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Histidine Kinase Two-Component Response Regulator Proteins Regulate Reproductive Development, Virulence, and Stress Responses of the Fungal Cereal Pathogens Cochliobolus heterostrophus and Gibberella zeae
<B>ABSTRACT</B><P>Histidine kinase (HK) phosphorelay signaling is a major mechanism by which fungi sense their environment. The maize pathogen Cochliobolus heterostrophus has 21 HK genes, 4 candidate response regulator (RR) genes (<I>SSK1</I>, <I>SKN7</I>, <I>RIM15</I>, <I>REC1</I>), and 1 gene (<I>HPT1</I>) encoding a histidine phosphotransfer domain protein. Because most HKs are expected to signal through RRs, these were chosen for deletion. Except for pigment and slight growth alterations for <I>rim15</I> mutants, no measurable altered phenotypes were detected in <I>rim15</I> or <I>rec1</I> mutants. Ssk1p is required for virulence and affects fertility and proper timing of sexual development of heterothallic C. heterostrophus. Pseudothecia from crosses involving <I>ssk1</I> mutants ooze masses of single ascospores, and tetrads cannot be found. Wild-type pseudothecia do not ooze. Ssk1p represses asexual spore proliferation during the sexual phase, and lack of it dampens asexual spore proliferation during vegetative growth, compared to that of the wild type. <I>ssk1</I> mutants are heavily pigmented. Mutants lacking Skn7p do not display any of the above phenotypes; however, both <I>ssk1</I> and <I>skn7</I> mutants are hypersensitive to oxidative and osmotic stresses and <I>ssk1 skn7</I> mutants are more exaggerated in their spore-type balance phenotype and more sensitive to stress than single mutants. <I>ssk1</I> mutant phenotypes largely overlap <I>hog1</I> mutant phenotypes, and in both types of mutant, the Hog1 target gene, <I>MST1</I>, is not induced. <I>ssk1</I> and <I>hog1</I> mutants were examined in the homothallic cereal pathogen Gibberella zeae, and pathogenic and reproductive phases of development regulated by Ssk1 and Hog1 were found to mirror, but also vary from, those of C. heterostrophus.</P>
Homothallic Didymella zeae-maydis undergoes sexual reproduction by selfing. Sequence analysis of the mating type (MAT) locus from this fungus revealed that MAT carries both MAT1-1-1 and MAT1-2-1 genes found in heterothallic Dothideomycetes, separated by ~1.0 kb of noncoding DNA. To understand the mechanistic basis of homothallism in D. zeae-maydis, each of the MAT genes was deleted and the effects on selfing and on ability to cross in a heterothallic manner were determined. The strain carrying an intact MAT1-1-1but defective MAT1-2-1 gene (MAT1-1-1;ΔMAT1-2-1) was self-sterile, however strains carrying an intact MAT1-2-1 but defective MAT1-1-1 gene (ΔMAT1-1-1;MAT1-2-1), when selfed, showed delayed production of a few ascospores. Attempts to cross the two MAT deletion strains yielded fewer ΔMAT1-1-1;MAT1-2-1 than MAT1-1-1;ΔMAT1-2-1 progeny and very few ascospores overall compared to WT selfs. This study demonstrates that, as in the other homothallic Dothideomycetes,both MAT genes are required for full fertility,but that, in contrast to other cases, the presence of a single MAT1-2-1 gene can induce homothallism, albeit inefficiently,in D. zeae-maydis.
Self-fertility in <i>Chromocrea spinulosa</i> is a consequence of direct repeat-mediated loss of <i>MAT1-2</i> , subsequent imbalance of nuclei differing in mating type, and recognition between unlike nuclei in a common cytoplasm
<▼1><P>The filamentous fungus <I>Chromocrea spinulosa</I> (<I>Trichoderma spinulosum</I>) exhibits both self-fertile (homothallic) and self-sterile (heterothallic) sexual reproductive behavior. Self-fertile strains produce progeny cohorts that are 50% homothallic, 50% heterothallic. Heterothallic progeny can mate only with homothallic strains, and progeny also segregate 50% homothallic, 50% heterothallic. Sequencing of the mating type (<I>MAT</I>) region of homothallic and heterothallic strains revealed that both carry an intact <I>MAT1-1</I> locus with three <I>MAT1-1</I> genes (<I>MAT1-1-1</I>, <I>MAT1-1-2</I>, <I>MAT1-1-3</I>), as previously described for the Sordariomycete group of filamentous fungi. Homothallic strains, however, have a second version of <I>MAT</I> with the <I>MAT1-2</I> locus genetically linked to <I>MAT1-1</I>. In this version, the <I>MAT1-1-1</I> open reading frame is split into a large and small fragment and the truncated ends are bordered by 115bp direct repeats (DR). The <I>MAT1-2-1</I> gene and additional sequences are inserted between the repeats. To understand the mechanism whereby <I>C</I>. <I>spinulosa</I> can exhibit both homothallic and heterothallic behavior, we utilized molecular manipulation to delete one of the DRs from a homothallic strain and insert <I>MAT1-2</I> into a heterothallic strain. Mating assays indicated that: i) the DRs are key to homothallic behavior, ii) looping out of <I>MAT1-2-1 via</I> intra-molecular homologous recombination between the DRs in self-fertile strains results in two nuclear types in an individual (one carrying both <I>MAT1-1</I> and <I>MAT1-2</I> and one carrying <I>MAT1-1</I> only), iii) self-fertility is achieved by inter-nuclear recognition between these two nuclear types before meiosis, iv) the two types of nuclei are in unequal proportion, v) having both an intact <I>MAT1-1-1</I> and <I>MAT1-2-1</I> gene in a single nucleus is not sufficient for self-fertility, and vi) the large truncated <I>MAT1-1-1</I> fragment is expressed. Comparisons with <I>MAT</I> regions of <I>Trichoderma reesei</I> and <I>Trichoderma virens</I> suggest that several crossovers between misaligned parental <I>MAT</I> chromosomes may have led to the <I>MAT</I> architecture of homothallic <I>C</I>. <I>spinulosa</I>.</P></▼1><▼2><P><B>Author summary</B></P><P>Fungi employ one of two mating tactics for sexual reproduction: self-sterile/heterothallic species can mate only with a genetically distinct partner while self-fertile/homothallic species do not require a partner. In ascomycetes, sexual reproduction is controlled by master regulators encoded by the mating-type (<I>MAT</I>) locus. The architecture of <I>MAT</I> differs in heterothallic <I>versus</I> homothallic species; heterothallics carry one of two forms (<I>MAT1-1</I> or <I>MAT1-2</I>) per nucleus, whereas most homothallics carry both <I>MAT</I> forms in a single nucleus. There are intriguing exceptions. For example, the yeast models, <I>Saccharomyces cerevisiae</I>, and <I>Schizosaccharomyces pombe</I> undergo reversible <I>MAT</I> switching, not demonstrated in filamentous fungi. Here, we describe the mating mechanism in <I>Chromocrea spinulosa</I> (<I>Trichoderma spinulosum</I>), a filamentous ascomycete that exhibits both homothallic and heterothallic behavior. Self-fertile strains produce progeny cohorts that are 50% homothallic, 50% heterothallic. Self-sterile strains can mate only with homothallic strains, and when this occurs, homothallic and heterothallic progeny are also produced in a 1:1 ratio. By <I>MAT</I> sequencing and manipulation, we discovered unique <I>MAT</I> architecture and determined that self-fertility is achieved by deletion of <I>MAT1-2</I> from most homothallic nuclei and subsequent inter-nuclear recognition between the resulting two, unevenly present, nuclear types in a common cytoplasm.</P></▼2>
Condon, Bradford J.,Elliott, Candace,Gonzá,lez, Jonathan B.,Yun, Sung Hwan,Akagi, Yasunori,Wiesner-Hanks, Tyr,Kodama, Motochiro,Turgeon, B. Gillian APS Press 2018 Molecular plant-microbe interactions Vol.31 No.11