Advancement in the Breeding, Biotechnological and Genomic Tools towards Development of Durable Genetic Resistance against the Rice Blast Disease

Abstract

Rice production needs to be sustained in the coming decades, as the changeable climatic conditions are becoming more conducive to disease outbreaks. The majority of rice diseases cause enormous economic damage and yield instability. Among them, rice blast caused by Magnaportheoryzae is a serious fungal disease and is considered one of the major threats to world rice production. This pathogen can infect the above-ground tissues of rice plants at any growth stage and causes complete crop failure under favorable conditions. Therefore, management of blast disease is essentially required to sustain global food production. When looking at the drawback of chemical management strategy, the development of durable, resistant varieties is one of the most sustainable, economic, and environment-friendly approaches to counter the outbreaks of rice blasts. Interestingly, several blast-resistant rice cultivars have been developed with the help of breeding and biotechnological methods. In addition, 146 R genes have been identified, and 37 among them have been molecularly characterized to date. Further, more than 500 loci have been identified for blast resistance which enhances the resources for developing blast resistance through marker-assisted selection (MAS), marker-assisted backcross breeding (MABB), and genome editing tools. Apart from these, a better understanding of rice blast pathogens, the infection process of the pathogen, and the genetics of the immune response of the host plant are very important for the effective management of the blast disease. Further, high throughput phenotyping and disease screening protocols have played significant roles in easy comprehension of the mechanism of disease spread. The present review critically emphasizes the pathogenesis, pathogenomics, screening techniques, traditional and molecular breeding approaches, and transgenic and genome editing tools to develop a broad spectrum and durable resistance against blast disease in rice. The updated and comprehensive information presented in this review would be definitely helpful for the researchers, breeders, and students in the planning and execution of a resistance breeding program in rice against this pathogen.

Keywords: Magnaporthe oryzae; R-genes; blast disease; broad-spectrum resistance; conventional breeding; genome editing; genomic tools; molecular breeding; rice.

Figure 1

Pathogenesis of the rice blast fungus Magnaporthe oryzae. (a). Infection of the rice blast fungus starts when a three-celled conidium lands on the rice leaf surface. (b). Spore tip mucilage (STM) present on conidium helps the spore to attach to hydrophobic cuticle of rice plant. (c). Conidium germinate using food reserve and produce a narrow germ tube. (d). Germ tube elongates and give rise to appressorium. (e). Autophagy occurs in three-celled conidium and it dies in a programmed process. (f). In appressorium turgor pressure increases with the help of melanin layer on cell wall and synthesized glycerol inside. Then penetration peg forms at the base, punctured the cuticle of rice and allows entry into the epidermis of plant. (g). Plant tissue invasion occurs by means of bulbous, invasive hyphae (IH) that invaginate the rice plasma membrane and invade epidermal cells. Penetration peg develops into two primary hyphae and separated from rice cytoplasm by extra-invasive hyphal membrane (EIHM). Primary hyphae develop into invasive hyphae. At the tip of primary IH, a new structure known as biotrophic interfacial complex (BIC) develops which is present within EIHM. (h). Invasive hyphae moves from one cell to another by plasmodesmata. (i). Disease lesions occur on plant and sporulation starts under humid conditions, Spores develops on conidiophores with sympodially manners.

Figure 2

Overview of rice defense system against blast pathogen Magnoporthe oryzae. (a). PAMPs molecules present on membrane of Magnaporthe spore. (b). PRR presents on the rice cell membrane help to recognize PAMP molecules. (c). A successful recognition by PRR triggers PTI (PAMP-triggered immunity) and activates resistance signaling cascade. (d). Resistance signaling activates defense gene in nucleus of rice. (e). Defense responses includes ROS production, Callose deposition, Cell wall fortification, Phytoalexin production, Hypersensitive response, Hormone signaling etc. (f). In order to avoid recognition of PAMP molecules by PRR, Magnaporthe secretes effectors molecules. (g). Effectors molecules inhibit PTI responses which known as effector triggered susceptibility (ETS). (h). Plant resistant gene recognize effectors of pathogen which known as effector triggered immunity (ETI). (i). Successful recognition of effectors molecules by R gene activates resistance signaling cascade.

Figure 3

Schematic representation of Uniform Blast Nursery for screening of rice genotypes for blast disease.

Figure 4

Schematic representation of various breeding biotechnological approaches used for development of blast-resistant rice varieties.

Figure 5

Future perspectives and way forward for developing blast resistance in rice.