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1. TAL Effectors with Avirulence Activity in African Strains of Xanthomonas oryzae pv. oryzae

   https://doi.org/10.1186/s12284-022-00553-9  

   Lachaux, Marlène ; Thomas, Emilie ; Bogdanove, Adam J. ; Szurek, Boris ; Hutin, Mathilde ( February 2022 , Rice)

   Xanthomonas oryzaepv.oryzae(Xoo) causes bacterial leaf blight, a devastating disease of rice. Among the type-3 effectors secreted byXooto support pathogen virulence, the Transcription Activator-Like Effector (TALE) family plays a critical role. Some TALEs are major virulence factors that activate susceptibility (S) genes, overexpression of which contributes to disease development. Host incompatibility can result from TALE-induced expression of so-called executor (E) genes leading to a strong and rapid resistance response that blocks disease development. In that context, the TALE functions as an avirulence (Avr) factor. To date no such avirulence factors have been identified in African strains ofXoo.

   With respect to the importance of TALEs in the Rice-Xoopathosystem, we aimed at identifying those that may act as Avr factor within AfricanXoo. We screened 86 rice accessions, and identified 12 that were resistant to two African strains while being susceptible to a well-studied Asian strain. In a gain of function approach based on the introduction of each of the ninetalgenes of the avirulent African strain MAI1 into the virulent Asian strain PXO99^A, four were found to trigger resistance on specific rice accessions. Loss-of-function mutational analysis further demonstrated theavractivity of two of them,talDandtalI,on the rice varieties IR64 and CT13432 respectively. Further analysis of TalI demonstrated the requirement of its activation domain for triggering resistance in CT13432. Resistance in 9 of the 12 rice accessions that were resistant against AfricanXoospecifically, including CT13432, could be suppressed or largely suppressed by trans-expression of the truncTALEtal2h, similarly to resistance conferred by theXa1gene which recognizes TALEs generally independently of their activation domain.

   We identified and characterized TalD and TalI as two AfricanXooTALEs with avirulence activity on IR64 and CT13432 respectively. Resistance of CT13432 against AfricanXooresults from the combination of two mechanisms, one relying on the TalI-mediated induction of an unknown executor gene and the other on anXa1-like gene or allele.

    

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2. Magnaporthe oryzae nucleoside diphosphate kinase is required for metabolic homeostasis and redox‐mediated host innate immunity suppression

   https://doi.org/10.1111/mmi.14580  

   Rocha, Raquel O. ; Wilson, Richard A. ( September 2020 , Molecular Microbiology)

   The fungusMagnaporthe oryzaecauses blast, the most devastating disease of cultivated rice. After penetrating the leaf cuticle,M. oryzaegrows as a biotroph in intimate contact with living rice epidermal cells before necrotic lesions develop. Biotrophic growth requires maintaining metabolic homeostasis while suppressing plant defenses, but the metabolic connections and requirements involved are largely unknown. Here, we characterized theM. oryzaenucleoside diphosphate kinase‐encoding geneNDK1and discovered it was essential for facilitating biotrophic growth by suppressing the host oxidative burst—the first line of plant defense. NDK enzymes reversibly transfer phosphate groups from tri‐ to diphosphate nucleosides. Correspondingly, intracellular nucleotide pools were perturbed inM. oryzaestrains lackingNDK1through targeted gene deletion, compared to WT. This affected metabolic homeostasis: TCA, purine and pyrimidine intermediates, and oxidized NADP⁺, accumulated in Δndk1. cAMP and glutathione were depleted. ROS accumulated in Δndk1hyphae. Functional appressoria developed on rice leaf sheath surfaces, but Δndk1invasive hyphal growth was restricted and redox homeostasis was perturbed, resulting in unsuppressed host oxidative bursts that triggered immunity. We conclude Ndk1 modulates intracellular nucleotide pools to maintain redox balance via metabolic homeostasis, thus quenching the host oxidative burst and suppressing rice innate immunity during biotrophy.

    

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3. Biosynthesis and secretion of the microbial sulfated peptide RaxX and binding to the rice XA21 immune receptor

   https://doi.org/10.1073/pnas.1818275116  

   Luu, Dee Dee ; Joe, Anna ; Chen, Yan ; Parys, Katarzyna ; Bahar, Ofir ; Pruitt, Rory ; Chan, Leanne Jade G. ; Petzold, Christopher J. ; Long, Kelsey ; Adamchak, Clifford ; et al ( April 2019 , Proceedings of the National Academy of Sciences)

   The rice immune receptor XA21 is activated by the sulfated microbial peptide required for activation of XA21-mediated immunity X (RaxX) produced byXanthomonas oryzaepv.oryzae(Xoo). Mutational studies and targeted proteomics revealed that the RaxX precursor peptide (proRaxX) is processed and secreted by the protease/transporter RaxB, the function of which can be partially fulfilled by a noncognate peptidase-containing transporter component B (PctB). proRaxX is cleaved at a Gly–Gly motif, yielding a mature peptide that retains the necessary elements for RaxX function as an immunogen and host peptide hormone mimic. These results indicate that RaxX is a prokaryotic member of a previously unclassified and understudied group of eukaryotic tyrosine sulfated ribosomally synthesized, posttranslationally modified peptides (RiPPs). We further demonstrate that sulfated RaxX directly binds XA21 with high affinity. This work reveals a complete, previously uncharacterized biological process: bacterial RiPP biosynthesis, secretion, binding to a eukaryotic receptor, and triggering of a robust host immune response.

    

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4. Resistance and susceptibility QTL identified in a rice MAGIC population by screening with a minor‐effect virulence factor from Xanthomonas oryzae pv. oryzae

   https://doi.org/10.1111/pbi.13438  

   Huerta, Alejandra I. ; Delorean, Emily E. ; Bossa‐Castro, Ana M. ; Tonnessen, Bradley W. ; Raghavan, Chitra ; Corral, Rene ; Pérez‐Quintero, Álvaro L. ; Leung, Hei ; Verdier, Valérie ; Leach, Jan E. ( July 2020 , Plant Biotechnology Journal)

   Effective and durable disease resistance for bacterial blight (BB) of rice is a continuous challenge due to the evolution and adaptation of the pathogen,Xanthomonas oryzaepv.oryzae(Xoo), on cultivated rice varieties. Fundamental to this pathogens’ virulence is transcription activator‐like (TAL) effectors that activate transcription of host genes and contribute differently to pathogen virulence, fitness or both. Host plant resistance is predicted to be more durable if directed at strategic virulence factors that impact both pathogen virulence and fitness. We characterized Tal7b, a minor‐effect virulence factor that contributes incrementally to pathogen virulence in rice, is a fitness factor to the pathogen and is widely present in geographically diverse strains ofXoo. To identify sources of resistance to this conserved effector, we used a highly virulent strain carrying a plasmid borne copy of Tal7b to screen an indica multi‐parent advanced generation inter‐cross (MAGIC) population. Of 18 QTL revealed by genome‐wide association studies and interval mapping analysis, six were specific to Tal7b (qBB‐tal7b). Overall, 150 predicted Tal7b gene targets overlapped with qBB‐tal7bQTL. Of these, 21 showed polymorphisms in the predicted effector binding element (EBE) site and 23 lost the EBE sequence altogether. Inoculation and bioinformatics studies suggest that the Tal7b target in one of the Tal7b‐specific QTL, qBB‐tal7b‐8, is a disease susceptibility gene and that the resistance mechanism for this locus may be through loss of susceptibility. Our work demonstrates that minor‐effect virulence factors significantly contribute to disease and provide a potential new approach to identify effective disease resistance.

    

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5. A protein kinase coordinates cycles of autophagy and glutaminolysis in invasive hyphae of the fungus Magnaporthe oryzae within rice cells

   https://doi.org/10.1038/s41467-023-39880-w  

   Li, Gang ; Gong, Ziwen ; Dulal, Nawaraj ; Marroquin-Guzman, Margarita ; Rocha, Raquel O. ; Richter, Michael ; Wilson, Richard A. ( July 2023 , Nature Communications)

   The blast fungusMagnaporthe oryzaeproduces invasive hyphae in living rice cells during early infection, separated from the host cytoplasm by plant-derived interfacial membranes. However, the mechanisms underpinning this intracellular biotrophic growth phase are poorly understood. Here, we show that theM. oryzaeserine/threonine protein kinase Rim15 promotes biotrophic growth by coordinating cycles of autophagy and glutaminolysis in invasive hyphae. Alongside inducing autophagy, Rim15 phosphorylates NAD-dependent glutamate dehydrogenase, resulting in increased levels of α-ketoglutarate that reactivate target-of-rapamycin (TOR) kinase signaling, which inhibits autophagy. DeletingRIM15attenuates invasive hyphal growth and triggers plant immunity; exogenous addition of α-ketoglutarate prevents these effects, while glucose addition only suppresses host defenses. Our results indicate that Rim15-dependent cycles of autophagic flux liberate α-ketoglutarate – via glutaminolysis – to reactivate TOR signaling and fuel biotrophic growth while conserving glucose for antioxidation-mediated host innate immunity suppression.

    

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