Deciphering Rice Lesion Mimic Mutants to Understand Molecular Network Governing Plant Immunity and Growth

Plant lesion mimic mutants(lmms)generally possess autoimmunity and hypersensitive response(HR)-like cell death in the absence of biotic or abiotic stress.They have attracted much attention because they are useful tools for deciphering the interaction between defense signaling and growth.Recent studies have identified more than 30 lmms involved in the plant immune response and cell death in rice.Genes underlying these lmms,coding for diverse types of proteins,mainly regulate transcription,protein translation and modification,vesicular trafficking and catalyzation of metabolism.Here,we presented an overview of the most recent advances on the study of lmms in rice and proposed a perspective on potential utilization of LMM genes in agriculture.

The gene encoding flavonol synthase contributes to lesion mimic in wheat

Lesion mimic often exhibits leaf disease-like symptoms even in the absence of pathogen infection,and is characterized by a hypersensitive-response(HR)that closely linked to plant disease resistance.Despite this,only a few lesion mimic genes have been identified in wheat.In this investigation,a lesion mimic wheat mutant named je0297 was discovered,showing no alteration in yield components when compared to the wild type(WT).Segregation ratio analysis of the F_(2)individuals resulting from the cross between the WT and the mutant revealed that the lesion mimic was governed by a single recessive gene in je0297.Using Bulked segregant analysis(BSA)and exome capture sequencing,we mapped the lesion mimic gene designated as lm6 to chromosome 6BL.Further gene fine mapping using 3315 F_(2)individuals delimited the lm6 within a 1.18 Mb region.Within this region,we identified 16 high-confidence genes,with only two displaying mutations in je0297.Notably,one of the two genes,responsible for encoding flavonol synthase,exhibited altered expression levels.Subsequent phenotype analysis of TILLING mutants confirmed that the gene encoding flavonol synthase was indeed the causal gene for lm6.Transcriptome sequencing analysis revealed that the DEGs between the WT and mutant were significantly enriched in KEGG pathways related to flavonoid biosynthesis,including flavone and flavonol biosynthesis,isoflavonoid biosynthesis,and flavonoid biosynthesis pathways.Furthermore,more than 30 pathogen infection-related(PR)genes exhibited upregulation in the mutant.Corresponding to this expression pattern,the flavonoid content in je0297 showed a significant decrease in the 4^(th)leaf,accompanied by a notable accumulation of reactive oxygen,which likely contributed to the development of lesion mimic in the mutant.This investigation enhances our comprehension of cell death signaling pathways and provides a valuable gene resource for the breeding of disease-resistant wheat.

Characterization and mapping of a novel light-dependent lesion mimic mutant lmm6 in rice(Oryza sativa L.)

A novel rice lesion mimic mutant (LMM) was isolated from an ethane methyl sulfonate (EMS)-induced 02428 mutant bank. The mutant, tentatively designated as lmm6, develops necrotic lesions in the whole growth period along with changes in several important agronomic traits. We found that the initiation of the lesions was induced by light and cel death occurred in lmm6 accompanied with accumulation of reactive oxygen species (ROS). The lower chlorophyl content, soluble protein content and superoxide dismutase (SOD) activity, the higher malondialdehyde (MDA) content were detected in lmm6 than in the wild type (WT). Moreover, the observation by transmission electronic microscope (TEM) demonstrated that some organel es were damaged and the stroma lamel a of chloroplast was irregular and loose in mesophyl cel of lmm6. In addition, lmm6 was more resistant than WT to rice blast fungus Magnaporthe grisea infection, which was consistent with increased expression of four genes involved in the defense-related reaction. Genetic analysis showed that mutant trait of lmm6 is inherited as a monogenic recessive nuclear gene located on the long arm of chromosome 6. Using simple sequence repeat (SSR) markers, the target gene was ifnal y delimited to an interval of 80.8 kb between markers MM2359 and MM2370, containing 7 annotated genes. Taken together, our results provide the information to identify a new gene involved in rice lesion mimic, which wil be helpful in clarifying the mechanism of cel death and disease resistance in rice.

Identification and Genetic Mapping of a Lesion Mimic Mutant in Rice

A lesion mimic stripe mutant, designated as Ims1 （lesion mimic stripe 1）, was obtained from the M2 progeny of a ^60Co y-radiation treated japonica rice variety Jiahua 1. The Ims1 mutant displayed propagation type lesions across the whole growth and developmental stages. Physiology and histochemistry analysis showed that the mutant exhibited a phenotype of white stripe when grown under high temperature （30 ℃）, and the lesion mimic caused by programmed cell death under low temperature （20 ℃）. The genetic analysis indicated that this lesion-mimic phenotype is controlled by a single locus recessive nuclear gene. Furthermore, by using simple sequence repeat markers and an F2 segregating population derived from two crosses of Ims1 ×93-11 and Ims1 ×Pei＇ai 64S, the Imsl gene was mapped between markers Indel1 and MM0112-4 with a physical distance of 400 kb on chromosome 6 in rice.

Deterioration of Antioxidant Competence in Barley Lesion Mimic Mutant 194

A barley mutant,194,was observed to exhibit a leaf spot phenotype over the whole course of its growing period.In this study,the phenotype and antioxidant competence were studied in the lesion mimic mutant 194.Plant height was slightly higher in mutant 194 than in the wild type(WT).In addition,leaf spot per plant in mutant 194 was significantly higher than in WT.Antioxidant competence,as indicated by reactive oxygen species(ROS)accumulation,antioxidant enzyme activity,and the expression of antioxidant enzyme-encoding genes was also assessed in mutant 194.Compared to the WT,mutant 194 displayed a relatively higher accumulation of ROS,accompanied by lower activities of some antioxidant enzymes and downregulation of antioxidant enzyme-encoding genes.This demonstrated reduced antioxidant competence in mutant 194.The results suggested that this lower antioxidant competence of mutant 194 could lead to the accumulation of excessive ROS.This excess of ROS could induce programmed cell death and has the potential to promote disease resistance in mutant 194.

Host Active Defense Responses Occur within 24 Hours after Pathogen Inoculation in the Rice Blast System

Phenotypical, cytological and molecular responses of rice to the fungus Magnaporthe grisea were studied using rice cultivars and lesion mimic plants. The cultivar Katy was susceptible to several virulent M. grisea isolates, and a Sekiguchi like-lesion mimic mutant of Katy （LmmKaty） showed enhanced resistance to these isolates. Lesion mimic phenotype of LmmKaty was rapidly induced by virulent M. grisea isolates or by avirulent ones only at high levels of inoculum. Autofluorescence （a sign of an active defense response） was visible under ultraviolet light 24 h after localized inoculation in the incompatible interaction, whereas, not evident in the compatible interaction. Autofluorescence was also observed in LmmKaty 20 h after pathogen inoculation, indicating that rapid cell death is a mechanism of LmmKaty to restrict pathogen invasion. Rapid accumulations of defense related （DR） gene transcripts, phenylalanine ammonia lyase and β-glucanase, were observed beginning at 6 h and were obvious at 16 h and 24 h after inoculation in an incompatible interaction. Rapid transcript accumulations of PR-1 and chitinase had occurred by 24 h after inoculation in an incompatible interaction. Accumulations of these transcripts were delayed in the compatible interaction. These results indicate that host active defense responses occur 24 h after pathogen inoculation and that LmmKaty exhibits enhanced resistance to M. grisea. It is suggested that the autofluorescence and expression of the DR genes after heavy inoculation are important cytological and molecular markers respectively for early determination of the host response to M. grisea in the rice blast system.

Genome editing enables defense-yield balance in rice

This brief article highlights the key findings of the study conducted by Sha et al.(Nature,doi:10.1038/s41586-023-06205-2,2023),focusing on the cloning of the RBL1 gene from rice,which is associated with lesion mimic mutant(LMM)traits.The RBL1 gene encodes a cytidine diphosphate diacylglycerol(CDP-DAG)synthase and plays a crucial role in regulating cell death and immunity by controlling phosphatidylinositol biosynthesis.The rbl1 mutant shows autoimmunity with multi-pathogen resistance but with severe yield penalty.Using genome editing techniques,the research team successfully generated an elite allele of RBL1 that not only restores rice yield but also provides broad-spectrum resistance against both bacterial and fungal pathogens.These findings demonstrate the potential of utilizing genome editing to enhance crop productivity and pathogen resistance.