Identification of a polyketide synthase gene responsible for ascochitine biosynthesis in Ascochyta fabae and its abrogation in sister taxa

Authors: KIM, WONYONG - Suncheon National University; LICHTENZVEIG, JUDITH - Curtin University; SYME, ROBERT - Curtin University; BERIM, ANNA - Washington State University; PEEVER, TOBIN - Washington State University; HUR, JAE-SEOUN - Suncheon National University; Chen, Weidong

Submitted to: mSphere
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/4/2019
Publication Date: 9/24/2019
Citation: Kim, W., Lichtenzveig, J., Syme, R.A., Berim, A., Peever, T., Hur, J., Chen, W. 2019. Identification of a polyketide synthase gene responsible for ascochitine biosynthesis in Ascochyta fabae and its abrogation in sister taxa. mSphere. 4:5. https://doi.org/10.1128/mSphere.00622-19.
DOI: https://doi.org/10.1128/mSphere.00622-19

Interpretive Summary: Fungal plant pathogens often produce compounds that are toxic to plants and are consequently assumed to be responsible for causing disease. Ascochyta blight pathogens (Ascochyta spp.) infect legume crops in a host selective manner and produce an array of toxic compounds. The chickpea pathogen Ascochyta rabiei produces solanopyrone, whereas the pathogen of faba bean Ascochyta fabae and some other Ascochyta species produce ascochitine, which has been assumed to be a factor in disease development. However, the lentil pathogen Ascochyta lentis and its sister species Ascochyta lentis var. lathyri, which are closely related to A. fabae, have lost the ability to produce ascochitine. We found by analyzing DNA sequences that Ascochyta fabae possesses a complete sequence of a polyketide synthase gene (denoted as pksAC), whereas the same genes in non-ascochitine producers had mutations that made the genes not work, which explains the lack of ascochitine production in several Ascochyta species. Although ascochitine is toxic to plants, mutants that lost ascochitine production could still cause disease, which suggests that the ability of these fungi to cause disease was not dependent on production of ascochytine.

Technical Abstract: A polyketide-derived secondary metabolite ascochitine is produced by many Ascochyta species infecting different legumes in a host-selective manner. Ascochitine is structurally similar to the well-known mycotoxin citrinin, and exhibits a broad spectrum of phytotoxicity and antimicrobial activities. Here we identified a polyketide synthase gene (denoted as pksAC) responsible for ascochitine production in the filamentous fungus Ascochyta fabae. pksAC deletion mutants lost production of ascochitine and its derivative ascochital in A. fabae. The putative ascochitine biosynthesis gene cluster consisted of eleven genes that have undergone gene rearrangements and gene gain and loss events in relation to the citrinin biosynthesis gene cluster in Monascus ruber. Interestingly, we also identified pksAC homologs in two recently diverged species, A. lentis and A. lentis var. lathyri, which are sister taxa to ascochitine producers such as A. fabae and A. viciae-villosae. However, nonsense mutations have been independently introduced in coding sequences of the pksAC homologs of A. lentis and A. lentis var. lathyri, resulting in loss of ascochitine production. Despite the reported phytotoxicity, ascochitine was not a pathogenicity factor in the A. fabae and faba bean (Vicia faba L.) pathosystem. The observation that ascochitine was mainly produced by aged hyphae where pycnidia formed suggests a protective role of the compound in nature against other microbial competitors. This study highlights evolution of gene clusters harnessing structural diversity of polyketides and a mechanism with the potential to alter secondary metabolite profiles through single nucleotide polymorphisms in closely-related fungal species.