Magnaporthe oryzae is the causal agent of rice blast disease, leading to enormous losses of rice production. Here, we characterized a basic leucine zipper (bZIP) transcription factor, Moatf1, in M. oryzae, a homolog of Schizosaccharomyces pombe ATF/CREB that regulates the oxidative stress response. Moatf1 deletion caused retarded vegetative growth of mycelia, and the Moatf1 mutant exhibited higher sensitivity to hydrogen peroxide (H(2)O(2)) than did the wild-type strain. The mutant showed severely reduced activity of extracellular enzymes and transcription level of laccases and peroxidases and exhibited significantly reduced virulence on rice cultivar CO-39. On rice leaf sheath, most of the infectious hyphae of the mutant became swollen and displayed restricted growth in primary infected cells. Defense response was strongly activated in plants infected by the mutant. Diamino benzidine staining revealed an accumulation of H(2)O(2) around Moatf1 mutant appressoria and rice cells with Moatf1 hyphae that was absent in the wild type. Inhibition of the plant NADPH oxidase by diphenyleneiodonium prevented host-derived H(2)O(2) accumulation and restored infectious hyphal growth of the mutant in rice cells. Thus, we conclude that Moatf1 is necessary for full virulence of M. oryzae by regulating the transcription of laccases and peroxidases to impair reactive oxygen species-mediated plant defense.

Magnaporthe oryzae MAPK MoMps1 plays a critical role in regulating various developmental processes including cell wall integrity, stress responses, and pathogenicity. To identify potential effectors of MoMps1, we characterized the function of MoSwi6, a homolog of Saccharomyces cerevisiae Swi6 downstream of MAPK Slt2 signaling. MoSwi6 interacted with MoMps1 both in vivo and in vitro, suggesting a possible functional link analogous to Swi6-Slt2 in S. cerevisiae. Targeted gene disruption of MoSWI6 resulted in multiple developmental defects, including reduced hyphal growth, abnormal formation of conidia and appressoria, and impaired appressorium function. The reduction in appressorial turgor pressure also contributed to an attenuation of pathogenicity. The ΔMoswi6 mutant also displayed a defect in cell wall integrity, was hypersensitive to the oxidative stress, and showed significant reduction in transcription and activities of extracellular enzymes including peroxidases and laccases. Collectively, these roles are similar to those of MoMps1, confirming that MoSwi6 functions in the MoMps1 pathway to govern growth, development, and full pathogenicity.

Summary Allopolyploidization, which entails interspecific hybridization and whole genome duplication (WGD), is associated with emergent genetic and epigenetic instabilities that are thought to contribute to adaptation and evolution. One frequent genomic consequence of nascent allopolyploidization is homoeologous exchange (HE), which arises from compromised meiotic fidelity and generates genetically and phenotypically variable progenies. Here, we used a genetically tractable synthetic rice segmental allotetraploid system to interrogate genome‐wide DNA methylation and gene expression responses and outcomes to the separate and combined effects of hybridization, WGD and HEs. Progenies of the tetraploid rice were genomically diverse due to genome‐wide HEs that affected all chromosomes, yet they exhibited overall methylome stability. Nonetheless, regional variation of cytosine methylation states was widespread in the tetraploids. Transcriptome profiling revealed genome‐wide alteration of gene expression, which at least in part associates with changes in DNA methylation. Intriguingly, changes of DNA methylation and gene expression could be decoupled from hybridity and sustained and amplified by HEs. Our results suggest that HEs, a prominent genetic consequence of nascent allopolyploidy, can exacerbate, diversify and perpetuate the effects of allopolyploidization on epigenetic and gene expression variation, and hence may contribute to allopolyploid evolution.

Although a distinct karyotype with defined chromosome number and structure characterizes each biological species, it is intrinsically labile. Polyploidy or whole-genome duplication has played a pervasive and ongoing role in the evolution of all eukaryotes, and is the most dramatic force known to cause rapid karyotypic reconfiguration, especially at the initial stage. However, issues concerning transgenerational propagation of karyotypic heterogeneity and its translation to phenotypic diversity in nascent allopolyploidy, at the population level, have yet to be studied in detail. Here, we report a large-scale examination of transgenerationally propagated karyotypic heterogeneity and its phenotypic manifestation in an artificially constructed allotetraploid with a genome composition of AADD, that is, involving two of the three progenitor genomes of polyploid wheat. Specifically, we show that 1) massive organismal karyotypic heterogeneity is precipitated after 12 consecutive generations of selfing from a single euploid founder individual, 2) there exist dramatic differences in aptitudes between subgenomes and among chromosomes for whole-chromosome gain and/or loss and structural variations, 3) majority of the numerical and structural chromosomal variations are concurrent due to mutual contingency and possible functional constraint, 4) purposed and continuous selection and propagation for euploidy over generations did not result in enhanced karyotype stabilization, and 5) extent of karyotypic variation correlates with variability of phenotypic manifestation. Together, our results document that allopolyploidization catalyzes rampant and transgenerationally heritable organismal karyotypic heterogeneity that drives population-level phenotypic diversification, which lends fresh empirical support to the still contentious notion that whole-genome duplication enhances organismal evolvability.

The Triticum/Aegilops complex includes hybrid species resulting from HHS (homoploid hybrid speciation) and AS (allopolyploid speciation). Sequential allotetra- and allohexaploidy events presumably result in two challenges for the hybrids, which involve (i) cytonuclear stoichiometric disruptions caused by combining two diverged nuclear genomes with the maternal inheritance of the cytoplasmic organellar donor; and (ii) incompatibility of chimeric protein complexes with diverged subunits from nuclear and cytoplasmic genomes. Here, we describe co-evolution of nuclear rbcS genes encoding the small subunits of Rubisco (ribulose 1,5-bisphosphate carboxylase/oxygenase) and nuclear genes encoding plastid translocons, which mediate recognition and translocation of nuclear-encoded proteins into plastids, in allopolyploid wheat species. We demonstrate that intergenomic paternal-to-maternal gene conversion specifically occurred in the genic region of the homoeologous rbcS3 gene from the D genome progenitor of wheat (abbreviated as rbcS3D) such that it encodes a maternal-like or B-subgenome-like SSU3D transit peptide in allohexaploid wheat but not in allotetraploid wheat. Divergent and limited interaction between SSU3D and the D-subgenomic TOC90D translocon subunit is implicated to underpin SSU3D targeting into the chloroplast of hexaploid wheat. This implicates early selection favoring individuals harboring optimal maternal-like organellar SSU3D targeting in hexaploid wheat. These data represent a novel dimension of cytonuclear evolution mediated by organellar targeting and transportation of nuclear proteins.