Variability in an effector gene promoter of a necrotrophic fungal pathogen dictates epistasis and effector-triggered susceptibility in wheat

Genome-wide survey of the bipartite structure and pathogenesis-related genes of Neostagonosporella sichuanensis, a causal agent of Fishscale bamboo rhombic-spot disease

Bamboo resources have garnered significant global attention due to their excellent capacity for regeneration and high yield. Rhombic-spot disease, a substantial threat to fishscale bamboo (Phyllostachys heteroclada), is primarily caused by Neostagonosporella sichuanensis. This study first reported the genome assemblies and characteristics of two N. sichuanensis isolates using PacBio and Illumina sequencing platforms. The genomes of N. sichuanensis strain SICAUCC 16-0001 and strain SICAUCC 23-0140, with sizes of 48.0 Mb and 48.4 Mb, respectively, revealed 10,289 and 10,313 protein-coding genes. Additionally, they contained 34.99 and 34.46% repetitive sequences within AT-rich regions, with notable repeat-induced point mutation activity. Comparative genome analysis identified 1,049 contracted and 45 expanded gene families in the genome of N. sichuanensis, including several related to pathogenicity. Several gene families involved in mycotoxin metabolism, secondary metabolism, sterol biosynthesis and transport, and cell wall degradation were contracted. Compared to most analyzed necrotrophic, hemibiotrophic, and phaeosphaeriacous pathogens, the genomes of two N. sichuanensis isolates exhibited fewer secondary metabolite enzymes, carbohydrate-active enzymes, plant cell wall degrading enzymes, secreted proteins, and effectors. Comparative genomics analysis suggested that N. sichuanensis shares more similar characteristics with hemibiotrophic pathogens. Based on single carbon source tests, N. sichuanensis strains demonstrated a higher potential for xylan decomposition than pectin and cellulose. The proportion of cell wall-degrading enzyme effectors occupied a high proportion of the total effectors of the N. sichuanensis genomes. These findings provide valuable insights into uncovering the pathogenesis of N. sichuanensis toward the efficient management of rhombic-spot disease of fishscale bamboo.

Regulatory insight for a Zn2Cys6 transcription factor controlling effector-mediated virulence in a fungal pathogen of wheat

The regulation of virulence in plant-pathogenic fungi has emerged as a key area of importance underlying host infections. Recent work has highlighted individual transcription factors (TFs) that serve important roles. A prominent example is PnPf2, a member of the Zn2Cys6 family of fungal TFs, which controls the expression of effectors and other virulence-associated genes in Parastagonospora nodorum during infection of wheat. PnPf2 orthologues are similarly important for other major fungal pathogens during infection of their respective host plants, and have also been shown to control polysaccharide metabolism in model saprophytes. In each case, the direct genomic targets and associated regulatory mechanisms were unknown. Significant insight was made here by investigating PnPf2 through chromatin-immunoprecipitation (ChIP) and mutagenesis approaches in P. nodorum. Two distinct binding motifs were characterised as positive regulatory elements and direct PnPf2 targets identified. These encompass known effectors and other components associated with the P. nodorum pathogenic lifestyle, such as carbohydrate-active enzymes and nutrient assimilators. The results support a direct involvement of PnPf2 in coordinating virulence on wheat. Other prominent PnPf2 targets included TF-encoding genes. While novel functions were observed for the TFs PnPro1, PnAda1, PnEbr1 and the carbon-catabolite repressor PnCreA, our investigation upheld PnPf2 as the predominant transcriptional regulator characterised in terms of direct and specific coordination of virulence on wheat, and provides important mechanistic insights that may be conserved for homologous TFs in other fungi.

The Velvet transcription factor PnVeA regulates necrotrophic effectors and secondary metabolism in the wheat pathogen Parastagonospora nodorum

The fungus Parastagonospora nodorum causes septoria nodorum blotch on wheat. The role of the fungal Velvet-family transcription factor VeA in P. nodorum development and virulence was investigated here. Deletion of the P. nodorum VeA ortholog, PnVeA, resulted in growth abnormalities including pigmentation, abolished asexual sporulation and highly reduced virulence on wheat. Comparative RNA-Seq and RT-PCR analyses revealed that the deletion of PnVeA also decoupled the expression of major necrotrophic effector genes. In addition, the deletion of PnVeA resulted in an up-regulation of four predicted secondary metabolite (SM) gene clusters. Using liquid-chromatography mass-spectrometry, it was observed that one of the SM gene clusters led to an accumulation of the mycotoxin alternariol. PnVeA is essential for asexual sporulation, full virulence, secondary metabolism and necrotrophic effector regulation.

The Role of Cytokinins and Abscisic Acid in the Growth, Development and Virulence of the Pathogenic Fungus Stagonospora nodorum (Berk.)

Cytokinins (CKs) and abscisic acid (ABA) play an important role in the life of both plants and pathogenic fungi. However, the role of CKs and ABA in the regulation of fungal growth, development and virulence has not been sufficiently studied. We compared the ability of two virulent isolates (SnB and Sn9MN-3A) and one avirulent isolate (Sn4VD) of the pathogenic fungus Stagonospora nodorum Berk. to synthesize three groups of hormones (CKs, ABA and auxins) and studied the effect of exogenous ABA and zeatin on the growth, sporulation and gene expression of necrotrophic effectors (NEs) and transcription factors (TFs) in them. Various isolates of S. nodorum synthesized different amounts of CKs, ABA and indoleacetic acid. Using exogenous ABA and zeatin, we proved that the effect of these hormones on the growth and sporulation of S. nodorum isolates can be opposite, depends on both the genotype of the isolate and on the concentration of the hormone and is carried out through the regulation of carbohydrate metabolism. ABA and zeatin regulated the expression of fungal TF and NE genes, but correlation analysis of these parameters showed that this effect depended on the genotype of the isolate. This study will contribute to our understanding of the role of the hormones ABA and CKs in the biology of the fungal pathogen S. nodorum.

The elicitor VP2 from Verticillium dahliae triggers defence response in cotton

Verticillium dahliae is a widespread and destructive soilborne vascular pathogenic fungus that causes serious diseases in dicot plants. Here, comparative transcriptome analysis showed that the number of genes upregulated in defoliating pathotype V991 was significantly higher than in the non-defoliating pathotype 1cd3-2 during the early response of cotton. Combined with analysis of the secretome during the V991-cotton interaction, an elicitor VP2 was identified, which was highly upregulated at the early stage of V991 invasion, but was barely expressed during the 1cd3-2-cotton interaction. Full-length VP2 could induce cell death in several plant species, and which was dependent on NbBAK1 but not on NbSOBIR1 in N. benthamiana. Knock-out of VP2 attenuated the pathogenicity of V991. Furthermore, overexpression of VP2 in cotton enhanced resistance to V. dahliae without causing abnormal plant growth and development. Several genes involved in JA, SA and lignin synthesis were significantly upregulated in VP2-overexpressing cotton. The contents of JA, SA, and lignin were also significantly higher than in the wild-type control. In summary, the identified elicitor VP2, recognized by the receptor in the plant membrane, triggers the cotton immune response and enhances disease resistance.

Till death do us pair: Co-evolution of plant-necrotroph interactions

Plants use programmed cell death as a potent defense response against biotrophic pathogens that require living host cells to thrive. However, cell death can promote infection by necrotrophic pathogens. This discrepancy creates specific co-evolutionary dynamics in the interaction between plants and necrotrophs. Necrotrophic pathogens produce diverse cell death-inducing effectors that act redundantly on several plant targets and sometimes suppress plant immune responses as an additional function. Plants use surface receptors that recognize necrotrophic effectors to increase quantitative disease resistance, some of which evolved independently in several plant lineages. Co-evolution has shaped molecular mechanisms involved in plant-necrotroph interactions into robust systems, relying on degenerate and multifunctional modules, general-purpose components, and compartmentalized functioning.

Surface frustration re-patterning underlies the structural landscape and evolvability of fungal orphan candidate effectors

Pathogens secrete effector proteins to subvert host physiology and cause disease. Effectors are engaged in a molecular arms race with the host resulting in conflicting evolutionary constraints to manipulate host cells without triggering immune responses. The molecular mechanisms allowing effectors to be at the same time robust and evolvable remain largely enigmatic. Here, we show that 62 conserved structure-related families encompass the majority of fungal orphan effector candidates in the Pezizomycotina subphylum. These effectors diversified through changes in patterns of thermodynamic frustration at surface residues. The underlying mutations tended to increase the robustness of the overall effector protein structure while switching potential binding interfaces. This mechanism could explain how conserved effector families maintained biological activity over long evolutionary timespans in different host environments and provides a model for the emergence of sequence-unrelated effector families with conserved structures.

Genetic co-option into plant-filamentous pathogen interactions

Plants are engaged in a coevolutionary arms race with their pathogens that drives rapid diversification and specialization of genes involved in resistance and virulence. However, some major innovations in plant-pathogen interactions, such as molecular decoys, trans-kingdom RNA interference, two-speed genomes, and receptor networks, evolved through the expansion of the functional landscape of genes. This is a typical outcome of genetic co-option, the evolutionary process by which available genes are recruited into new biological functions. Co-option into plant-pathogen interactions emerges generally from (i) cis-regulatory variation, (ii) horizontal gene transfer (HGT), (iii) mutations altering molecular promiscuity, and (iv) rewiring of gene networks and protein complexes. Understanding these molecular mechanisms is key for the functional and predictive biology of plant-pathogen interactions.