The Phantom Menace: latest findings on effector biology in the rice blast fungus

Comparative transcriptome profiling reveals the key genes and molecular mechanisms involved in rice under blast infection

The aim of this study was to analyze the resistance genes and molecular mechanisms involved in rice blast infection. The contents of seven hormones and eight biochemical indicators in the leaves and spikes were at dynamic levels after inoculation with rice blast strains over time. The mRNA and protein expression of the six genes were consistent with the transcriptome analysis results. In addition, KEGG enrichment analysis showed that Os03g0132000, Os06g0215600, and Os06g0215500 were significantly enriched in the alpha-linolenic acid metabolism KEGG pathway, whereas Os05g0311801 was significantly enriched in the zeatin biosynthesis KEGG pathway. Furthermore, Os03g0180900 and Os09g0439200 were significantly enriched in the plant hormone signal transduction KEGG pathways. Therefore, blast infection could alter the hormones, biochemical indicators, and traits of rice. Moreover, genes including Os03g0132000, Os03g0180900, and Os05g0311801 were identified as rice blast resistance genes, and the mechanism might involve alpha-linolenic acid metabolism, zeatin biosynthesis, and plant hormone signal transduction KEGG pathways.

Membrane fluidity control by the Magnaporthe oryzae acyl-CoA binding protein sets the thermal range for host rice cell colonization

Following leaf cuticle penetration by specialized appressorial cells, the devastating blast fungus Magnaporthe oryzae grows as invasive hyphae (IH) in living rice cells. IH are separated from host cytoplasm by plant-derived membranes forming an apoplastic compartment and a punctate biotrophic interfacial complex (BIC) that mediate the molecular host-pathogen interaction. What molecular and cellular processes determine the temperature range for this biotrophic growth stage is an unanswered question pertinent to a broader understanding of how phytopathogens may cope with environmental stresses arising under climate change. Here, we shed light on thermal adaptation in M. oryzae by disrupting the ACB1 gene encoding the single acyl-CoA-binding protein, an intracellular transporter of long-chain acyl-CoA esters. Loss of ACB1 affected fatty acid desaturation levels and abolished pathogenicity at optimal (26°C) and low (22°C) but not elevated (29°C) infection temperatures (the latter following post-penetration shifts from 26°C). Relative to wild type, the Δacb1 mutant strain exhibited poor vegetative growth and impaired membrane trafficking at 22°C and 26°C, but not at 29°C. In planta, Δacb1 biotrophic growth was inhibited at 26°C-which was accompanied by a multi-BIC phenotype-but not at 29°C, where BIC formation was normal. Underpinning the Δacb1 phenotype was impaired membrane fluidity at 22°C and 26°C but not at elevated temperatures, indicating Acb1 suppresses membrane rigidity at optimal- and suboptimal- but not supraoptimal temperatures. Deducing a temperature-dependent role for Acb1 in maintaining membrane fluidity homeostasis reveals how the thermal range for rice blast disease is both mechanistically determined and wider than hitherto appreciated.

Effectors and environment modulating rice blast disease: from understanding to effective control

Rice blast is a highly destructive crop disease that requires the interplay of three essential factors: the virulent blast fungus, the susceptible rice plant, and favorable environmental conditions. Although previous studies have focused mainly on the pathogen and rice, recent research has shed light on the molecular mechanisms by which the blast fungus and environmental conditions regulate host resistance and contribute to blast disease outbreaks. This review summarizes significant achievements in understanding the sophisticated modulation of blast resistance by Magnaporthe oryzae effectors and the dual regulatory mechanisms by which environmental conditions influence rice resistance and virulence of the blast fungus. Furthermore, it emphasizes potential strategies for developing blast-resistant rice varieties to effectively control blast disease.

The structural landscape and diversity of Pyricularia oryzae MAX effectors revisited

Magnaporthe AVRs and ToxB-like (MAX) effectors constitute a family of secreted virulence proteins in the fungus Pyricularia oryzae (syn. Magnaporthe oryzae), which causes blast disease on numerous cereals and grasses. In spite of high sequence divergence, MAX effectors share a common fold characterized by a ß-sandwich core stabilized by a conserved disulfide bond. In this study, we investigated the structural landscape and diversity within the MAX effector repertoire of P. oryzae. Combining experimental protein structure determination and in silico structure modeling we validated the presence of the conserved MAX effector core domain in 77 out of 94 groups of orthologs (OG) identified in a previous population genomic study. Four novel MAX effector structures determined by NMR were in remarkably good agreement with AlphaFold2 (AF2) predictions. Based on the comparison of the AF2-generated 3D models we propose a classification of the MAX effectors superfamily in 20 structural groups that vary in the canonical MAX fold, disulfide bond patterns, and additional secondary structures in N- and C-terminal extensions. About one-third of the MAX family members remain singletons, without strong structural relationship to other MAX effectors. Analysis of the surface properties of the AF2 MAX models also highlights the high variability within the MAX family at the structural level, potentially reflecting the wide diversity of their virulence functions and host targets.

Unveiling the Roles of LncRNA MOIRAs in Rice Blast Disease Resistance

Rice blast disease, caused by the fungal pathogen Magnaporthe oryzae, is a major threat to rice production worldwide. This study investigates the role of long non-coding RNAs (lncRNAs) in rice's response to this destructive disease, with a focus on their impacts on disease resistance and yield traits. Three specific lncRNAs coded by M. oryzae infection-responsive lncRNAs (MOIRAs), MOIRA1, MOIRA2, and MOIRA3, were identified as key regulators of rice's response to M. oryzae infection. Strikingly, when MOIRA1 and MOIRA2 were overexpressed, they exhibited a dual function: they increased rice's susceptibility to blast fungus, indicating a negative role in disease resistance, while simultaneously enhancing tiller numbers and single-plant yield, with no adverse effects on other yield-related traits. This unexpected improvement in productivity suggests the possibility of overcoming the traditional trade-off between disease resistance and crop yield. These findings provide a novel perspective on crop enhancement, offering a promising solution to global food security challenges by developing rice varieties that effectively balance disease resistance and increased productivity.

GWAS reveals a rapidly evolving candidate avirulence effector in the Cercospora leaf spot pathogen

The major resistance gene BvCR4 recently bred into sugar beet hybrids provides a high level of resistance to Cercospora leaf spot caused by the fungal pathogen Cercospora beticola. The occurrence of pathogen strains that overcome BvCR4 was studied using field trials in Switzerland conducted under natural disease pressure. Virulence of a subset of these strains was evaluated in a field trial conducted under elevated artificial disease pressure. We created a new C. beticola reference genome and mapped whole genome sequences of 256 isolates collected in Switzerland and Germany. These were combined with virulence phenotypes to conduct three separate genome-wide association studies (GWAS) to identify candidate avirulence genes. We identified a locus associated with avirulence containing a putative avirulence effector gene named AvrCR4. All virulent isolates either lacked AvrCR4 or had nonsynonymous mutations within the gene. AvrCR4 was present in all 74 isolates from non-BvCR4 hybrids, whereas 33 of 89 isolates from BvCR4 hybrids carried a deletion. We also mapped genomic data from 190 publicly available US isolates to our new reference genome. The AvrCR4 deletion was found in only one of 95 unique isolates from non-BvCR4 hybrids in the United States. AvrCR4 presents a unique example of an avirulence effector in which virulent alleles have only recently emerged. Most likely these were selected out of standing genetic variation after deployment of BvCR4. Identification of AvrCR4 will enable real-time screening of C. beticola populations for the emergence and spread of virulent isolates.

Recent Advances in Effector Research of Magnaporthe oryzae

Recalcitrant rice blast disease is caused by Magnaporthe oryzae, which has a significant negative economic reverberation on crop productivity. In order to induce the disease onto the host, M. oryzae positively generates many types of small secreted proteins, here named as effectors, to manipulate the host cell for the purpose of stimulating pathogenic infection. In M. oryzae, by engaging with specific receptors on the cell surface, effectors activate signaling channels which control an array of cellular activities, such as proliferation, differentiation and apoptosis. The most recent research on effector identification, classification, function, secretion, and control mechanism has been compiled in this review. In addition, the article also discusses directions and challenges for future research into an effector in M. oryzae.