Identification of Novel Alleles of the Rice Blast-Resistance Gene Pi9 through Sequence-Based Allele Mining

Genetic engineering, including genome editing, for enhancing broad-spectrum disease resistance in crops

Plant diseases, caused by a wide range of pathogens, severely reduce crop yield and quality, posing a significant threat to global food security. Developing broad-spectrum resistance (BSR) in crops is a key strategy for controlling crop diseases and ensuring sustainable crop production. Cloning disease-resistance (R) genes and understanding their underlying molecular mechanisms provide new genetic resources and strategies for crop breeding. Novel genetic engineering and genome editing tools have accelerated the study and engineering of BSR genes in crops, which is the primary focus of this review. We first summarize recent advances in understanding the plant immune system, followed by an examination of the molecular mechanisms underlying BSR in crops. Finally, we highlight diverse strategies employed to achieve BSR, including gene stacking to combine multiple R genes, multiplexed genome editing of susceptibility genes and promoter regions of executor R genes, editing cis-regulatory elements to fine-tune gene expression, RNA interference, saturation mutagenesis, and precise genomic insertions. The genetic studies and engineering of BSR are accelerating the breeding of disease-resistant cultivars, contributing to crop improvement and enhancing global food security.

Allelic variation in rice blast resistance: a pathway to sustainable disease management

Rice blast is a major problem in agriculture, affecting rice production and threatening food security worldwide. This disease, caused by the fungus Magnaporthe oryzae, has led to a lot of research since the discovery of the first resistance gene, pib, in 1999. Researchers have now identified more than 50 resistance genes on eight of the twelve chromosomes in rice, each targeting different strains of the pathogen.These genes are spread out across seventeen different loci. These genes, which primarily code for nucleotide-binding and leucine-rich repeat proteins, play an important part in the defense of rice against the pathogen, either alone or in combination with other genes. An important characteristic of these genes is the allelic or paralogous interactions that exist within these loci. These relationships contribute to the gene's increased capacity for evolutionary adaptation. The ability of resistance proteins to recognize and react to novel effectors is improved by the frequent occurrence of variations within the domains that are responsible for recognizing pathogen effectors. The purpose of this review is to summarize the progress that has been made in identifying these essential genes and to investigate the possibility of utilizing the allelic variants obtained from these genes in future rice breeding efforts to increase resistance to rice blast.

Detection and Evaluation of Blast Resistance Genes in Backbone Indica Rice Varieties from South China

Rice blast caused by the pathogenic fungus Magnaporthe oryzae poses a significant threat to rice cultivation. The identification of robust resistance germplasm is crucial for breeding resistant varieties. In this study, we employed functional molecular markers for 10 rice blast resistance genes, namely Pi1, Pi2, Pi5, Pi9, Pia, Pid2, Pid3, Pigm, Pikh, and Pita, to assess blast resistance across 91 indica rice backbone varieties in South China. The results showed a spectrum of resistance levels ranging from highly resistant (HR) to highly susceptible (HS), with corresponding frequencies of 0, 19, 40, 27, 5, and 0, respectively. Yearly correlations in blast resistance genes among the 91 key indica rice progenitors revealed Pid2 (60.44%), Pia (50.55%), Pita (45.05%), Pi2 (32.97%), Pikh (4.4%), Pigm (2.2%), Pi9 (2.2%), and Pi1 (1.1%). Significant variations were observed in the distribution frequencies of these 10 resistance genes among these progenitors across different provinces. Furthermore, as the number of aggregated resistance genes increased, parental resistance levels correspondingly improved, though the efficacy of different gene combinations varied significantly. This study provides the initial steps toward strategically distributing varieties of resistant indica rice genotypes across South China.

Understanding the Dynamics of Blast Resistance in Rice-Magnaporthe oryzae Interactions

Rice is a global food grain crop for more than one-third of the human population and a source for food and nutritional security. Rice production is subjected to various stresses; blast disease caused by Magnaporthe oryzae is one of the major biotic stresses that has the potential to destroy total crop under severe conditions. In the present review, we discuss the importance of rice and blast disease in the present and future global context, genomics and molecular biology of blast pathogen and rice, and the molecular interplay between rice–M. oryzae interaction governed by different gene interaction models. We also elaborated in detail on M. oryzae effector and Avr genes, and the role of noncoding RNAs in disease development. Further, rice blast resistance QTLs; resistance (R) genes; and alleles identified, cloned, and characterized are discussed. We also discuss the utilization of QTLs and R genes for blast resistance through conventional breeding and transgenic approaches. Finally, we review the demonstrated examples and potential applications of the latest genome-editing tools in understanding and managing blast disease in rice.

The Pid Family Has Been Diverged into Xian and Geng Type Resistance Genes against Rice Blast Disease

Rice blast (the causative agent the fungus Magnaporthe oryzae) represents a major constraint on the productivity of one of the world’s most important staple food crops. Genes encoding resistance have been identified in both the Xian and Geng subspecies genepools, and combining these within new cultivars represents a rational means of combating the pathogen. In this research, deeper allele mining was carried out on Pid2, Pid3, and Pid4 via each comprehensive FNP marker set in three panels consisting of 70 Xian and 58 Geng cultivars. Within Pid2, three functional and one non-functional alleles were identified; the former were only identified in Xian type entries. At Pid3, four functional and one non-functional alleles were identified; once again, all of the former were present in Xian type entries. However, the pattern of variation at Pid4 was rather different: here, the five functional alleles uncovered were dispersed across the Geng type germplasm. Among all the twelve candidate functional alleles, both Pid2-ZS and Pid3-ZS were predominant. Furthermore, the resistance functions of both Pid2-ZS and Pid3-ZS were assured by transformation test. Profiting from the merits of three comprehensive FNP marker sets, the study has validated all three members of the Pid family as having been strictly diverged into Xian and Geng subspecies: Pid2 and Pid3 were defined as Xian type resistance genes, and Pid4 as Geng type. Rather limited genotypes of the Pid family have been effective in both Xian and Geng rice groups, of which Pid2-ZS_Pid3-ZS has been central to the Chinese rice population.

Development of a Temperate Climate-Adapted indica Multi-stress Tolerant Rice Variety by Pyramiding Quantitative Trait Loci

Successful cultivation of rice (Oryza sativa L.) in many Asian countries requires submergence stress tolerance at the germination and early establishment stages. Two quantitative trait loci, Sub1 (conferring submergence tolerance) and AG1 (conferring anaerobic germination), were recently pyramided into a single genetic background, without compromising any desirable agronomic traits, leading to the development of Ciherang-Sub1 + AG1 (CSA). However, little research has been conducted to enhance plant tolerance to abiotic stress (submergence) and biotic stress (rice blast), which occur in a damp climate following flooding. The BC₂F₅ breeding line was phenotypically characterized using the AvrPi9 isolate. The biotic and abiotic stress tolerance of selected lines was tested under submergence stress and anaerobic germination conditions, and lines tolerant to each stress condition were identified through phenotypic and gene expression analyses. The Ciherang-Sub1 + AG1 + Pi9 (CSA-Pi9) line showed similar agronomic performance to its recurrent parent, CSA, but had significantly reduced chalkiness in field trials conducted in temperate regions. Unexpectedly, the CSA-Pi9 line also showed salinity tolerance. Thus, the breeding line newly developed in this study, CSA-Pi9, functioned under stress conditions, in which Sub1, AG1, and Pi9 play a role and had superior grain quality traits compared to its recurrent parent in temperate regions. We speculate that CSA-Pi9 will enable the establishment of climate-resilient rice cropping systems, particularly in East Asia.

Recent Progress in Rice Broad-Spectrum Disease Resistance

Rice is one of the most important food crops in the world. However, stable rice production is constrained by various diseases, in particular rice blast, sheath blight, bacterial blight, and virus diseases. Breeding and cultivation of resistant rice varieties is the most effective method to control the infection of pathogens. Exploitation and utilization of the genetic determinants of broad-spectrum resistance represent a desired way to improve the resistance of susceptible rice varieties. Recently, researchers have focused on the identification of rice broad-spectrum disease resistance genes, which include R genes, defense-regulator genes, and quantitative trait loci (QTL) against two or more pathogen species or many isolates of the same pathogen species. The cloning of broad-spectrum disease resistance genes and understanding their underlying mechanisms not only provide new genetic resources for breeding broad-spectrum rice varieties, but also promote the development of new disease resistance breeding strategies, such as editing susceptibility and executor R genes. In this review, the most recent advances in the identification of broad-spectrum disease resistance genes in rice and their application in crop improvement through biotechnology approaches during the past 10 years are summarized.

Integrated Strategies for Durable Rice Blast Resistance in Sub-Saharan Africa

Rice is a key food security crop in Africa. The importance of rice has led to increasing country-specific, regional, and multinational efforts to develop germplasm and policy initiatives to boost production for a more food-secure continent. Currently, this critically important cereal crop is predominantly cultivated by small-scale farmers under suboptimal conditions in most parts of sub-Saharan Africa (SSA). Rice blast disease, caused by the fungus Magnaporthe oryzae, represents one of the major biotic constraints to rice production under small-scale farming systems of Africa, and developing durable disease resistance is therefore of critical importance. In this review, we provide an overview of the major advances by a multinational collaborative research effort to enhance sustainable rice production across SSA and how it is affected by advances in regional policy. As part of the multinational effort, we highlight the importance of joint international partnerships in tackling multiple crop production constraints through integrated research and outreach programs. More specifically, we highlight recent progress in establishing international networks for rice blast disease surveillance, farmer engagement, monitoring pathogen virulence spectra, and the establishment of regionally based blast resistance breeding programs. To develop blast-resistant, high yielding rice varieties for Africa, we have established a breeding pipeline that utilizes real-time data of pathogen diversity and virulence spectra, to identify major and minor blast resistance genes for introgression into locally adapted rice cultivars. In addition, the project has developed a package to support sustainable rice production through regular stakeholder engagement, training of agricultural extension officers, and establishment of plant clinics.