A novel, conditional, lesion mimic phenotype in cotton cotyledons due to the expression of an endochitinase gene from Trichoderma virens

A Novel Single Base Mutation in OsSPL42 Leads to the Formation of Leaf Lesions in Rice

Rice spotted-leaf mutants serve as valuable resources for studying plant programmed cell death (PCD) and disease resistance mechanisms, making them crucial for research on disease resistance in rice. Map-based cloning was used to identify and clone the spotted-leaf gene OsSPL42. Then, functional complementation and CRISPR/Cas9 techniques were also employed to further validate the function of this gene. By applying leaf clippings for bacterial blight (BB) inoculation, the BB resistance of different rice lines was assessed. The results in this study were as follows: The OsSPL42 behaved as a recessive nuclear gene and was narrowed down to a 111 kb region on chromosome 8. All T0 transgenic rice plants in the complementation experiments exhibited a wild-type phenotype, without any lesion spots on the rice leaves. This suggests that the LOC_Os08g06100 encoding O-methyltransferase is the candidate gene for the mutant spl42. The OsSpl42 is widely expressed and the OsSPL42-GFP protein is mainly localized in the cytoplasm. OsSPL42 overexpression lines are more susceptible to BBs, which indicates that OsSPL42 may act as a negative regulator of rice resistance to BB. In summary, we speculate that OsSPL42 plays an important role in the regulation of pathogen response, providing new insights into plant defense mechanisms.

The Gain-of-Function Mutation, OsSpl26, Positively Regulates Plant Immunity in Rice

Rice spotted-leaf mutants are ideal materials to study the molecular mechanism underlying programmed cell death and disease resistance in plants. LOC_Os07g04820 has previously been identified as the candidate gene responsible for the spotted-leaf phenotype in rice Spotted-leaf 26 (Spl26) mutant. Here, we cloned and validated that LOC_Os07g04820 is the locus controlling the spotted-leaf phenotype of Spl26 by reverse functional complementation and CRISPR/Cas9-mediated knockout of the mutant allele. The recessive wild-type spl26 allele (Oryza sativa spotted-leaf 26, Osspl26) is highly conservative in grass species and encodes a putative G-type lectin S-receptor-like serine/threonine protein kinase with 444 amino acid residuals. OsSPL26 localizes to the plasma membrane and can be detected constitutively in roots, stems, leaves, sheaths and panicles. The single base substitution from T to A at position 293 leads to phenylalanine/tyrosine replacement at position 98 in the encoded protein in the mutant and induces excessive accumulation of H₂O₂, leading to oxidative damage to cells, and finally, formation of the spotted-leaf phenotype in Spl26. The formation of lesions not only affects the growth and development of the plants but also activates the defense response and enhances the resistance to the bacterial blight pathogen, Xanthomonas oryzae pv. oryzae. Our results indicate that the gain-of-function by the mutant allele OsSpl26 positively regulates cell death and immunity in rice.

A dominant spotted leaf gene TaSpl1 activates endocytosis and defense-related genes causing cell death in the absence of dominant inhibitors

The spotted leaf lesion mimic trait simulates cell death in a plant responding to pathogen infection. Some spotted leaf genes are recessive, while others are dominant. A small number of plants with a lesion mimic phenotype appeared in a segregating population obtained by crossing two normal green wheat strains, XN509 and N07216. Here, we clarified the genetic model and its breeding value. Phenotyping of the consecutive progeny populations over six cropping seasons showed that the spotted leaf lesion mimic phenotype was controlled by a dominant gene designated TaSpl1, which was inhibited by two other dominant genes, designated TaSpl1-I₁ and TaSpl1-I₂. Using bulked segregant analysis RNA-seq (BSR-Seq) and newly developed KASP-PCR markers, the TaSpl1 and TaSpl1-I₁ loci in N07216 were mapped to the end of chromosomes 3DS and 3BS, respectively. Plants with the spotted phenotype showed lower levels of stripe rust and powdery mildew than those with the normal green phenotype. Compared with normal leaves, the differentially expressed genes in spotted leaves were significantly enriched in plant-pathogen interaction and endocytosis pathways. There were no differences in the yield parameters of the F₅ and F₆ sister lines, N13039S with TaSpl1 and N13039 N without TaSpl1. These results provide a greater understanding of spotted leaf phenotyping and the breeding value of the lesion mimic allele in developing disease-resistance varieties.

Rice Lesion Mimic Mutants (LMM): The Current Understanding of Genetic Mutations in the Failure of ROS Scavenging during Lesion Formation

Rice lesion mimic mutants (LMMs) form spontaneous lesions on the leaves during vegetative growth without pathogenic infections. The rice LMM group includes various mutants, including spotted leaf mutants, brown leaf mutants, white-stripe leaf mutants, and other lesion-phenotypic mutants. These LMM mutants exhibit a common phenotype of lesions on the leaves linked to chloroplast destruction caused by the eruption of reactive oxygen species (ROS) in the photosynthesis process. This process instigates the hypersensitive response (HR) and programmed cell death (PCD), resulting in lesion formation. The reasons for lesion formation have been studied extensively in terms of genetics and molecular biology to understand the pathogen and stress responses. In rice, the lesion phenotypes of most rice LMMs are inherited according to the Mendelian principles of inheritance, which remain in the subsequent generations. These rice LMM genetic traits have highly developed innate self-defense mechanisms. Thus, although rice LMM plants have undesirable agronomic traits, the genetic principles of LMM phenotypes can be used to obtain high grain yields by deciphering the efficiency of photosynthesis, disease resistance, and environmental stress responses. From these ailing rice LMM plants, rice geneticists have discovered novel proteins and physiological causes of ROS in photosynthesis and defense mechanisms. This review discusses recent studies on rice LMMs for the Mendelian inheritances, molecular genetic mapping, and the genetic definition of each mutant gene.

Characterization and Identification of a woody lesion mimic mutant lmd, showing defence response and resistance to Alternaria alternate in birch

Lesion mimic mutants (LMM) usually show spontaneous cell death and enhanced defence responses similar to hypersensitive response (HR) in plants. Many LMM have been reported in rice, wheat, maize, barley, Arabidopsis, etc., but little was reported in xylophyta. BpGH3.5 is an early auxin-response factor which regulates root elongation in birch. Here, we found a T-DNA insertion mutant in a BpGH3.5 transgenic line named lmd showing typical LMM characters and early leaf senescence in Betula platyphylla × B. pendula. lmd showed H₂O₂ accumulation, increased SA level and enhanced resistance to Alternaria alternate, compared with oe21 (another BpGH3.5 transgenic line) and NT (non-transgenic line). Cellular structure observation showed that programmed cell death occurred in lmd leaves. Stereomicroscope observation and Evans’ blue staining indicated that lmd is a member of initiation class of LMM. Transcriptome analysis indicated that defence response-related pathways were enriched. Southern-blot indicated that there were two insertion sites in lmd genome. Genome re-sequencing and thermal asymmetric interlaced PCR (TAIL-PCR) confirmed the two insertion sites, one of which is a T-DNA insertion in the promoter of BpEIL1 that may account for the lesion mimic phenotype. This study will benefit future research on programmed cell death, HR and disease resistance in woody plants.

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

Lesion mimics (LMs) that exhibit spontaneous disease-like lesions in the absence of pathogen attack might confer enhanced plant disease resistance to a wide range of pathogens. The LM mutant, lm3 was derived from a single naturally mutated individual in the F₁ population of a 3-1/Jing411 cross, backcrossed six times with 3–1 as the recurrent parent and subsequently self-pollinated twice. The leaves of young seedlings of the lm3 mutant exhibited small, discrete white lesions under natural field conditions. The lesions first appeared at the leaf tips and subsequently expanded throughout the entire leaf blade to the leaf sheath. The lesions were initiated through light intensity and day length. Histochemical staining revealed that lesion formation might reflect programmed cell death (PCD) and abnormal accumulation of reactive oxygen species (ROS). The chlorophyll content in the mutant was significantly lower than that in wildtype, and the ratio of chlorophyll a/b was increased significantly in the mutant compared with wildtype, indicating that lm3 showed impairment of the biosynthesis or degradation of chlorophyll, and that Chlorophyll b was prone to damage during lesion formation. The lm3 mutant exhibited enhanced resistance to wheat powdery mildew fungus (Blumeria graminis f. sp. tritici; Bgt) infection, which was consistent with the increased expression of seven pathogenesis-related (PR) and two wheat chemically induced (WCI) genes involved in the defense-related reaction. Genetic analysis showed that the mutation was controlled through a single partially dominant gene, which was closely linked to Xbarc203 on chromosome 3BL; this gene was delimited to a 40 Mb region between SSR3B450.37 and SSR3B492.6 using a large derived segregating population and the available Chinese Spring chromosome 3B genome sequence. Taken together, our results provide information regarding the identification of a novel wheat LM gene, which will facilitate the additional fine-mapping and cloning of the gene to understand the mechanism underlying LM initiation and disease resistance in common wheat.

Trichoderma genes in plants for stress tolerance- status and prospects

Many filamentous fungi from the genus Trichoderma are well known for their anti-microbial properties. Certain genes from Trichoderma spp. have been identified and transferred to plants for improving biotic and abiotic stress tolerance, as well for applications in bioremediation. Several Trichoderma genomes have been sequenced and many are in the pipeline, facilitating high throughput gene analysis and increasing the availability of candidate transgenes. This, coupled with improved plant transformation systems, is expected to usher in a new era in plant biotechnology where several genes from these antagonistic fungi can be transferred into plants to achieve enhanced stress tolerance, bioremediation activity, herbicide tolerance, and reduction of phytotoxins. In this review, we illustrate the major achievements made by transforming plants with Trichoderma genes as well as their possible mode of action. Moreover, examples of efficient application of genetically modified plants as biofactories to produce active Trichoderma enzymes are indicated.

Enhanced resistance against Thielaviopsis basicola in transgenic cotton plants expressing Arabidopsis NPR1 gene

Black root rot, caused by Thielaviopsis basicola, is an important disease in several crops including cotton. We studied the response of Arabidopsis NPR1 (AtNPR1)-expressing cotton lines, previously shown to be highly resistant to a diverse set of pathogens, to a challenge from T. basicola. In four different experiments, we found significant degree of tolerance in the transgenic lines to black root rot. Although transformants showed the typical root discoloration symptoms similar to the wild-type control plants following infection, their roots tended to recover faster and resumed normal growth. Better performance of transgenic plants is reflected by the fact that they have significantly higher shoot and root mass, longer shoot length, and greater number of boll-set. Transcriptional analysis of the defense response showed that the roots of AtNPR1-overexpressing transgenic plants exhibited stronger and faster induction of most of these defense-related genes, particularly PR1, thaumatin, glucanase, LOX1, and chitinase. The results obtained in this investigation provide further support for a broad-spectrum nature of the resistance conferred by overexpression of AtNPR1 gene in cotton.