Characterization and Genetic Analysis of a Novel Light-Dependent Lesion Mimic Mutant, lm3, Showing Adult-Plant Resistance to Powdery Mildew in Common Wheat

The development of pleiotropic phenotypes in powdery mildew-resistant barley and Arabidopsis thaliana mlo mutants is linked to nitrogen availability

Powdery mildew-resistant barley (Hordeum vulgare) and Arabidopsis thaliana mlo mutant plants exhibit pleiotropic phenotypes such as the spontaneous formation of callose-rich cell wall appositions and early leaf chlorosis and necrosis, indicative of premature leaf senescence. The exogenous factors governing the occurrence of these undesired side effects remain poorly understood. Here, we characterised the formation of these symptoms in detail. Ultrastructural analysis revealed that the callose-rich cell wall depositions spontaneously formed in A. thaliana mlo mutants are indistinguishable from those induced by the bacterial pattern epitope, flagellin 22 (flg22). We further found that increased plant densities during culturing enhance the extent of the leaf senescence syndrome in A. thaliana mlo mutants. Application of a liquid fertiliser rescued the occurrence of leaf chlorosis and necrosis in both A. thaliana and barley mlo mutant plants. Controlled fertilisation experiments uncovered nitrogen as the macronutrient whose deficiency promotes the extent of pleiotropic phenotypes in A. thaliana mlo mutants. Light intensity and temperature had a modulatory impact on the incidence of leaf necrosis in the case of barley mlo mutant plants. Collectively, our data indicate that the development of pleiotropic phenotypes associated with mlo mutants is governed by various exogenous factors.

Integration of transcriptomics, metabolomics, and hormone analysis revealed the formation of lesion spots inhibited by GA and CTK was related to cell death and disease resistance in bread wheat (Triticum aestivum L.)

BACKGROUND

Wheat is one of the important grain crops in the world. The formation of lesion spots related to cell death is involved in disease resistance, whereas the regulatory pathway of lesion spot production and resistance mechanism to pathogens in wheat is largely unknown.

RESULTS

In this study, a pair of NILs (NIL-Lm5W and NIL-Lm5M) was constructed from the BC1F4 population by the wheat lesion mimic mutant MC21 and its wild genotype Chuannong 16. The formation of lesion spots in NIL-Lm5M significantly increased its resistance to stripe rust, and NIL-Lm5M showed superiour agronomic traits than NIL-Lm5W under stripe rust infection.Whereafter, the NILs were subjected to transcriptomic (stage N: no spots; stage S, only a few spots; and stage M, numerous spots), metabolomic (stage N and S), and hormone analysis (stage S), with samples taken from normal plants in the field. Transcriptomic analysis showed that the differentially expressed genes were enriched in plant-pathogen interaction, and defense-related genes were significantly upregulated following the formation of lesion spots. Metabolomic analysis showed that the differentially accumulated metabolites were enriched in energy metabolism, including amino acid metabolism, carbohydrate metabolism, and lipid metabolism. Correlation network diagrams of transcriptomic and metabolomic showed that they were both enriched in energy metabolism. Additionally, the contents of gibberellin A7, cis-Zeatin, and abscisic acid were decreased in leaves upon lesion spot formation, whereas the lesion spots in NIL-Lm5M leaves were restrained by spaying GA and cytokinin (CTK, trans-zeatin) in the field.

CONCLUSION

The formation of lesion spots can result in cell death and enhance strip rust resistance by protein degradation pathway and defense-related genes overexpression in wheat. Besides, the formation of lesion spots was significantly affected by GA and CTK. Altogether, these results may contribute to the understanding of lesion spot formation in wheat and laid a foundation for regulating the resistance mechanism to stripe rust.

Disruption of the Novel Small Protein RBR7 Leads to Enhanced Plant Resistance to Blast Disease

Plant disease is a threat to global food security. Breeding crops carrying broad-spectrum resistance loci is an effective way to control infectious disease. Disease-resistant mutants are valuable resources for deciphering the underlying mechanisms of plant immunity and could provide genetic loci to generate disease-resistant crops. Here, we identified a rice mutant, rbr7 (rice blast resistance 7), that confers resistance against different strains of Magnaporthe oryzae. Disease-mimicking necrotic lesions started to appear on the leaves of rbr7 four weeks after sowing. Histochemical analysis revealed reactive oxygen species accumulation and cell death accompanied by spontaneous lesion formation in rbr7. Map-based cloning and bulk segregation analysis showed a 2855 bp fragment deletion on chromosome 5, leading to the disruption of the LOC_Os05g28480-coding protein. Transgenic rbr7 complementation plants showed compromised resistance to rice blast, indicating that LOC_Os05g28480, or Rbr7, regulates the rice immune response. Rbr7 encodes a small protein of unknown function with 85 amino acids. Transcriptomic analysis revealed that disruption of RBR7 led to the upregulation of genes responding to salicylic acid, systemic acquired resistance and pathogenesis-related genes. Taken together, our findings reveal insights into a novel small protein involved in regulating plant resistance to rice blast and provide a potential target for crop breeding.

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.

A dominant gene Ihrl1 is tightly linked to and inhibits the gene Ndhrl1 mediating nitrogen-dependent hypersensitive reaction-like phenotype in wheat

Identification and mapping of an inhibitor of Ndhrl1 mediating nitrogen-dependent hypersensitive reaction-like phenotype in wheat. Hypersensitive reaction-like (HRL) traits are characteristic of spontaneous lesions including yellowish spots, brown spots or white-stripe that appeared randomly and dispersedly on all the leaves in the absence of plant pathogens. Our previous studies have shown that the wheat line P7001 showed an HRL trait at low nitrogen supply, and this trait was controlled by the gene Ndhrl1 (Nitrogen-dependent hypersensitive reaction-like 1). In order to investigate the robustness of the trait expression mediated by Ndhrl1 under different genetic backgrounds, seven genetic populations, with P7001 being the common female parent, were constructed and analyzed. F₁ plants from six of the seven combinations showed HRL trait and Ndhrl1 segregated in a dominant way of HRL: non-HRL = 3:1 in the six populations (F₂). Exceptionally, the F₁ plants of P7001/Fielder combination showed non-HRL trait and HRL trait in the F₂ population showed a contrasting recessive segregation ratio of HRL: non-HRL = 1:3, suggesting Fielder may have another HRL-related gene. Using 55 K SNP array and PCR-based markers, the HRL-related gene in Fielder was mapped to an interval of 5.63-12.91 Mb on the short arm of chromosome 2B with the flanking markers Yzu660R075552 and Yzu660F075941. A recombinant with genomic region of Fielder at Ndhrl1 locus showing HRL trait demonstrated that Fielder also harbored Ndhrl1 same as P7001. Thus, Fielder carries a single dominant suppressor of Ndhrl1, designated as Ihrl1 (Inhibitor of hypersensitive reaction-like). Interestingly, Ihrl1 is tightly linked to Ndhrl1 and may be also involved in nitrogen metabolic and (or) signaling pathways.

ATP-citrate lyase B (ACLB) negatively affects cell death and resistance to Verticillium wilt

BACKGROUND

ATP-citrate lyase (ACL) plays a pivotal role in histone acetylation and aerobic glycolysis. In plant, ACL is a heteromeric enzyme composed of ACLA (45 kD) and ACLB (65 kD). So far, the function of ACL genes in cotton still remains unknown.

RESULTS

Here, we identified three ACLA homologous sequences and two ACLB homologous in each genome/sub-genome of cotton species. Silencing ACLB in cotton led to cell death at newly-grown leaves and stem apexes. Simultaneously, in ACLB-silenced plants, transcription factors related to senescence including SGR, WRKY23 and Osl57 were observed to be activated. Further investigation showed that excessive H₂O₂ was accumulated, salicylic acid-dependent defense response and pathogenesis-related gene expressions were evidently enhanced in ACLB-silenced plants, implying that knockdown of ACLB genes leads to hypersensitive response-like cell death in cotton seedlings. However, as noted, serious cell death happened in newly-grown leaves and stem apexes in ACLB-silenced plants, which led to the failure of subsequent fungal pathogenicity assays. To confirm the role of ACLB gene in regulating plant immune response, the dicotyledonous model plant Arabidopsis was selected for functional verification of ACLB gene. Our results indicate the resistance to Verticillium dahliae infection in the Arabidopsis mutant aclb-2 were enhanced without causing strong cell death. Ectopic expression of GausACLB-2 in Arabidopsis weakened its resistance to V. dahliae either in Col-0 or in aclb-2 background, in which the expression level of ACLB is negatively correlated with the resistance to V. dahliae.

CONCLUSIONS

These results indicate that ACLB has a new function in negatively affecting the induction of plant defense response and cell death in cotton, which provides theoretical guidance for developing cotton varieties with resistance against Verticillium wilt.

Cloning the barley nec3 disease lesion mimic mutant using complementation by sequencing

Disease lesion mimic (DLM) or necrotic mutants display necrotic lesions in the absence of pathogen infections. They can show improved resistance to some pathogens and their molecular dissection can contribute to revealing components of plant defense pathways. Although forward-genetics strategies to find genes causal to mutant phenotypes are available in crops, these strategies require the production of experimental cross populations, mutagenesis, or gene editing and are time- and resource-consuming or may have to deal with regulated plant materials. In this study, we described a collection of 34 DLM mutants in barley (Hordeum vulgare L.) and applied a novel method called complementation by sequencing (CBS), which enables the identification of the gene responsible for a mutant phenotype given the availability of two or more chemically mutagenized individuals showing the same phenotype. Complementation by sequencing relies on the feasibility to obtain all induced mutations present in chemical mutants and on the low probability that different individuals share the same mutated genes. By CBS, we identified a cytochrome P450 CYP71P1 gene as responsible for orange blotch DLM mutants, including the historical barley nec3 locus. By comparative phylogenetic analysis we showed that CYP71P1 gene family emerged early in angiosperm evolution but has been recurrently lost in some lineages including Arabidopsis thaliana (L.) Heynh. Complementation by sequencing is a straightforward cost-effective approach to clone genes controlling phenotypes in a chemically mutagenized collection. The TILLMore (TM) collection will be instrumental for understanding the molecular basis of DLM phenotypes and to contribute knowledge about mechanisms of host-pathogen interaction.

Characterization and fine mapping of a lesion mimic mutant (Lm5) with enhanced stripe rust and powdery mildew resistance in bread wheat (Triticum aestivum L.)

A novel light intensity-dependent lesion mimic mutant with enhanced disease resistance was physiologically, biochemically, and genetically characterized, and the causative gene was fine mapped to a 1.28 Mbp interval containing 17 high-confidence genes. Lesion mimic mutants are ideal for studying disease resistance and programmed cell death photosynthesis in plants to improve crop yield. In this study, a novel light intensity-dependent lesion mimic mutant (MC21) was obtained from the wheat variety Chuannong16 (CN16) by ethyl methane sulfonate treatment. The mutant initially developed tiny lesion spots on the basal part of the leaves, which then gradually proceeded down to leaf sheaths, stems, shells, and awns at the flowering stage. The major agronomic traits were significantly altered in the mutant compared to that in the wild-type CN16. Furthermore, the mutant exhibited a lesion phenotype with degenerated chloroplast structure, decreased chlorophyll content, increased level of reactive oxygen species, and increased resistance to stripe rust and powdery mildew. Genetic analysis indicated that the lesion phenotype was controlled by a novel single semi-dominant nuclear gene. The target gene was mapped on chromosome arm 2AL located between Kompetitive Allele Specific PCR (KASP) markers, KASP-4211 and KASP-5353, and tentatively termed as lesion mimic 5 (Lm5). The fine mapping suggested that Lm5 was located in a 1.28 Mbp interval between markers KASP-5825 and KASP-9366; 17 high-confidence candidate genes were included in this genomic region. This study provides an important foundational step for further cloning of Lm5 using a map-based approach.

Cotton GhSSI2 isoforms from the stearoyl acyl carrier protein fatty acid desaturase family regulate Verticillium wilt resistance

Lipids are major and essential constituents of plant cells and provide energy for various metabolic processes. However, the function of the lipid signal in defence against Verticillium dahliae, a hemibiotrophic pathogen, remains unknown. Here, we characterized 19 conserved stearoyl-ACP desaturase family proteins from upland cotton (Gossypium hirsutum). We further confirmed that GhSSI2 isoforms, including GhSSI2-A, GhSSI2-B, and GhSSI2-C located on chromosomes A10, D10, and A12, respectively, played a dominant role to the cotton 18:1 (oleic acid) pool. Suppressing the expression of GhSSI2s reduced the 18:1 level, which autoactivated the hypersensitive response (HR) and enhanced cotton Verticillium wilt and Fusarium wilt resistance. We found that low 18:1 levels induced phenylalanine ammonia-lyase-mediated salicylic acid (SA) accumulation and activated a SA-independent defence response in GhSSI2s-silenced cotton, whereas suppressing expression of GhSSI2s affected PDF1.2-dependent jasmonic acid (JA) perception but not the biosynthesis and signalling cascade of JA. Further investigation showed that structurally divergent resistance-related genes and nitric oxide (NO) signal were activated in GhSSI2s-silenced cotton. Taken together, these results indicate that SA-independent defence response, multiple resistance-related proteins, and elevated NO level play an important role in GhSSI2s-regulated Verticillium wilt resistance. These findings broaden our knowledge regarding the lipid signal in disease resistance and provide novel insights into the molecular mechanism of cotton fungal disease resistance.

Enhanced SA and Ca2+ signaling results in PCD-mediated spontaneous leaf necrosis in wheat mutant wsl

Leaf is the major photosynthesis organ and the key source of wheat (Triticum aestivum L.) grain. Spotted leaf (spl) mutant is a kind of leaf lesion mimic mutants (LMMs) in plants, which is an ideal material for studying the mechanisms of leaf development. In this study, we report the leaf abnormal development molecular mechanism of a spl mutant named white stripe leaf (wsl) derived from wheat cultivar Guomai 301 (WT). Histochemical observation indicated that the leaf mesophyll cells of the wsl were destroyed in the necrosis regions. To explore the molecular regulatory network of the leaf development in mutant wsl, we employed transcriptome analysis, histochemistry, quantitative real-time PCR (qRT-PCR), and observations of the key metabolites and photosynthesis parameters. Compared to WT, the expressions of the chlorophyll synthesis and photosynthesis-related homeotic genes were repressed; many genes in the WRKY transcription factor (TF) families were highly expressed; the salicylic acid (SA) and Ca²⁺ signal transductions were enhanced in wsl. Both the chlorophyll contents and the photosynthesis rate were lower in wsl. The contents of SA and reactive oxygen species (ROS) were significantly higher, and the leaf rust resistance was enhanced in wsl. Based on the experimental data, a primary molecular regulatory model for leaf development in wsl was established. The results indicated that the SA accumulation and enhanced Ca²⁺ signaling led to programmed cell death (PCD), and ultimately resulted in spontaneous leaf necrosis of wsl. These results laid a solid foundation for further research on the molecular mechanism of leaf development in wheat.

Commonly and Specifically Activated Defense Responses in Maize Disease Lesion Mimic Mutants Revealed by Integrated Transcriptomics and Metabolomics Analysis

Disease lesion mimic (Les/les) mutants display disease-like spontaneous lesions in the absence of pathogen infection, implying the constitutive activation of defense responses. However, the genetic and biochemical bases underlying the activated defense responses in those mutants remain largely unknown. Here, we performed integrated transcriptomics and metabolomics analysis on three typical maize Les mutants Les4, Les10, and Les17 with large, medium, and small lesion size, respectively, thereby dissecting the activated defense responses at the transcriptional and metabolomic level. A total of 1,714, 4,887, and 1,625 differentially expressed genes (DEGs) were identified in Les4, Les10, and Les17, respectively. Among them, 570, 3,299, and 447 specific differentially expressed genes (SGs) were identified, implying a specific function of each LES gene. In addition, 480 common differentially expressed genes (CGs) and 42 common differentially accumulated metabolites (CMs) were identified in all Les mutants, suggesting the robust activation of shared signaling pathways. Intriguingly, substantial analysis of the CGs indicated that genes involved in the programmed cell death, defense responses, and phenylpropanoid and terpenoid biosynthesis were most commonly activated. Genes involved in photosynthetic biosynthesis, however, were generally repressed. Consistently, the dominant CMs identified were phenylpropanoids and flavonoids. In particular, lignin, the phenylpropanoid-based polymer, was significantly increased in all three mutants. These data collectively imply that transcriptional activation of defense-related gene expression; increase of phenylpropanoid, lignin, flavonoid, and terpenoid biosynthesis; and inhibition of photosynthesis are generalnatures associated with the lesion formation and constitutively activated defense responses in those mutants. Further studies on the identified SGs and CGs will shed new light on the function of each LES gene as well as the regulatory network of defense responses in maize.

Plant Defense Responses to Biotic Stress and Its Interplay With Fluctuating Dark/Light Conditions

Plants are subjected to a plethora of environmental cues that cause extreme losses to crop productivity. Due to fluctuating environmental conditions, plants encounter difficulties in attaining full genetic potential for growth and reproduction. One such environmental condition is the recurrent attack on plants by herbivores and microbial pathogens. To surmount such attacks, plants have developed a complex array of defense mechanisms. The defense mechanism can be either preformed, where toxic secondary metabolites are stored; or can be inducible, where defense is activated upon detection of an attack. Plants sense biotic stress conditions, activate the regulatory or transcriptional machinery, and eventually generate an appropriate response. Plant defense against pathogen attack is well understood, but the interplay and impact of different signals to generate defense responses against biotic stress still remain elusive. The impact of light and dark signals on biotic stress response is one such area to comprehend. Light and dark alterations not only regulate defense mechanisms impacting plant development and biochemistry but also bestow resistance against invading pathogens. The interaction between plant defense and dark/light environment activates a signaling cascade. This signaling cascade acts as a connecting link between perception of biotic stress, dark/light environment, and generation of an appropriate physiological or biochemical response. The present review highlights molecular responses arising from dark/light fluctuations vis-à-vis elicitation of defense mechanisms in plants.

Molecular mapping of a novel lesion mimic gene (lm4) associated with enhanced resistance to stripe rust in bread wheat

BACKGROUND

Lesion mimics (LMs) are disease-like symptoms that occur randomly on plant green leaves in the absence of pathogens. A previous study showed that LMs are related to enhanced resistance to a broad spectrum of diverse pathogen races and programmed cell death (PCD). Stripe rust is a globally epidemic fungal disease that can substantially reduce the quality and yield of crops. The development of resistant cultivars is an economical and environmentally friendly way to enhance the adaptability and yield stability of crops instead of the use of fungicide applications.

RESULTS

In this study, a novel LM gene affording Pst resistance was identified and mapped with molecular markers developed for marker-assisted selection (MAS)-based wheat breeding. In this study, a novel LM gene named lm4, which is closely linked (8.06 cM) to SSR markers Xgwm210 and Xgwm455, was identified by using a Yanzhan 1/Neixiang 188 RIL population. The genetic distance of lm4 was then narrowed such that it was flanked by SSR markers with 0.51 cM and 0.77 cM intervals. Two SSR markers, lm4_01_cib and lm4_02_cib, were developed based on the content in the Chinese Spring genome database and wheat 660 K SNP results; these markers can be used to conduct MAS of LMs in wheat. The results also showed that lm4 significantly improved the resistance of stripe rust in wheat.

CONCLUSIONS

Therefore, lm4 is associated with stripe rust resistance, which may provide theoretical support for future crop disease-resistance breeding and for understanding the plant apoptosis mechanism.

Genetic characterization for lesion mimic and other traits in relation to spot blotch resistance in spring wheat

Lesion mimic (Lm) mutants display hypersensitive responses (HR) without any pathogen attack; their symptoms are similar to those produced by a pathogen and result in cell death. In wheat, such mutants have been reported to be resistant against leaf rust due to their biotrophic nature. However, Lm mutants tend to encourage spot blotch (SB) disease caused by Bipolarissorokiniana since dead cells facilitate pathogen multiplication. In this study, 289 diverse wheat germplasm lines were phenotyped in three consecutive growing seasons (2012–2015). Genotype data was generated using the Illumina iSelect beadchip assay platform for wheat germplasm lines. A total of 13,589 single-nucleotide polymorphisms (SNPs) were selected andused for further association mapping. Lm was positively associated with Area Under Disease Progress Curve (AUDPC) for SB but negatively with glaucous index (GI), leaf tip necrosis (Ltn) and latent period (LP). Ltn had a negative association with AUDPC and Lm but a positive one with LP. In a genome-wide association study (GWAS), 29 markers were significantly associated with these traits and 27 were an notated. Seven SNP markers associated with Lm were on chromosome 6A; another on 1B was found to be linked with Ltn. Like wise, seven SNP markers were associated with GI; one on chromosome 6A with the others on 6B. Five SNP markers on chromosomes 3B and 3Dwere significantly correlated with LP, while nine SNP markers on chromosomes 5A and 5B were significantly associated with AUDPC for SB. This study is the first to explore the interaction in wheat between Lm mutants and the hemibiotrophic SB pathogen B.sorokiniana.

Identification and characterization of a spotted-leaf mutant spl40 with enhanced bacterial blight resistance in rice

BACKGROUND

Spotted leaf mutants show typical necrotic lesions that appear spontaneously in the absence of any pathogen attack. These mutants are often characterized to exhibit programmed cell death (PCD) and activation of plant defense responses resulting in enhanced disease resistance to multiple pathogens. Here, we reported a novel spotted-leaf mutant, spl40 that showed enhanced disease resistance response.

RESULTS

Initially lesions appeared at leaf tips during seedling stage and gradually covered the whole leaf at the tillering stage. The lesion development was light-dependent. spl40 showed obvious cell death at and around the lesion, and burst of reactive oxygen species (ROS) was accompanied by disturbed ROS scavenging system. Photosynthetic capacity was compromised as evidenced by significant reductions in chlorophyll content, important photosynthesis parameters and downregulated expression of photosynthesis-related genes which ultimately led to poor performance of major agronomic traits. spl40 exhibited enhanced resistance to 14 out of 16 races of bacterial blight pathogen of rice, caused by Xanthomonas oryzae pv. oryzae, most probably though activation of SA and JA signaling pathways, owing to upregulated expression of SA and JA signaling genes, though the exact mechanism remain to be elucidated. The spotted-leaf phenotype was controlled by a novel single recessive nuclear gene. Genetic mapping combined with high throughput sequencing analysis identified Os05G0312000 as the most probable candidate gene. Sequencing of ORF revealed a single SNP change from C to T that resulted in non-synonymous change in amino acid residue from leucine to phenylalanine. Interestingly, the complementation plants did not display lesions before heading but showed lesions at the heading stage and the transgenic T₁ progenies could be classified into 3 categories based on their lesion intensity, indicating the complex genetic nature of the spl40 mutation.

CONCLUSION

The results obtained here clearly show that genes related to defense and PCD were upregulated in accordance with enhanced disease resistance and occurrence of PCD, whereas the photosynthetic capacity and overall ROS homeostasis was compromised in spl40. Our data suggest that a novel spotted-leaf mutant, spl40, would help to elucidate the mechanism behind lesion development involving programmed cell death and associated defense responses.

LMM24 Encodes Receptor-Like Cytoplasmic Kinase 109, Which Regulates Cell Death and Defense Responses in Rice

Lesion mimic mutants are excellent models for research on molecular mechanisms of cell death and defense responses in rice. We identified a new rice lesion mimic mutant lmm24 from a mutant pool of indica rice cultivar “ZhongHui8015”. The LMM24 gene was identified by MutMap, and LMM24 was confirmed as a receptor-like cytoplasmic kinase 109 by amino acid sequence analysis. The lmm24 mutant displayed dark brown lesions in leaves and growth retardation that were not observed in wild-type ZH8015. The results of histochemical staining and TUNEL assays showed enhanced ROS accumulation and cell death in lmm24. Chloroplast degradation was observed in lmm24 leaves, with decreased expression of photosynthesis-related genes and increased expression of the senescence-induced STAYGREEN (SGR) gene and other senescence-associated genes. Furthermore, lmm24 exhibited enhanced resistance to rice blast fungus Magnaporthe oryzae (M. oryzae) and up-regulation of defense response genes. Our data demonstrate that LMM24 regulates cell death and defense responses in rice.

Semi-dominant mutation in the cysteine-rich receptor-like kinase gene, ALS1, conducts constitutive defence response in rice

Plants have evolved a sophisticated two-branch defence system to prevent the growth and spread of pathogen infection. The novel Cys-rich repeat (CRR) containing receptor-like kinases, known as CRKs, were reported to mediate defence resistance in plants. For rice, there are only two reports of CRKs. A semi-dominant lesion mimic mutant als1 (apoptosis leaf and sheath 1) in rice was identified to demonstrate spontaneous lesions on the leaf blade and sheath. A map-based cloning strategy was used for fine mapping and cloning of ALS1, which was confirmed to be a typical CRK in rice. Functional studies of ALS1 were conducted, including phylogenetic analysis, expression analysis, subcellular location and blast resistance identification. Most pathogenesis-related (PR) genes and other defence-related genes were activated and up-regulated to a high degree. ALS1 was expressed mainly in the leaf blade and sheath, in which further study revealed that ALS1 was present in the vascular bundles. ALS1 was located in the cell membrane of rice protoplasts, and its mutation did not change its subcellular location. Jasmonic acid (JA) and salicylic acid (SA) accumulation were observed in als1, and enhanced blast resistance was also observed. The mutation of ALS1 caused a constitutively activated defence response in als1. The results of our study imply that ALS1 participates in a defence response resembling the common SA-, JA- and NH1-mediated defence responses in rice.

Combined Effect of CO2 and Temperature on Wheat Powdery Mildew Development

The effect of simulated climate changes by applying different temperatures and CO2 levels was investigated in the Blumeria graminis f. sp. tritici/wheat pathosystem. Healthy and inoculated plants were exposed in single phytotrons to six CO2+temperature combinations: (1) 450 ppm CO2/18-22°C (ambient CO2 and low temperature), (2) 850 ppm CO2/18-22°C (elevated CO2 and low temperature), (3) 450 ppm CO2/22-26°C (ambient CO2 and medium temperature), (4) 850 ppm CO2/22-26°C (elevated CO2 and medium temperature), (5) 450 ppm CO2/26-30°C (ambient CO2 and high temperature), and (6) 850 ppm CO2/26-30°C (elevated CO2 and high temperature). Powdery mildew disease index, fungal DNA quantity, plant death incidence, plant expression of pathogenesis-related (PR) genes, plant growth parameters, carbohydrate and chlorophyll content were evaluated. Both CO2 and temperature, and their interaction significantly influenced powdery mildew development. The most advantageous conditions for the progress of powdery mildew on wheat were low temperature and ambient CO2. High temperatures inhibited pathogen growth independent of CO2 conditions, and no typical powdery mildew symptoms were observed. Elevated CO2 did not stimulate powdery mildew development, but was detrimental for plant vitality. Similar abundance of three PR transcripts was found, and the level of their expression was different between six phytotron conditions. Real time PCR quantification of Bgt was in line with the disease index results, but this technique succeeded to detect the pathogen also in asymptomatic plants. Overall, future global warming scenarios may limit the development of powdery mildew on wheat in Mediterranean area, unless the pathogen will adapt to higher temperatures.

Excessive UDPG resulting from the mutation of UAP1 causes programmed cell death by triggering reactive oxygen species accumulation and caspase-like activity in rice

Lesion mimic mutants are valuable to unravel the mechanisms governing the programmed cell death (PCD) process. Uridine 5'-diphosphoglucose-glucose (UDPG) functions as a signaling molecule activating multiple pathways in animals, but little is known about its function in plants. Two novel allelic mutants of spl29 with typical PCD characters and reduced pollen viability were obtained by ethane methyl sulfonate mutagenesis in rice cv Kitaake. The enzymatic analyses showed that UDP-N-acetylglucosamine pyrophosphorylase 1 (UAP1) irreversibly catalyzed the decomposition of UDPG. Its activity was severely destroyed and caused excessive UDPG accumulation, with the lesion occurrence associated with the enhanced caspase-like activities in spl29-2. At the transcriptional level, several key genes involved in endoplasmic reticulum stress and the unfolded protein response were abnormally expressed. Moreover, exogenous UDPG could aggravate lesion initiation and development in spl29-2. Importantly, exogenous UDPG and its derivative UDP-N-acetylglucosamine could induce reactive oxygen species (ROS) accumulation and lesion mimics in Kitaake seedlings. These results suggest that the excessive accumulation of UDPG, caused by the mutation of UAP1, was a key biochemical event resulting in the lesion mimics in spl29-2. Thus, our findings revealed that UDPG might be an important component involved in ROS accumulation, PCD execution and lesion mimicking in rice, which also provided new clues for investigating the connection between sugar metabolism and PCD process.

Identification and Map-Based Cloning of the Light-Induced Lesion Mimic Mutant 1 (LIL1) Gene in Rice

The hypersensitive response (HR) is a mechanism by which plants prevent the spread of pathogen. Despite extensive study, the molecular mechanisms underlying HR remain poorly understood. Lesion mimic mutants (LMMs), such as LIL1 that was identified in an ethylmethane sulfonate mutagenized population of Indica rice (Oryza sativa L. ssp. Indica) 93-11, can be used to study the HR. Under natural field conditions, the leaves of LIL1 mutant plants exhibited light-induced, small, rust-red lesions that first appeared at the leaf tips and subsequently expanded throughout the entire leaf blade to the leaf sheath. Histochemical staining indicated that LIL1 lesions displayed an abnormal accumulation of reactive oxygen species (ROS) and resulted from programmed cell death (PCD). The LIL1 mutants also displayed increased expression of defense-related genes and enhanced resistance to rice blast fungus (Magnaporthe grisea). Genetic analysis showed that mutation of LIL1 created a semi-dominant allele. Using 1,758 individuals in the F2 population, LIL1 was mapped in a 222.3 kb region on the long arm of chromosome 7. That contains 12 predicted open reading frames (ORFs). Sequence analysis of these 12 candidate genes revealed a G to A base substitution in the fourth exon of LOC_Os07g30510, a putative cysteine-rich receptor-like kinase (CRK), which led to an amino acid change (Val 429 to Ile) in the LIL1 protein. Comparison of the transcript accumulation of the 12 candidate genes between LIL1 and 93-11 revealed that LOC_Os07g30510 was up-regulated significantly in LIL1. Overexpression of the LOC_Os07g30510 gene from LIL1 induced a LIL1-like lesion phenotype in Nipponbare. Thus, LIL1 is a novel LMM in rice that will facilitate the further study of the molecular mechanisms of HR and the rice blast resistance.

Silencing of RBOHF2 Causes Leaf Age-Dependent Accelerated Senescence, Salicylic Acid Accumulation, and Powdery Mildew Resistance in Barley

Plant RBOH (RESPIRATORY BURST OXIDASE HOMOLOGS)-type NADPH oxidases produce superoxide radical anions and have a function in developmental processes and in response to environmental challenges. Barley RBOHF2 has diverse reported functions in interaction with the biotrophic powdery mildew fungus Blumeria graminis f. sp. hordei. Here, we analyzed, in detail, plant leaf level- and age-specific susceptibility of stably RBOHF2-silenced barley plants. This revealed enhanced susceptibility to fungal penetration of young RBOHF2-silenced leaf tissue but strongly reduced susceptibility of older leaves when compared with controls. Loss of susceptibility in old RBOHF2-silenced leaves was associated with spontaneous leaf-tip necrosis and constitutively elevated levels of free and conjugated salicylic acid. Additionally, these leaves more strongly expressed pathogenesis-related genes, both constitutively and during interaction with B. graminis f. sp. hordei. Together, this supports the idea that barley RBOHF2 contributes to basal resistance to powdery mildew infection in young leaf tissue but is required to control leaf cell death, salicylic acid accumulation, and defense gene expression in older leaves, explaining leaf age-specific resistance of RBOHF2-silenced barley plants.

5-Aminolevulinic Acid Dehydratase Gene Dosage Affects Programmed Cell Death and Immunity

Programmed cell death (PCD) is an important form to protect plants from pathogen attack. However, plants must precisely control the PCD process under microbe attacks to avoid detrimental effects. The complexity of how plants balance the defense activation and PCD requires further clarification. Lesion mimic mutants constitute an excellent material to study the crosstalk between them. Here, we identified a Gossypium hirsutum (cotton) lesion mimic mutant (Ghlmm), which exhibits necrotic leaf damage and enhanced disease resistance. Map-based cloning demonstrated that GhLMMD, encoding 5-aminolevulinic acid dehydratase and located on chromosome D5, was responsible for the phenotype. The mutant was resulted from a nonsense mutation within the coding region of GhLMMD It exhibited an overaccumulation of the 5-aminolevulinic acid, elevated levels of reactive oxygen species and salicylic acid, along with constitutive expression of pathogenesis-related genes and enhanced resistance to the Verticillium dahliae infection. Interestingly, GhLMM plays a dosage-dependent role in regulating PCD of cotton leaves and resistance to V. dahliae infection. This study provides a new strategy on the modulation of plant immunity, particularly in polyploidy plants.