Tal6b/AvrXa27A, a hidden TALE targeting the susceptibility gene OsSWEET11a and the resistance gene Xa27 in rice.

Xanthomonas oryzae pv. oryzae (Xoo) secretes transcription activator-like effectors (TALEs) to activate rice susceptibility (S) genes, causing bacterial blight (BB), as well as resistance (R) genes, leading to defense against BB. This activation follows a gene-for-gene paradigm that results in an arms race between the TALE of the pathogen and effector-binding elements (EBEs) in the promoters of host genes. In this study, we characterized a novel TALE, designated Tal6b/AvrXa27A, that activates the rice S gene OsSWEET11a and the rice R gene Xa27. Tal6b/AvrXa27A is a member of the AvrXa27/TalAO class and contains 16 repeat variable diresidues (RVDs); one RVD is altered and one is deleted in Tal6b/AvrXa27A compared with AvrXa27, a known avirulence (avr) effector of Xa27. Tal6b/AvrXa27A can transcriptionally activate the expression of Xa27 and OsSWEET11a via EBEs in their corresponding promoters, leading to effector-triggered immunity and susceptibility, respectively. The 16 RVDs in Tal6b/AvrXa27A have no obvious similarity to the 24 RVDs in the effector PthXo1, but EBETal6b and EBEPthXo1 are overlapped in the OsSWEET11a promoter. Tal6b/AvrXa27A is prevalent among Asian Xoo isolates, but PthXo1 has only been reported in the Philippine strain PXO99A. Genome editing of EBETal6b in the OsSWEET11a promoter further confirmed the requirement for OsSWEET11a expression in Tal6b/AvrXa27A-dependent susceptibility to Xoo. Moreover, Tal6b/AvrXa27A resulted in higher transcription of Xa27 than of OsSWEET11a, which led to a strong, rapid resistance response that blocked disease development. These findings suggest that Tal6b/AvrXa27A has a dual function: triggering resistance by activating Xa27 gene expression as an avirulence factor and inducing transcription of the S gene OsSWEET11a, resulting in virulence. Intriguingly, Tal6b/AvrXa27A, but not AvrXa27, can bind to the promoter of OsSWEET11a. The underlying recognition mechanism for this binding remains unclear but appears to deviate from the currently accepted TALE code.

A New Biocontrol Agent Bacillus velezensis SF334 against Rubber Tree Fungal Leaf Anthracnose and Its Genome Analysis of Versatile Plant Probiotic Traits.

Natural rubber is an important national strategic and industrial raw material. The leaf anthracnose of rubber trees caused by the Colletotrichum species is one of the important factors restricting the yields of natural rubber. In this study, we isolated and identified strain Bacillus velezensis SF334, which exhibited significant antagonistic activity against both C. australisinense and C. siamense, the dominant species of Colletotrichum causing rubber tree leaf anthracnose in the Hainan province of China, from a pool of 223 bacterial strains. The cell suspensions of SF334 had a significant prevention effect for the leaf anthracnose of rubber trees, with an efficacy of 79.67% against C. siamense and 71.8% against C. australisinense. We demonstrated that SF334 can lead to the lysis of C. australisinense and C. siamense mycelia by causing mycelial expansion, resulting in mycelial rupture and subsequent death. B. velezensis SF334 also harbors some plant probiotic traits, such as secreting siderophore, protease, cellulase, pectinase, and the auxin of indole-3-acetic acid (IAA), and it has broad-spectrum antifungal activity against some important plant pathogenic fungi. The genome combined with comparative genomic analyses indicated that SF334 possesses most genes of the central metabolic and gene clusters of secondary metabolites in B. velezensis strains. To our knowledge, this is the first time a Bacillus velezensis strain has been reported as a promising biocontrol agent against the leaf anthracnose of rubber trees caused by C. siamense and C. australisinense. The results suggest that B. velezensis could be a potential candidate agent for the leaf anthracnose of rubber trees.

Antagonistic transcriptome profile reveals potential mechanisms of action on Xanthomonas oryzae pv. oryzicola by the cell-free supernatants of Bacillus velezensis 504, a versatile plant probiotic bacterium.

Bacterial leaf streak (BLS) of rice is a severe disease caused by the bacterial pathogen Xanthomonas oryzae pv. oryzicola (Xoc) that has gradually become the fourth major disease on rice in some rice-growing regions in southern China. Previously, we isolated a Bacillus velezensis strain 504 that exhibited apparent antagonistic activity against the Xoc wild-type strain RS105, and found that B. velezensis 504 was a potential biocontrol agent for BLS. However, the underlying mechanisms of antagonism and biocontrol are not completely understood. Here we mine the genomic data of B. velezensis 504, and the comparative transcriptomic data of Xoc RS105 treated by the cell-free supernatants (CFSs) of B. velezensis 504 to define differentially expressed genes (DEGs). We show that B. velezensis 504 shares over 89% conserved genes with FZB42 and SQR9, two representative model strains of B. velezensis, but 504 is more closely related to FZB42 than SQR9, as well as B. velezensis 504 possesses the secondary metabolite gene clusters encoding the essential anti-Xoc agents difficidin and bacilysin. We conclude that approximately 77% of Xoc RS105 coding sequences are differentially expressed by the CFSs of B. velezensis 504, which significantly downregulates genes involved in signal transduction, oxidative phosphorylation, transmembrane transport, cell motility, cell division, DNA translation, and five physiological metabolisms, as well as depresses an additional set of virulence-associated genes encoding the type III secretion, type II secretion system, type VI secretion system, type IV pilus, lipopolysaccharides and exopolysaccharides. We also show that B. velezensis 504 is a potential biocontrol agent for bacterial blight of rice exhibiting relative control efficiencies over 70% on two susceptible cultivars, and can efficiently antagonize against some important plant pathogenic fungi including Colletotrichum siamense and C. australisinense that are thought to be the two dominant pathogenic species causing leaf anthracnose of rubber tree in Hainan province of China. B. velezensis 504 also harbors some characteristics of plant growth-promoting rhizobacterium such as secreting protease and siderophore, and stimulating plant growth. This study reveals the potential biocontrol mechanisms of B. velezensis against BLS, and also suggests that B. velezensis 504 is a versatile plant probiotic bacterium.

The MinCDE Cell Division System Participates in the Regulation of Type III Secretion System (T3SS) Genes, Bacterial Virulence, and Motility in Xanthomonas oryzae pv. oryzae.

Xanthomonas oryzae pv. oryzae (Xoo) causes bacterial leaf blight (BLB) in rice, which is one of the most severe bacterial diseases in rice in some Asian countries. The type III secretion system (T3SS) of Xoo encoded by the hypersensitive response and pathogenicity (hrp) genes is essential for its pathogenicity in host rice. Here, we identified the Min system (MinC, MinD, and MinE), a negative regulatory system for bacterial cell division encoded by minC, minD, and minE genes, which is involved in negative regulation of hrp genes (hrpB1 and hrpF) in Xoo. We found that the deletion of minC, minD, and minCDE resulted in enhanced hrpB1 and hrpF expression, which is dependent on two key hrp regulators HrpG and HrpX. The minC, minD, and minCDE mutants exhibited elongated cell lengths, and the classic Min system-defective cell morphology including minicells and short filamentations. Mutation of minC in Xoo resulted in significantly impaired virulence in host rice, swimming motility, and enhanced biofilm formation. Our transcriptome profiling also indicated some virulence genes were differentially expressed in the minC mutants. To our knowledge, this is the first report about the Min system participating in the regulation of T3SS expression. It sheds light on the understanding of Xoo virulence mechanisms.