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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. OsSWEET11b , a potential sixth leaf blight susceptibility gene involved in sugar transport‐dependent male fertility

   https://doi.org/10.1111/nph.18054  

   Wu, Lin‐Bo ; Eom, Joon‐Seob ; Isoda, Reika ; Li, Chenhao ; Char, Si Nian ; Luo, Dangping ; Schepler‐Luu, Van ; Nakamura, Masayoshi ; Yang, Bing ; Frommer, Wolf B. ( March 2022 , New Phytologist)

   SWEETs play important roles in intercellular sugar transport. Induction of SWEET sugar transporters by Transcription Activator‐Like effectors (TALe) ofXanthomonasssp. is key for virulence in rice, cassava and cotton.

   We identified OsSWEET11b with roles in male fertility and potential bacterial blight (BB) susceptibility in rice. While singleossweet11aor11bmutants were fertile, double mutants were sterile. As clade III SWEETs can transport gibberellin (GA), a key hormone for spikelet fertility, sterility and BB susceptibility might be explained by GA transport deficiencies. However, in contrast with the Arabidopsis homologues, OsSWEET11b did not mediate detectable GA transport. Fertility and susceptibility therefore are likely to depend on sucrose transport activity.

   Ectopic induction ofOsSWEET11bby designer TALe enabled TALe‐freeXanthomonas oryzaepv.oryzae(Xoo) to cause disease, identifyingOsSWEET11bas a potential BB susceptibility gene and demonstrating that the induction of host sucrose uniporter activity is key to virulence ofXoo. Notably, only three of six clade III SWEETs are targeted by knownXoostrains from Asia and Africa.

   The identification of OsSWEET11b is relevant for fertility and for protecting rice against emergingXoostrains that targetOsSWEET11b.

    

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3. High‐efficiency prime editing enables new strategies for broad‐spectrum resistance to bacterial blight of rice

   https://doi.org/10.1111/pbi.14049  

   Gupta, Ajay ; Liu, Bo ; Chen, Qi‐Jun ; Yang, Bing ( May 2023 , Plant Biotechnology Journal)

   Using genetic resistance against bacterial blight (BB) caused byXanthomonas oryzaepathovaroryzae(Xoo) is a major objective in rice breeding programmes. Prime editing (PE) has the potential to create novel germplasm againstXoo. Here, we use an improved prime‐editing system to implement two new strategies for BB resistance. Knock‐in of TAL effector binding elements (EBE) derived from the BB susceptible geneSWEET14into the promoter of a dysfunctional executorRgenexa23reaches 47.2% with desired edits including biallelic editing at 18% in T₀generation that enables an inducible TALE‐dependent BB resistance. Editing the transcription factor TFIIA geneTFIIAγ5required for TAL effector‐dependent BB susceptibility recapitulates the resistance ofxa5at an editing efficiency of 88.5% with biallelic editing rate of 30% in T₀generation. The engineered loci provided resistance against multipleXoostrains in T₁generation. Whole‐genome sequencing detected noOsMLH1dn‐associated random mutations and no off‐target editing demonstrating high specificity of this PE system. This is the first‐ever report to use PE system to engineer resistance against biotic stress and to demonstrate knock‐in of 30‐nucleotides cis‐regulatory element at high efficiency. The new strategies hold promises to fend rice off the evolvingXoostrains and protect it from epidemics.

    

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4. New insights into bsr‐d1 ‐mediated broad‐spectrum resistance to rice blast

   https://doi.org/10.1111/mpp.12941  

   Zhu, Ziwei ; Yin, Junjie ; Chern, Mawsheng ; Zhu, Xiaobo ; Yang, Chao ; He, Kaiwei ; Liu, Yuchen ; He, Min ; Wang, Jing ; Song, Li ; et al ( May 2020 , Molecular Plant Pathology)

   bsr‐d1, an allele encoding a transcription factor identified from the rice cultivar Digu, confers durable, broad‐spectrum resistance to infections by strains ofMagnaporthe oryzae.bsr‐d1was predicted to inhibitM. oryzae‐induced expression ofBsr‐d1RNA and degradation of hydrogen peroxide to achieve resistance toM. oryzae. However, the global effect of biological process and molecular function on blast resistance mediated byBsr‐d1remains unknown. In this study, we compared transcriptomic profiling betweenBsr‐d1knockout (Bsr‐d1KO) lines and the wild type, TP309. Our study revealed thatbsr‐d1mainly regulates the redox state of plant cells, but also affects amino acid and unsaturated fatty acid metabolism. We further found that BSR‐D1 indirectly regulates salicylic acid biosynthesis, metabolism, and signal transduction downstream of the activation of H₂O₂signalling in thebsr‐d1‐mediated immune response. Furthermore, we identified a novel peroxidase‐encoding gene,Perox3, as a new BSR‐D1 target gene that reduces resistance toM. oryzaewhen overexpressed in TP309. These results provide new insights into thebsr‐d1‐mediated blast resistance.

    

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5. The Barley HvWRKY6 Transcription Factor Is Required for Resistance Against Pyrenophora teres f. teres

   https://doi.org/10.3389/fgene.2020.601500  

   Tamang, Prabin ; Richards, Jonathan K. ; Solanki, Shyam ; Ameen, Gazala ; Sharma Poudel, Roshan ; Deka, Priyanka ; Effertz, Karl ; Clare, Shaun J. ; Hegstad, Justin ; Bezbaruah, Achintya ; et al ( January 2021 , Frontiers in Genetics)

   Barley is an important cereal crop worldwide because of its use in the brewing and distilling industry. However, adequate supplies of quality malting barley are threatened by global climate change due to drought in some regions and excess precipitation in others, which facilitates epidemics caused by fungal pathogens. The disease net form net blotch caused by the necrotrophic fungal pathogen Pyrenophora teres f. teres ( Ptt ) has emerged as a global threat to barley production and diverse populations of Ptt have shown a capacity to overcome deployed genetic resistances. The barley line CI5791 exhibits remarkably effective resistance to diverse Ptt isolates from around the world that maps to two major QTL on chromosomes 3H and 6H. To identify genes involved in this effective resistance, CI5791 seed were γ-irradiated and two mutants, designated CI5791-γ3 and CI5791-γ8, with compromised Ptt resistance were identified from an M 2 population. Phenotyping of CI5791-γ3 and -γ8 × Heartland F 2 populations showed three resistant to one susceptible segregation ratios and CI5791-γ3 × -γ8 F 1 individuals were susceptible, thus these independent mutants are in a single allelic gene. Thirty-four homozygous mutant (susceptible) CI5791-γ3 × Heartland F 2 individuals, representing 68 recombinant gametes, were genotyped via PCR genotype by sequencing. The data were used for single marker regression mapping placing the mutation on chromosome 3H within an approximate 75 cM interval encompassing the 3H CI5791 resistance QTL. Sequencing of the mutants and wild-type (WT) CI5791 genomic DNA following exome capture identified independent mutations of the HvWRKY6 transcription factor located on chromosome 3H at ∼50.7 cM, within the genetically delimited region. Post transcriptional gene silencing of HvWRKY6 in barley line CI5791 resulted in Ptt susceptibility, confirming that it functions in NFNB resistance, validating it as the gene underlying the mutant phenotypes. Allele analysis and transcript regulation of HvWRKY6 from resistant and susceptible lines revealed sequence identity and upregulation upon pathogen challenge in all genotypes analyzed, suggesting a conserved transcription factor is involved in the defense against the necrotrophic pathogen. We hypothesize that HvWRKY6 functions as a conserved signaling component of defense mechanisms that restricts Ptt growth in barley. 

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