Lesion mimic mutant 8 balances disease resistance and growth in rice

Lesion-mimic mutants (LMM) spontaneously produce necrotic spots, a process not affected by environmental stress or pathogen infection. In this study, we identified a LMM, lesion mimic mutant 8 (lmm8) in rice (Oryza sativa). The lmm8 mutant produces brown and off-white lesions on its leaves during the second- and third-leaf stages. The lesion mimic phenotype of the lmm8 mutant was enhanced by light. At the mature stage, lmm8 mutant are shorter and exhibit inferior agronomic traits than the wild type. Contents of photosynthetic pigments and chloroplast fluorescence were significantly reduced in lmm8 leaves, along with increased production of reactive oxygen species and programmed cell death compared to the wild type. The mutated gene was identified as LMM8 (LOC_Os01g18320) by map-based cloning. A point mutation occurred in LMM8, causing a Leu to Arg mutation of the 146th amino acid of LMM8. It is an allele of SPRL1, encoding a protoporphyrinogen IX oxidase (PPOX) located in chloroplasts and involved in the biosynthesis of tetrapyrrole in chloroplasts. The lmm8 mutant showed enhanced resistance and broad-spectrum resistance. Together, our results demonstrate the importance of rice LMM8 protein in defense responses and plant growth in rice, and provides theoretical support for resistance breeding to improve rice yield.

Role of reactive oxygen species in lesion mimic formation and conferred basal resistance to Fusarium graminearum in barley lesion mimic mutant 5386

This study investigated the barley lesion mimic mutant (LMM) 5386, evidenced by a leaf brown spot phenotype localized on the chromosome 3H, and its conferred basal resistance to Fusarium graminearum. RNA-seq analysis identified 1453 genes that were differentially expressed in LMM 5386 compared to those in the wild type. GO and KEGG functional annotations suggested that lesion mimic formation was mediated by pathways involving oxidation reduction and glutathione metabolism. Additionally, reactive oxygen species (ROS) accumulation in brown spots was substantially higher in LMM 5386 than in the wild-type plant; therefore, antioxidant competence, which is indicated by ROS accumulation, was significantly lower in LMM 5386. Furthermore, the reduction of glycine in LMM 5386 inhibited glutathione biosynthesis. These results suggest that the decrease in antioxidant competence and glutathione biosynthesis caused considerable ROS accumulation, leading to programmed cell death, which eventually reduced the yield components in LMM 5386.

LMPA Regulates Lesion Mimic Leaf and Panicle Development Through ROS-Induced PCD in Rice

Leaf and panicle are important nutrient and yield organs in rice, respectively. Although several genes controlling lesion mimic leaf and panicle abortion have been identified, a few studies have reported the involvement of a single gene in the production of both the traits. In this study, we characterized a panicle abortion mutant, lesion mimic leaf and panicle apical abortion (lmpa), which exhibits lesions on the leaf and causes degeneration of apical spikelets. Molecular cloning revealed that LMPA encodes a proton pump ATPase protein that is localized in the plasma membrane and is highly expressed in leaves and panicles. The analysis of promoter activity showed that the insertion of a fragment in the promoter of lmpa caused a decrease in the transcription level. Cellular and histochemistry analysis indicated that the ROS accumulated and cell death occurred in lmpa. Moreover, physiological experiments revealed that lmpa was more sensitive to high temperatures and salt stress conditions. These results provide a better understanding of the role of LMPA in panicle development and lesion mimic formation by regulating ROS homeostasis.

AhNPR3 regulates the expression of WRKY and PR genes, and mediates the immune response of the peanut (Arachis hypogaea L.)

Systemic acquired resistance is an essential immune response that triggers a broad-spectrum disease resistance throughout the plant. In the present study, we identified a peanut lesion mimic mutant m14 derived from an ethyl methane sulfonate-mutagenized mutant pool of peanut cultivar "Yuanza9102." Brown lesions were observed in the leaves of an m14 mutant from seedling stage to maturity. Using MutMap together with bulked segregation RNA analysis approaches, a G-to-A point mutation was identified in the exon region of candidate gene Arahy.R60CUW, which is the homolog of AtNPR3 (Nonexpresser of PR genes) in Arabidopsis. This point mutation caused a transition from Gly to Arg within the C-terminal transactivation domain of AhNPR3A. The mutation of AhNPR3A showed no effect in the induction of PR genes when treated with salicylic acid. Instead, the mutation resulted in upregulation of WRKY genes and several PR genes, including pathogenesis-related thaumatin- and chitinase-encoding genes, which is consistent with the resistant phenotype of m14 to leaf spot disease. Further study on the AhNPR3A gene will provide valuable insights into understanding the molecular mechanism of systemic acquired resistance in peanut. Moreover, our results indicated that a combination of MutMap and bulked segregation RNA analysis is an effective method for identifying genes from peanut mutants.

OsLMP1, Encoding a Deubiquitinase, Regulates the Immune Response in Rice

Lesion mimic mutants have become an effective material for understanding plant-microbe interactions and the immune mechanism in plants. Although many mechanisms responsible for the lesion mimic phenotype have been clarified in plants, the mechanism by which lesion mimic is regulated by posttranslational modification remained largely elusive, especially in rice. In this study, a mutant with the lesion mimic phenotype was obtained and named lmp1-1. Physiological measurements and quantitative real-time PCR analysis showed that the defense response was activated in the mutants. Transcriptome analysis showed that the phenylalanine ammonia lyase (PAL) pathway was activated in the mutant, causing the accumulation of salicylic acid (SA). The results of mapping based cloning showed that OsLMP1 encodes a deubiquitinase. OsLMP1 can cleave ubiquitination precursors. Furthermore, OsLMP1 epigenetically modifies SA synthetic pathway genes by deubiquitinating H₂B and regulates the immune response in rice. In summary, this study deepens our understanding of the function of OsLMP1 in the plant immune response and provides further insight into the relationship between plants and pathogenic microorganisms.

OsNBL3, a mitochondrion-localized pentatricopeptide repeat protein, is involved in splicing nad5 intron 4 and its disruption causes lesion mimic phenotype with enhanced resistance to biotic and abiotic stresses

Lesion mimic mutants are used to elucidate mechanisms controlling plant responses to pathogen attacks and environmental stresses. Although dozens of genes had been functionally demonstrated to be involved in lesion mimic phenotype in several plant species, the molecular mechanisms underlying the hypersensitive response are largely unknown. Here, a rice (Oryza sativa) lesion mimic mutant natural blight leaf 3 (nbl3) was identified from T-DNA insertion lines. The causative gene, OsNBL3, encodes a mitochondrion-localized pentatricopeptide repeat (PPR) protein. The nbl3 mutant exhibited spontaneous cell death response and H2 O2 accumulation, and displayed enhanced resistance to the fungal and bacterial pathogens Magnaporthe oryzae and Xanthomonas oryzae pv. oryzae. This resistance was consistent with the up-regulation of several defence-related genes; thus, defence responses were induced in nbl3. RNA interference lines of OsNBL3 exhibited enhanced disease resistance similar to that of nbl3, while the disease resistance in overexpression lines did not differ from that of the wild type. In addition, nbl3 displayed improved tolerance to salt, accompanied by up-regulation of several salt-associated marker genes. OsNBL3 was found to mainly participate in the splicing of mitochondrial gene nad5 intron 4. Disruption of OsNBL3 leads to the reduction in complex I activity, the elevation of alternative respiratory pathways and the destruction of mitochondrial morphology. Overall, the results demonstrated that the PPR protein-coding gene OsNBL3 is essential for mitochondrial development and functions, and its disruption causes the lesion mimic phenotype and enhances disease resistance and tolerance to salt in rice.

A teosinte-derived allele of a MYB transcription repressor confers multiple disease resistance in maize

Natural alleles that control multiple disease resistance (MDR) are valuable for crop breeding. However, only one MDR gene has been cloned in maize, and the molecular mechanisms of MDR remain unclear in maize. In this study, through map-based cloning we cloned a teosinte-derived allele of a resistance gene, Mexicana lesion mimic 1 (ZmMM1), which causes a lesion mimic phenotype and confers resistance to northern leaf blight (NLB), gray leaf spot (GLS), and southern corn rust (SCR) in maize. Strong MDR conferred by the teosinte allele is linked with polymorphisms in the 3' untranslated region of ZmMM1 that cause increased accumulation of ZmMM1 protein. ZmMM1 acts as a transcription repressor and negatively regulates the transcription of specific target genes, including ZmMM1-target gene 3 (ZmMT3), which functions as a negative regulator of plant immunity and associated cell death. The successful isolation of the ZmMM1 resistance gene will help not only in developing broad-spectrum and durable disease resistance but also in understanding the molecular mechanisms underlying MDR.

Altered Expression of OsAAP3 Influences Rice Lesion Mimic and Leaf Senescence by Regulating Arginine Transport and Nitric Oxide Pathway

Persistent lesion mimic can cause leaf senescence, affecting grain yield in crops. However, knowledge about the regulation of lesion mimic and leaf senescence in crop plants is still limited. Here, we report that the amino acid transporter OsAAP3, a negative regulator of tiller bud elongation and rice grain yield, is involved in lesion mimic and leaf senescence. Altered expression of OsAAP3 can initiate the nitric oxide signaling pathway through excessive accumulation of arginine in rice leaves, influencing ROS accumulation, antioxidant enzymes activities, proline concentration, and malondialdehyde concentration. This finally triggers cell death which ultimately leads to lesion mimic and leaf senescence by regulating the degradation of chloroplast and the expression abundance of components in the photosynthetic pathway. Overall, the results not only provide initial insights into the regulatory role of amino acid transport genes in rice growth and development, but also help to understand the factors regulating the leaf senescence.

A novel glycine-rich domain protein, GRDP1, functions as a critical feedback regulator for controlling cell death and disease resistance in rice

Lesion mimic mutants constitute a valuable genetic resource for unraveling the signaling pathways and molecular mechanisms governing the programmed cell death and defense responses of plants. Here, we identified a lesion mimic mutant, spl-D, from T-DNA insertion rice lines. The mutant exhibited higher accumulation of H2O2, spontaneous cell death, decreased chlorophyll content, up-regulation of defense-related genes, and enhanced disease resistance. The causative gene, OsGRDP1, encodes a cytosol- and membrane-associated glycine-rich domain protein. OsGRDP1 was expressed constitutively in all of the organs of the wild-type plant, but was up-regulated throughout plant development in the spl-D mutant. Both the overexpression and knockdown (RNAi) of OsGRDP1 resulted in the lesion mimic phenotype. Moreover, the intact-protein level of OsGRDP1 was reduced in the spotted leaves from both overexpression and RNAi plants, suggesting that the disruption of intact OsGRDP1 is responsible for lesion formation. OsGRDP1 interacted with an aspartic proteinase, OsAP25. In the spl-D and overexpression plants, proteinase activity was elevated, and lesion formation was partially suppressed by an aspartic proteinase inhibitor. Taken together, our results reveal that OsGRDP1 is a critical feedback regulator, thus contributing to the elucidation of the mechanism underlying cell death and disease resistance.

OsCUL3a-Associated Molecular Switches Have Functions in Cell Metabolism, Cell Death, and Disease Resistance

This study applies parallel reaction monitoring (PRM) proteomics and CRISPR-Cas9 mutagenesis to identify relationships between cell metabolism, cell death, and disease resistance. In oscul3a (oscullin3a) mutants, OsCUL3a-associated molecular switches are responsible for disrupted cell metabolism that leads to increased total lipid content in rice grain, a late accumulation of H₂O₂ in leaves, enhanced Xanthomonas oryzae pv. oryzae disease resistance, and suppressed panicle and first internode growth. In oscul3a mutants, PRM-confirmed upregulated molecular switch proteins include lipoxygenases (CM-LOX1 and CM-LOX2), suggesting a novel connection between ferroptosis and rice lesion mimic formation. Rice immunity-associated proteins OsNPR1 and OsNPR3 were shown to interact with each other and have opposing regulatory effects based on the cell death phenotype of osnpr1/oscul3a and osnpr3/oscul3a double mutants. Together, these results describe a network that regulates plant growth, disease resistance, and grain quality that includes the E3 ligase OsCUL3a, cell metabolism-associated molecular switches, and immunity switches OsNPR1 and OsNPR3.

Regulation of ABA dependent wound induced spreading cell death by MYB108

Wounding results in the controlled cell death of a few rows of cells adjacent to disrupted cells resulting in physical wound closure, which combined with phenolic compound deposition, prevents water loss and pathogen entry. The control of these processes remains uncharacterized. Cell death in a mutant of Arabidopsis thaliana lacking BOTRYTIS SENSITIVE1/MYB108 (BOS1/MYB108) function was characterized utilizing physiological, cell biological and genetic methods. The bos1 mutant has a wound induced runaway cell death that includes enhanced reactive oxygen species (ROS) production that followed the extent of enhanced cell death. Exogenous abscisic acid (ABA) enhanced wound induced cell death in Col-0 plants and was sufficient to trigger cell death in bos1. Uncontrolled cell death was dependent of the production and perception of ABA. Furthermore, bos1 had altered sensitivity to and accumulation of ABA. Arabidopsis possesses a genetic program controlling the extent of wound inducible cell death. BOS1 acts as a negative regulator of ABA induced cell death, which functions in the control of this wound sealing program. This program is distinct from other known cell death programs in that it is ABA dependent, but independent of salicylate biosynthesis, ethylene, jasmonate, metacaspases and ROS derived from RBOHD and RBOHF.

The exocyst subunit Exo70B1 is involved in the immune response of Arabidopsis thaliana to different pathogens and cell death

Components of the vesicle trafficking machinery are central to the immune response in plants. The role of vesicle trafficking during pre-invasive penetration resistance has been well documented. However, emerging evidence also implicates vesicle trafficking in early immune signaling. Here we report that Exo70B1, a subunit of the exocyst complex which mediates early tethering during exocytosis is involved in resistance. We show that exo70B1 mutants display pathogen-specific immuno-compromised phenotypes. We also show that exo70B1 mutants display lesion-mimic cell death, which in combination with the reduced responsiveness to pathogen-associated molecular patterns (PAMPs) results in complex immunity-related phenotypes.