The telomere-linked helicase (TLH) gene family in Magnaporthe oryzae: revised gene structure reveals a novel TLH-specific protein motif

The Histone Deacetylases MoRpd3 and MoHst4 Regulate Growth, Conidiation, and Pathogenicity in the Rice Blast Fungus Magnaporthe oryzae

As the causal agent of the blast disease, Magnaporthe oryzae is one of the most destructive fungal pathogens of rice. Histone acetylation/deacetylation is important for remodeling of chromatin superstructure and thus altering gene expression. In this study, two genes encoding histone deacetylases, namely, MoRPD3 and MoHST4, were identified and functionally characterized in M. oryzae. MoHst4 was required for proper mycelial growth and pathogenicity, whereas overproduction of MoRpd3 led to loss of pathogenicity, likely due to a block in conidial cell death and restricted invasive growth within the host plants. Green fluorescent protein (GFP)-MoRpd3 localized to the nucleus and cytoplasm in vegetative hyphae and developing conidia. By comparative transcriptomics analysis, we identified potential target genes epigenetically regulated by histone deacetylases (HDACs) containing MoRpd3 or MoHst4, which may contribute to conidia formation and/or conidial cell death, which is a prerequisite for successful appressorium-mediated host invasion. Taken together, our results suggest that histone deacetylases MoRpd3 and MoHst4 differentially regulate mycelial growth, asexual development, and pathogenesis in M. oryzae. IMPORTANCE HDACs (histone deacetylases) regulate various aspects of growth, development, and pathogenesis in plant-pathogenic fungi. Most members of HDAC classes I to III have been functionally characterized, except for orthologous Rpd3 and Hst4, in the rice blast fungus Magnaporthe oryzae. In this study, we assessed the function of MoRpd3 and MoHst4 by reverse genetics and found that they differentially regulate M. oryzae vegetative growth, asexual development, and infection. Particularly, MoRpd3 negatively regulates M. oryzae pathogenicity, likely through suppression of conidial cell death, which we recently reported as being critical for appressorium maturation and functioning. Overall, this study broadens our understanding of fungal pathobiology and its critical regulation by histone modification(s) during cell death and in planta differentiation.

Comprehensive study of the genes involved in chlorophyll synthesis and degradation pathways in some monocot and dicot plant species

Chlorophyll (Chl) biosynthesis is one of the most important cellular processes essential for plant photosynthesis. Chl degradation pathway is also important catabolic process occurs during leaf senescence, fruit ripening and under biotic or abiotic stress conditions. Here we have systematically investigated the molecular evolution, gene structure, compositional analysis along with ENc plot, correspondence analysis and codon usage bias of the proteins and encoded genes involved in Chl metabolism from monocots and dicots. The gene and species specific phylogenetic trees using amino acid sequences showed clear clustering formation of the selected species based on monocots and dicots but not supported by 18S rRNA. Nucleotide composition of the encoding genes showed that average GC%, GC1%, GC2% and GC3% were higher in monocots. RSCU analysis depicts that genes from monocots for both pathways and genes for synthesis pathway from dicots only biased to G/C-ending synonymous codons but in degradation pathway most optimal codons (except UUG) in dicots biased to A/U-ending synonymous codons. We found strong evidence of episodic diversifying selection at several amino acid sites in all genes investigated. Conserved domain and gene structures were observed for the genes with varying lengths of introns and exons, involved in Chl metabolism along with some intronless genes within synthesis pathway. ENc and correspondence analyses suggested the mutational or selection constraint on the genes to shape the codon usage. These comprehensive studies may be helpful in further research in molecular phylogenetics and genomics and to better understand the evolutionary dynamics of Chl metabolic pathway.Communicated by Ramaswamy H. Sarma.

Identification of the group IIa WRKY subfamily and the functional analysis of GhWRKY17 in upland cotton (Gossypium hirsutum L.)

WRKY transcription factors play important roles in plant defense, stress response, leaf senescence, and plant growth and development. Previous studies have revealed the important roles of the group IIa GhWRKY genes in cotton. To comprehensively analyze the group IIa GhWRKY genes in upland cotton, we identified 15 candidate group IIa GhWRKY genes in the Gossypium hirsutum genome. The phylogenetic tree, intron-exon structure, motif prediction and Ka/Ks analyses indicated that most group IIa GhWRKY genes shared high similarity and conservation and underwent purifying selection during evolution. In addition, we detected the expression patterns of several group IIa GhWRKY genes in individual tissues as well as during leaf senescence using public RNA sequencing data and real-time quantitative PCR. To better understand the functions of group IIa GhWRKYs in cotton, GhWRKY17 (KF669857) was isolated from upland cotton, and its sequence alignment, promoter cis-acting elements and subcellular localization were characterized. Moreover, the over-expression of GhWRKY17 in Arabidopsis up-regulated the senescence-associated genes AtWRKY53, AtSAG12 and AtSAG13, enhancing the plant’s susceptibility to leaf senescence. These findings lay the foundation for further analysis and study of the functions of WRKY genes in cotton.

Molecular variability and genetic relationship among Brazilian strains of the sugarcane smut fungus

Sporisorium scitamineum is the fungus that causes sugarcane smut disease. Despite of the importance of sugarcane for Brazilian agribusiness and the persistence of the pathogen in most cropping areas, genetic variation studies are still missing for Brazilian isolates. In this study, sets of isolates were analyzed using two molecular markers (AFLP and telRFLP) and ITS sequencing. Twenty-two whips were collected from symptomatic plants in cultivated sugarcane fields of Brazil. A total of 41 haploid strains of compatible mating types were selected from individual teliospores and used for molecular genetic analyses. telRFLP and ITS analyses were expanded to six Argentine isolates, where the sugarcane smut was first recorded in America. Genetic relationship among strains suggests the human-mediated dispersal of S. scitamineum within the Brazilian territory and between the two neighboring countries. Two genetically distinct groups were defined by the combined analysis of AFLP and telRFLP. The opposite mating-type strains derived from single teliospores were clustered together into these main groups, but had not always identical haplotypes. telRFLP markers analyzed over two generations of selfing and controlled outcrossing confirmed the potential for emergence of new variants and occurrence of recombination, which are relevant events for evolution of virulence and environmental adaptation.

Directional Selection from Host Plants Is a Major Force Driving Host Specificity in Magnaporthe Species

One major threat to global food security that requires immediate attention, is the increasing incidence of host shift and host expansion in growing number of pathogenic fungi and emergence of new pathogens. The threat is more alarming because, yield quality and quantity improvement efforts are encouraging the cultivation of uniform plants with low genetic diversity that are increasingly susceptible to emerging pathogens. However, the influence of host genome differentiation on pathogen genome differentiation and its contribution to emergence and adaptability is still obscure. Here, we compared genome sequence of 6 isolates of Magnaporthe species obtained from three different host plants. We demonstrated the evolutionary relationship between Magnaporthe species and the influence of host differentiation on pathogens. Phylogenetic analysis showed that evolution of pathogen directly corresponds with host divergence, suggesting that host-pathogen interaction has led to co-evolution. Furthermore, we identified an asymmetric selection pressure on Magnaporthe species. Oryza sativa-infecting isolates showed higher directional selection from host and subsequently tends to lower the genetic diversity in its genome. We concluded that, frequent gene loss or gain, new transposon acquisition and sequence divergence are host adaptability mechanisms for Magnaporthe species, and this coevolution processes is greatly driven by directional selection from host plants.

The Chp1-Tas3 core is a multifunctional platform critical for gene silencing by RITS

RNA interference (RNAi) is critical for the assembly of heterochromatin at fission yeast centromeres. Central to this process is the RNA-induced Initiation of Transcriptional gene Silencing (RITS) complex, which physically anchors small non-coding RNAs to chromatin. RITS includes Ago1, the chromodomain protein Chp1, and Tas3, which bridges between Chp1 and Ago1. Chp1 is a large protein with, apart from its chromodomain, no recognizable domains. Here we describe how the structured C-terminal half of Chp1 binds the Tas3 N-terminal domain, revealing Chp1's tight embrace of Tas3. The structure also reveals a PIN domain at the C-terminal tip of Chp1 that controls subtelomeric transcripts through a post-transcriptional mechanism. We suggest that the Chp1-Tas3 complex provides a solid and versatile platform to recruit both RNAi-dependent and RNAi-independent gene-silencing pathways for locus-specific regulation of heterochromatin.

Contrasting codon usage patterns and purifying selection at the mating locus in putatively asexual alternaria fungal species

Sexual reproduction in heterothallic ascomycete fungi is controlled by a single mating-type locus called MAT1 with two alternate alleles or idiomorphs, MAT1-1 and MAT1-2. These alleles lack sequence similarity and encode different transcriptional regulators. A large number of phytopathogenic fungi including Alternaria spp. are considered asexual, yet still carry expressed MAT1 genes. The molecular evolution of Alternaria MAT1 was explored using nucleotide diversity, nonsynonymous vs. synonymous substitution (dn/ds) ratios and codon usage statistics. Likelihood ratio tests of site-branch models failed to detect positive selection on MAT1-1-1 or MAT1-2-1. Codon-site models demonstrated that both MAT1-1-1 and MAT1-2-1 are under purifying selection and significant differences in codon usage were observed between MAT1-1-1 and MAT1-2-1. Mean GC content at the third position (GC3) and effective codon usage (ENC) were significantly different between MAT1-1-1 and MAT1-2-1 with values of 0.57 and 48 for MAT1-1-1 and 0.62 and 46 for MAT1-2-1, respectively. In contrast, codon usage of Pleospora spp. (anamorph Stemphylium), a closely related Dothideomycete genus, was not significantly different between MAT1-1-1 and MAT1-2-1. The purifying selection and biased codon usage detected at the MAT1 locus in Alternaria spp. suggest a recent sexual past, cryptic sexual present and/or that MAT1 plays important cellular role(s) in addition to mating.

Telomere position effect is regulated by heterochromatin-associated proteins and NkuA in Aspergillus nidulans

Gene-silencing mechanisms are being shown to be associated with an increasing number of fungal developmental processes. Telomere position effect (TPE) is a eukaryotic phenomenon resulting in gene repression in areas immediately adjacent to telomere caps. Here, TPE is shown to regulate expression of transgenes on the left arm of chromosome III and the right arm of chromosome VI in Aspergillus nidulans. Phenotypes found to be associated with transgene repression included reduction in radial growth and the absence of sexual spores; however, these pleiotropic phenotypes were remedied when cultures were grown on media with appropriate supplementation. Simple radial growth and ascosporogenesis assays provided insights into the mechanism of TPE, including a means to determine its extent. These experiments revealed that the KU70 homologue (NkuA) and the heterochromatin-associated proteins HepA, ClrD and HdaA were partially required for transgene silencing. This study indicates that TPE extends at least 30 kb on chromosome III, suggesting that this phenomenon may be important for gene regulation in subtelomeric regions of A. nidulans.

The choanoflagellate Monosiga brevicollis karyotype revealed by the genome sequence: telomere-linked helicase genes resemble those of some fungi

The approximately 42 Mbp assembled genome sequence for the choanoflagellate Monosiga brevicollis reveals that most of the large scaffolds of 300-2,600 kb represent entire chromosomes or chromosome arms. Telomeres are partially assembled at the termini of 37 scaffolds, while another 43 scaffolds end in telomere-associated regions containing distinctive gene sets. Potential centromeric regions were identified on 39 scaffolds. Together, these observations suggest a karyotype of approximately 40 metacentric and submetacentric chromosomes averaging 1 Mbp in size. Genes encoding RecQ family DNA helicases, along with ankyrin-domain proteins and serine/threonine kinases, are associated with most telomeres, a feature shared with some fungi. This telomere-linked helicase gene arrangement might be ancestral to both fungi and choanoflagellates in the super-kingdom Opisthokonta; however, the great lability of telomere architecture suggests that it could also be a convergent feature.