Research Progress on Cloning and Function of Xa Genes Against Rice Bacterial Blight

Revisiting growth-defence trade-offs and breeding strategies in crops

Plants have evolved a multi-layered immune system to fight off pathogens. However, immune activation is costly and is often associated with growth and development penalty. In crops, yield is the main breeding target and is usually affected by high disease resistance. Therefore, proper balance between growth and defence is critical for achieving efficient crop improvement. This review highlights recent advances in attempts designed to alleviate the trade-offs between growth and disease resistance in crops mediated by resistance (R) genes, susceptibility (S) genes and pleiotropic genes. We also provide an update on strategies for optimizing the growth-defence trade-offs to breed future crops with desirable disease resistance and high yield.

A Community-Curated DokuWiki Resource on Diagnostics, Diversity, Pathogenicity, and Genetic Control of Xanthomonads

Xanthomonads, including Xanthomonas and Xylella species, constitute a large and significant group of economically and ecologically important plant pathogens. Up-to-date knowledge of these pathogens and their hosts is essential for the development of suitable control measures. Traditional review articles or book chapters have inherent limitations, including static content and rapid obsolescence. To address these challenges, we have developed a Web-based knowledge platform dedicated to xanthomonads, inspired by the concept of living systematic reviews. This platform offers a dynamic resource that encompasses bacterial virulence factors, plant resistance genes, and tools for diagnostics and genetic diversity studies. Our goal is to facilitate access for newcomers to the field, provide continuing education opportunities for students, assist plant protection services with diagnostics, provide valuable information to breeders on sources of resistance and breeding targets, and offer comprehensive expert knowledge to other stakeholders interested in plant-pathogenic xanthomonads. This resource is available for queries and updates at https://euroxanth.ipn.pt. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Establishment of Functional PCR-Based Markers against Bacterial Leaf Blight Disease in Rice Landraces of Yunnan Province of China

Bacterial leaf blight is a devastating disease of rice worldwide. The resistant genes are routinely transferred from landraces to cultivated varieties through backcross breeding along with marker-assisted selection. In the present study, we use the gene-specific markers to screen the rice landraces in Yunnan Province of China. We collected 404 representative samples of 24 different rice landraces from Yunnan Province of China. The initial PCR-based screening suggested that the leaf blight resistance was not evenly distributed in Yunnan Province. Our results indicate that there is a complete loss of resistance for landraces based on xa5 and xa13 genes. On the other hand, landraces harboring Xa7 and Xa21 showed a high level of resistance. Using gene-specific PCR-based data, we were able to identify the resistant, susceptible and heterozygous populations across Yunnan Province. The widely used Xa21 gene alone showed a remarkable level of resistance throughout the province, indicating its potential to develop broad-spectrum resistance in rice germplasm. The key aspects of bacterial blight spread according to local sites in Yunnan Province and the resistance conferred by different landraces due to the presence of different resistance genes are discussed.

Integrated Molecular and Bioinformatics Approaches for Disease-Related Genes in Plants

Modern plant pathology relies on bioinformatics approaches to create novel plant disease diagnostic tools. In recent years, a significant amount of biological data has been generated due to rapid developments in genomics and molecular biology techniques. The progress in the sequencing of agriculturally important crops has made it possible to develop a better understanding of plant-pathogen interactions and plant resistance. The availability of host-pathogen genome data offers effective assistance in retrieving, annotating, analyzing, and identifying the functional aspects for characterization at the gene and genome levels. Physical mapping facilitates the identification and isolation of several candidate resistance (R) genes from diverse plant species. A large number of genetic variations, such as disease-causing mutations in the genome, have been identified and characterized using bioinformatics tools, and these desirable mutations were exploited to develop disease resistance. Moreover, crop genome editing tools, namely the CRISPR (clustered regulatory interspaced short palindromic repeats)/Cas9 (CRISPR-associated) system, offer novel and efficient strategies for developing durable resistance. This review paper describes some aspects concerning the databases, tools, and techniques used to characterize resistance (R) genes for plant disease management.

Genome-Wide Association Study Identifies Resistance Loci for Bacterial Blight in a Collection of Asian Temperate Japonica Rice Germplasm

Growing resistant rice cultivars is the most effective strategy to control bacterial blight (BB), a devastating disease caused by Xanthomonas oryzae pv. oryzae (Xoo). Screening resistant germplasm and identifying resistance (R) genes are prerequisites for breeding resistant rice cultivars. We conducted a genome-wide association study (GWAS) to detect quantitative trait loci (QTL) associated with BB resistance using 359 East Asian temperate Japonica accessions inoculated with two Chinese Xoo strains (KS6-6 and GV) and one Philippine Xoo strain (PXO99^A). Based on the 55K SNPs Array dataset of the 359 Japonica accessions, eight QTL were identified on rice chromosomes 1, 2, 4, 10, and 11. Four of the QTL coincided with previously reported QTL, and four were novel loci. Six R genes were localized in the qBBV-11.1, qBBV-11.2, and qBBV-11.3 loci on chromosome 11 in this Japonica collection. Haplotype analysis revealed candidate genes associated with BB resistance in each QTL. Notably, LOC_Os11g47290 in qBBV-11.3, encoding a leucine-rich repeat receptor-like kinase, was a candidate gene associated with resistance to the virulent strain GV. Knockout mutants of Nipponbare with the susceptible haplotype of LOC_Os11g47290 exhibited significantly improved BB resistance. These results will be useful for cloning BB resistance genes and breeding resistant rice cultivars.

Complete Genomic Sequence of Xanthomonas oryzae pv. oryzae Strain, LA20, for Studying Resurgence of Rice Bacterial Blight in the Yangtze River Region, China

Xanthomonas oryzae pv. oryzae (Xoo) is a causative agent of rice bacterial blight (BB). In 2020-2022, BB re-emerged, and there was a break out in the Yangtze River area, China. The pandemic Xoo strain, LA20, was isolated and identified from cultivar Quanyou1606 and demonstrated to be the Chinese R9 Xoo strain, which is able to override the widely adopted xa5-, Xa7- and xa13-mediated resistance in rice varieties in Yangtze River. Here, we report the complete genome of LA20 by PacBio and Illumina sequencing. The assembled genome consists of one circular chromosome of 4,960,087 bp, sharing 99.65% sequence identity with the traditional representative strain, YC11 (R5), in the Yangtze River. Comparative genome analysis of LA20 and YC11 revealed the obvious variability in Tal genes (the uppermost virulence determinants) in numbers and sequences. Particularly, six Tal genes were only found in LA20, but not in YC11, among which Tal1b (pthXo1)/Tal4 (pthXo6), along with the lost one, pthXo3 (avrXa7), might be the major factors for LA20 to overcome xa5-, Xa7- and xa13-mediated resistance, thus, leading to the resurgence of BB. This complete genome of the new pandemic Xoo strain will provide novel insights into pathogen evolution, the traits of pathogenicity on genomic level and the epidemic disease status in China.

Dynamic UAV Phenotyping for Rice Disease Resistance Analysis Based on Multisource Data

Bacterial blight poses a threat to rice production and food security, which can be controlled through large-scale breeding efforts toward resistant cultivars. Unmanned aerial vehicle (UAV) remote sensing provides an alternative means for the infield phenotype evaluation of crop disease resistance to relatively time-consuming and laborious traditional methods. However, the quality of data acquired by UAV can be affected by several factors such as weather, crop growth period, and geographical location, which can limit their utility for the detection of crop disease and resistant phenotypes. Therefore, a more effective use of UAV data for crop disease phenotype analysis is required. In this paper, we used time series UAV remote sensing data together with accumulated temperature data to train the rice bacterial blight severity evaluation model. The best results obtained with the predictive model showed an R _p ² of 0.86 with an RMSE_p of 0.65. Moreover, model updating strategy was used to explore the scalability of the established model in different geographical locations. Twenty percent of transferred data for model training was useful for the evaluation of disease severity over different sites. In addition, the method for phenotypic analysis of rice disease we built here was combined with quantitative trait loci (QTL) analysis to identify resistance QTL in genetic populations at different growth stages. Three new QTLs were identified, and QTLs identified at different growth stages were inconsistent. QTL analysis combined with UAV high-throughput phenotyping provides new ideas for accelerating disease resistance breeding.

Evaluation of rice bacterial blight severity from lab to field with hyperspectral imaging technique

Hyperspectral imaging technique combined with machine learning is a powerful tool for the evaluation of disease phenotype in rice disease-resistant breeding. However, the current studies are almost carried out in the lab environment, which is difficult to apply to the field environment. In this paper, we used visible/near-infrared hyperspectral images to analysis the severity of rice bacterial blight (BB) and proposed a novel disease index construction strategy (NDSCI) for field application. A designed long short-term memory network with attention mechanism could evaluate the BB severity robustly, and the attention block could filter important wavelengths. Best results were obtained based on the fusion of important wavelengths and color features with an accuracy of 0.94. Then, NSDCI was constructed based on the important wavelength and color feature related to BB severity. The correlation coefficient of NDSCI extended to the field data reached -0.84, showing good scalability. This work overcomes the limitations of environmental conditions and sheds new light on the rapid measurement of phenotype in disease-resistant breeding.

OsASR6 Alleviates Rice Resistance to Xanthomonas oryzae via Transcriptional Suppression of OsCIPK15

The plant-specific ASR (abscisic acid, stress and ripening) transcription factors are pivotal regulators of plant responses to abiotic stresses. However, their functions in plant disease resistance remain largely unknown. In this study, we revealed the role of OsASR6 in rice plants’ resistance to two important bacterial diseases caused by Xanthomonas oryzae pv. oryzae (Xoo) and X. oryzae pv. oryzicola (Xoc) and elucidated the mechanisms underlying OsASR6-regulated resistance. The expression of OsASR6 was strongly elevated in response to both Xoo and Xoc challenges. Silencing of OsASR6 in OsASR6-RNAi transgenic plants markedly enhanced rice resistance to the two bacterial pathogens. Moreover, comparative transcriptome analyses for OsASR6-RNAi and wild-type plants inoculated and uninoculated with Xoc demonstrated that OsASR6 suppressed rice resistance to Xoc by comprehensively fine-tuning CIPK15- and WRKY45-1-mediated immunity, SA signaling and redox homeostasis. Further luciferase reporter assays confirmed that OsASR6 negatively regulated CIPK15 but not WRKY45-1 expression in planta. Overexpression of OsCIPK15 strongly enhanced rice resistance to Xoo and Xoc. Collectively, these results reveal that OsASR6 alleviates rice resistance through the transcriptional suppression of OsCIPK15, and thus links calcium signaling to rice resistance against X. oryzae. Our findings provide insight into the mechanisms underlying OsASR6-mediated regulation of rice resistance to X. oryzae.