these genes should afford a more thorough evaluation of the role of phytoalexin detoxification in the pathogenesis of N. haematococca MP VI on chickpea.

Chromosome-length polymorphism in fungi.

“…In this organism, Pda genes, necessary for the detoxification of the phytoalexin pisatin, are unexpectedly lost from some of the progeny in some crosses. This property seems to be inherent in all of the Pda genes so far characterized (78,133), and in at least one case, the entire chromosome is apparently dispensable (79). The gene Pda6 maps to a 1.6-Mb chromosome.…”

The Fusarium solani species complex: ubiquitous pathogens of agricultural importance

“…MAK1 encodes a flavin adenine dinucleotide (FAD)-containing monooxygenase of 460 amino acids in length (Covert et al, 1996), which is responsible for the detoxification of maackiain to the less toxic compound 1a-hydroxymaackiain (Covert et al, 1996;Miao and VanEtten, 1992). Despite the divergence between the MAK1 and MAK2 genes, MAK2 is also responsible for converting maackiain to 1a-hydroxymaackiain, whereas MAK3 converts the phytoalexin to 6a-hydroxymaackiain (Miao and VanEtten, 1992). Although all three MAK genes encode enzymes capable of metabolizing maackiain, only MAK1 and MAK2 correlate with high virulence on chickpea (Miao and VanEtten, 1992).…”

“…Isolates of FSSC 11 were able to perform three metabolic modifications on maackiain and medicarpin via monooxygenases and were able to detoxify the phytoalexins to the less inhibitory (–)‐6a‐hydroxypterocarpan, the 1a‐hydroxydienone and the isoflavanone derivatives (Denny and VanEtten, , ; Lucy et al ., ). Three maackiain detoxification genes ( MAK 1– MAK 3) have been identified by genetic studies (Covert et al ., ; Miao and VanEtten, ). All FSSC 11 isolates that are capable of metabolizing maackiain are also able to modify medicarpin to analogous byproducts, suggesting that the enzymes encoded by the MAK genes are responsible for this modification (Denny and VanEtten, ).…”

Fungal Resistance to Plant Antibiotics as a Mechanism of Pathogenesis

“…Maackiain and medicarpin are believed to be detoxified by the same enzymes, but only maackian detoxification has been examined in detail (33,114,130). Four genes for maackiain detoxification have been identified in N. haematococca MPVI (MAK1 to MAK4) (33,129,130). Mak1 and Mak2 convert maackiain to 1(a)-hydroxymaackiain ( Fig.…”