Inheritance and Quantitative Trait Locus Mapping of Fusarium Wilt Resistance in Cucumber

Joint metabolomic and transcriptomic analysis identify unique phenolic acid and flavonoid compounds associated with resistance to fusarium wilt in cucumber (Cucumis sativus L.)

Introduction

Fusarium wilt (FW) caused by Fusarium oxysporum f. sp. cucumerinum (Foc) is a destructive soil-borne disease in cucumber (Cucumis sativus. L). However, there remains limited knowledge on the molecular mechanisms underlying FW resistance-mediated defense responses in cucumber.

Methods

In this study, metabolome and transcriptome profiling were carried out for two FW resistant (NR) and susceptible (NS), near isogenic lines (NILs) before and after Foc inoculation. NILs have shown consistent and stable resistance in multiple resistance tests conducted in the greenhouse and in the laboratory. A widely targeted metabolomic analysis identified differentially accumulated metabolites (DAMs) with significantly greater NR accumulation in response to Foc infection, including many phenolic acid and flavonoid compounds from the flavonoid biosynthesis pathway.

Results

Transcriptome analysis identified differentially expressed genes (DEGs) between the NILs upon Foc inoculation including genes for secondary metabolite biosynthesis and transcription factor genes regulating the flavonoid biosynthesis pathway. Joint analysis of the metabolomic and transcriptomic data identified DAMs and DEGs closely associated with the biosynthesis of phenolic acid and flavonoid DAMs. The association of these compounds with NR-conferred FW resistance was exemplified by in vivo assays. These assays found two phenolic acid compounds, bis (2-ethylhexyl) phthalate and diisooctyl phthalate, as well as the flavonoid compound gallocatechin 3-O-gallate to have significant inhibitory effects on Foc growth. The antifungal effects of these three compounds represent a novel finding.

Discussion

Therefore, phenolic acids and flavonoids play important roles in NR mediated FW resistance breeding in cucumber.

Fine mapping and analysis of a candidate gene controlling Phytophthora blight resistance in cucumber

Cucumber blight is a destructive disease. The best way to control this disease is resistance breeding. This study focuses on disease resistance gene mapping and molecular marker development. We used the resistant cucumber, JSH, and susceptible cucumber, B80, as parents to construct F2 populations. Bulked segregant analysis (BSA) combined with specific length amplified fragment sequencing (SLAF-seq) were used, from which we developed cleaved amplified polymorphic sequence (CAPs) markers to map the resistance gene. Resistance in F2 individuals showed a segregation ratio of resistance:susceptibility close to 3:1. The gene in JSH resistant cucumber was mapped to an interval of 9.25 kb, and sequencing results for the three genes in the mapped region revealed three mutations at base sites 225, 302, and 591 in the coding region of Csa5G139130 between JSH and B80, but no mutations in coding regions of Csa5G139140 and Csa5G139150. The mutations caused changes in amino acids 75 and 101 of the protein encoded by Csa5G139130, suggesting that Csa5G139130 is the most likely resistance candidate gene. We developed a molecular marker, CAPs-4, as a closely linked marker for the cucumber blight resistance gene. This is the first report on mapping of a cucumber blight resistance gene and will provideg a useful marker for molecular breeding of cucumber resistance to Phytophthora blight.

The exocyst subunit CsExo70B promotes both fruit length and disease resistance via regulating receptor kinase abundance at plasma membrane in cucumber

Plant defence against pathogens generally occurs at the expense of growth and yield. Uncoupling the inverse relationship between growth and defence is of great importance for crop breeding, while the underlying genes and regulatory mechanisms remain largely elusive. The exocytosis complex was shown to play an important role in the trafficking of receptor kinases (RKs) to the plasma membrane (PM). Here, we found a Cucumis sativus exocytosis subunit Exo70B (CsExo70B) regulates the abundance of both development and defence RKs at the PM to promote fruit elongation and disease resistance in cucumber. Knockout of CsExo70B resulted in shorter fruit and susceptibility to pathogens. Mechanistically, CsExo70B associates with the developmental RK CsERECTA, which promotes fruit longitudinal growth in cucumber, and contributes to its accumulation at the PM. On the other side, CsExo70B confers to the spectrum resistance to pathogens in cucumber via a similar regulatory module of defence RKs. Moreover, CsExo70B overexpression lines showed an increased fruit yield as well as disease resistance. Collectively, our work reveals a regulatory mechanism that CsExo70B promotes both fruit elongation and disease resistance by maintaining appropriate RK levels at the PM and thus provides a possible strategy for superior cucumber breeding with high yield and robust pathogen resistance.

Isolation and identification of the endophytic fungus J2-3 and its disease-preventive and growth-promoting effects on cucumber

There are many problems that result from the use of a large number of chemical pesticides to control plant diseases, including pathogenic bacteria resistance, environmental contamination, and human health effects. Recently, endophytic fungi have become a significant source of bioactive fungicide products and an invaluable resource for excavating microbial pesticides. In this study, endophytic fungi with biocontrol potential were isolated and screened from Mikania micrantha leaves, stems, and roots. Fifty endophytic fungi were isolated and their antagonistic activity was studied in vitro using the confrontation culture method. The J2-3 strains from stems exhibit broad-spectrum and high activity. The strain's biological characteristics were determined by various culture conditions, and it was identified as Fusarium proliferatum by both morphological and ITS sequence analysis. Biological characteristics of the J2-3 strain were also tested. The optimum temperature for mycelium growth and sporulation was 25 °C and 30 °C, respectively. For mycelium growth, starch was the optimum carbon source, and peptone was the optimum nitrogen source for sucrose, mycelium growth, and sporulation. Mycelium growth was killed by a temperature of 60 °C, and sporulation was killed by a temperature of 55 °C. The light aided mycelium growth, and the light alternated between light and dark cycles for sporulation. Further, pot experiments were conducted to determine the antagonistic and viable effects of highly antagonistic strains on cucumber. The spore suspension's final control efficacy on cucumber wilt disease was up to 62.79% and it also promoted cucumber growth significantly. The results show that the entophytic fungus J2-3 from M. micrantha can protect cucumbers from wilt disease and promote growth.

Genetic analysis of mixed models of fruit sugar-acid fractions in a cross between jujube (Ziziphus jujuba Mill.) and wild jujube (Z. acido jujuba)

Chinese jujube (Ziziphus jujuba Mill.), an economically significant species in the Rhamnaceae family, is a popular fruit tree in Asia. The sugar and acid concentrations in jujube are considerably higher than those in other plants. Due to the low kernel rate, it is extremely difficult to establish hybrid populations. Little is known about jujube evolution and domestication, particularly with regard to the role of the sugar and acid components of jujube. Therefore, we used cover net control as a hybridization technique for the cross-breeding of Ziziphus jujuba Mill and 'JMS2' and (Z. acido jujuba) 'Xing16' to obtain an F₁ population (179 hybrid progeny). The sugar and acid levels in the F₁ and parent fruit were determined by HPLC. The coefficient of variation ranged from 28.4 to 93.9%. The sucrose and quinic acid levels in the progeny were higher than those in the parents. The population showed continuous distributions with transgressive segregation on both sides. Analysis by the mixed major gene and polygene inheritance model was performed. It was found that glucose is controlled by one additive-dominant major gene and polygenes, malic acid is controlled by two additive-dominant major genes and polygenes, and oxalic acid and quinic acid are controlled by two additive-dominant-epistatic major genes and polygenes. The results of this study provide insights into the genetic predisposition and molecular mechanisms underlying the role of sugar acids in jujube fruit.

Research Advances in Genetic Mechanisms of Major Cucumber Diseases Resistance

Cucumber (Cucumis sativus L.) is an important economic vegetable crop worldwide that is susceptible to various common pathogens, including powdery mildew (PM), downy mildew (DM), and Fusarium wilt (FM). In cucumber breeding programs, identifying disease resistance and related molecular markers is generally a top priority. PM, DM, and FW are the major diseases of cucumber in China that cause severe yield losses and the genetic-based cucumber resistance against these diseases has been developed over the last decade. Still, the molecular mechanisms of cucumber disease resistance remain unclear. In this review, we summarize recent findings on the inheritance, molecular markers, and quantitative trait locus mapping of cucumber PM, DM, and FM resistance. In addition, several candidate genes, such as PM, DM, and FM resistance genes, with or without functional verification are reviewed. The data help to reveal the molecular mechanisms of cucumber disease resistance and provide exciting new opportunities for further resistance breeding.