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

BpEIN3.1 represses leaf senescence by inhibiting synthesis of ethylene and abscisic acid in Betula platyphylla

Leaf senescence and abscission play crucial role in annual plant adapting to seasonal alteration and climate changes by shortening life cycle and development process in response to abiotic and/or biotic stressors underlying phytohormones and environmental signals. Ethylene and abscisic acid are the major phytohormones that promotes leaf senescence, involving various transcription factors, such as EIN3 (ethylene-insensitive 3) and EIL (ethylene-insensitive 3-like) gene family, controlling leaf senescence through metabolite biosynthesis and signal transduction pathways. However, the roles of EIN3 regulating leaf senescence responding to environmental changes in perennial plant, especially forestry tree, remain unclear. In this study, we found that BpEIN3.1 from a subordinated to EIL3 subclade, is a transcription repressor and regulated light-dependent premature leaf senescence in birch (Betula platyphylla). BpEIN3.1 might inhibits the transcription of BpATPS1 by binding to its promoter. Shading suppressed premature leaf senescence in birch ein3.1 mutant line. Ethylene and abscisic acid biosynthesis were also reduced. In addition, abscisic acid positively regulated the expression of BpEIN3.1. This was demonstrated by the hormone-response element analysis of BpEIN3.1 promoter and its gene expression after the hormone treatments. Moreover, our results showed that abscisic acid is also involved in maintaining homeostasis. The molecular mechanism of leaf senescence provides a possibility to increasing wood production by delaying of leaf senescence.

A Phosphoproteomics Study of the Soybean root necrosis 1 Mutant Revealed Type II Metacaspases Involved in Cell Death Pathway

The soybean root necrosis 1 (rn1) mutation causes progressive browning of the roots soon after germination and provides increased tolerance to the soil-borne oomycete pathogen Phytophthora sojae in soybean. Toward understanding the molecular basis of the rn1 mutant phenotypes, we conducted tandem mass tag (TMT)-labeling proteomics and phosphoproteomics analyses of the root tissues of the rn1 mutant and progenitor T322 line to identify potential proteins involved in manifestation of the mutant phenotype. We identified 3,160 proteins. When the p-value was set at ≤0.05 and the fold change of protein accumulation between rn1 and T322 at ≥1.5 or ≤0.67, we detected 118 proteins that showed increased levels and 32 proteins decreased levels in rn1 as compared to that in T322. The differentially accumulated proteins (DAPs) are involved in several pathways including cellular processes for processing environmental and genetic information, metabolism and organismal systems. Five pathogenesis-related proteins were accumulated to higher levels in the mutant as compared to that in T322. Several of the DAPs are involved in hormone signaling, redox reaction, signal transduction, and cell wall modification processes activated in plant-pathogen interactions. The phosphoproteomics analysis identified 22 phosphopeptides, the levels of phosphorylation of which were significantly different between rn1 and T322 lines. The phosphorylation levels of two type II metacaspases were reduced in rn1 as compared to T322. Type II metacaspase has been shown to be a negative regulator of hypersensitive cell death. In absence of the functional Rn1 protein, two type II metacaspases exhibited reduced phosphorylation levels and failed to show negative regulatory cell death function in the soybean rn1 mutant. We hypothesize that Rn1 directly or indirectly phosphorylates type II metacaspases to negatively regulate the cell death process in soybean roots.

BpEIL1 negatively regulates resistance to Rhizoctonia solani and Alternaria alternata in birch

Pathogen attacks affect tree health, causing considerable economic losses as well as serious damage to the surrounding environment. Understanding the disease resistance mechanisms of trees is important for tree breeding. In previous studies on birch (Betula platyphylla × B. pendula), we identified a lesion mimic mutant called lmd. We found that reduced expression of BpEIL1 was responsible for the phenotype in lmd. Following cloning, we acquired several BpEIL1 overexpression and suppression lines in birch. In this study, we cloned the BpEIL1 promoter and found that BpEIL1 was primarily expressed in leaves, particularly in veins. We further studied the traits of transgenic lines and the function of BpEIL1 in disease resistance in birch using the BpEIL1 overexpression line OE9, the suppression line SE13 and the non-transgenic line NT. We found that hydrogen peroxide accumulated in SE13 leaves. Ascorbate peroxidase and catalase activity significantly increased in SE13. SE13 was more resistant to the fungal pathogens Alternaria alternata and Rhizoctonia solani than were the OE9 and NT lines. RNA-seq indicated that pathways related to signal transduction, disease resistance and plant immunity were enriched in SE13. BpEIL1 is thus a negative regulatory transcription factor for disease resistance in birch. This study provides a reference for disease resistance of birch and other trees.

Endophytic Streptomyces hygroscopicus OsiSh-2-Mediated Balancing between Growth and Disease Resistance in Host Rice

Plants fine-tune the growth-defense trade-off to survive when facing pathogens. Meanwhile, plant-associated microbes, such as the endophytes inside plant tissues, can benefit plant growth and stress resilience. However, the mechanisms for the beneficial microbes to increase stress resistance with little yield penalty in host plants remain poorly understood. In the present study, we report that endophytic Streptomyces hygroscopicus OsiSh-2 can form a sophisticated interaction with host rice, maintaining cellular homeostasis under pathogen-infection stress, and optimize plant growth and disease resistance in rice. Four-year field trials consistently showed that OsiSh-2 could boost host resistance to rice blast pathogen Magnaporthe oryzae while still maintaining a high yield. The integration of the proteomic, physiological, and transcriptional profiling analysis revealed that OsiSh-2 induced rice defense priming and controlled the expression of energy-consuming defense-related proteins, thus increasing the defense capability with the minimized costs of plant immunity. Meanwhile, OsiSh-2 improved the chloroplast development and optimally maintained the expression of proteins related to plant growth under pathogen stress, thus promoting the crop yield. Our results provided a representative example of an endophyte-mediated modulation of disease resistance and fitness in the host plant. The multilayer effects of OsiSh-2 implicate a promising future of using endophytic actinobacteria for disease control and crop yield promotion.

Characterization and Rapid Gene-Mapping of Leaf Lesion Mimic Phenotype of spl-1 Mutant in Soybean (Glycine max (L.) Merr.)

In plants, lesion mimic mutants (LMMs) reveal spontaneous disease-like lesions in the absence of pathogen that constitutes powerful genetic material to unravel genes underlying programmed cell death (PCD), particularly the hypersensitive response (HR). However, only a few LMMs are reported in soybean, and no related gene has been cloned until now. In the present study, we isolated a new LMM named spotted leaf-1 (spl-1) from NN1138-2 cultivar through ethyl methanesulfonate (EMS) treatment. The present study revealed that lesion formation might result from PCD and excessive reactive oxygen species (ROS) accumulation. The chlorophyll content was significantly reduced but antioxidant activities, viz., superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT), as well as the malondialdehyde (MDA) contents, were detected higher in spl-1 than in the wild-type. According to segregation analysis of mutant phenotype in two genetic populations, viz., W82×spl-1 and PI378692×spl-1, the spotted leaf phenotype of spl-1 is controlled by a single recessive gene named lm1. The lm1 locus governing mutant phenotype of spl-1 was first identified in 3.15 Mb genomic region on chromosome 04 through MutMap analysis, which was further verified and fine mapped by simple sequence repeat (SSR) marker-based genetic mapping. Genetic linkage analysis narrowed the genomic region (lm1 locus) for mutant phenotype to a physical distance of ~76.23 kb. By searching against the Phytozome database, eight annotated candidate genes were found within the lm1 region. qRT-PCR expression analysis revealed that, among these eight genes, only Glyma.04g242300 showed highly significant expression levels in wild-type relative to the spl-1 mutant. However, sequencing data of the CDS region showed no nucleotide difference between spl-1 and its wild type within the coding regions of these genes but might be in the non-coding regions such as 5' or 3' UTR. Hence, the data of the present study are in favor of Glyma.04g242300 being the possible candidate genes regulating the mutant phenotype of spl-1. However, further validation is needed to prove this function of the gene as well as its role in PCD, which in turn would be helpful to understand the mechanism and pathways involved in HR disease resistance of soybean.