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The Gy14 cucumber (Cucumis sativus) is resistant to oomyceteous downy mildew (DM), bacterial angular leaf spot (ALS) and fungal anthracnose (AR) pathogens, but the underlying molecular mechanisms are unknown. Quantitative trait locus (QTL) mapping for the disease resistances in Gy14 and further map-based cloning identified a candidate gene for the resistant loci, which was validated and functionally characterized by spatial-temporal gene expression profiling, allelic diversity and phylogenetic analysis, as well as local association studies. We showed that the triple-disease resistances in Gy14 were controlled by the cucumber STAYGREEN (CsSGR) gene. A single nucleotide polymorphism (SNP) in the coding region resulted in a nonsynonymous amino acid substitution in the CsSGR protein, and thus disease resistance. Genes in the chlorophyll degradation pathway showed differential expression between resistant and susceptible lines in response to pathogen inoculation. The causal SNP was significantly associated with disease resistances in natural and breeding populations. The resistance allele has undergone selection in cucumber breeding. The durable, broad-spectrum disease resistance is caused by a loss-of-susceptibility mutation of CsSGR. Probably, this is achieved through the inhibition of reactive oxygen species over-accumulation and phytotoxic catabolite over-buildup in the chlorophyll degradation pathway. The CsSGR-mediated host resistance represents a novel function of this highly conserved gene in plants.

Host resistances in PI 197088 cucumber to downy and powdery mildew pathogens are conferred by 11 (3 with major effect) and 4 (1 major effect) QTL, respectively, and three of which are co-localized. The downy mildew (DM) and powdery mildew (PM) are the two most important foliar diseases of cucurbit crops worldwide. The cucumber accession PI 197088 exhibits high-level resistances to both pathogens. Here, we reported QTL mapping results for DM and PM resistances with 148 recombinant inbred lines from a cross between PI 197088 and the susceptible line 'Coolgreen'. Phenotypic data on responses to natural DM and PM infection were collected in multi-year and multi-location replicated field trials. A high-density genetic map with 2780 single nucleotide polymorphisms (SNPs) from genotyping-by-sequencing and 55 microsatellite markers was developed, which revealed genomic regions with segregation distortion and mis-assemblies in the '9930' cucumber draft genome. QTL analysis identified 11 and 4 QTL for DM and PM resistances accounting for more than 73.5 and 63.0% total phenotypic variance, respectively. Among the 11 DM resistance QTL, dm5.1, dm5.2, and dm5.3 were major-effect contributing QTL, whereas dm1.1, dm2.1, and dm6.2 conferred susceptibility. Of the 4 QTL for PM resistance, pm5.1 was the major-effect QTL explaining 32.4% phenotypic variance and the minor-effect QTL pm6.1 contributed to disease susceptibility. Three PM QTL, pm2.1, pm5.1, and pm6.1, were co-localized with DM QTL dm2.1, dm5.2, and dm6.1, respectively, which was consistent with the observed linkage of PM and DM resistances in PI 197088. The genetic architecture of DM resistance in PI 197088 and another resistant line WI7120 (PI 330628) was compared, and the potential of using PI 197088 in cucumber breeding for downy and powdery mildew resistances is discussed.

Downy mildew, caused by the oomycete pathogen Pseudoperonospora cubensis (Berkeley & Curtis) Rostov, is a major foliar disease of cucumber. Ten years after the reemergence of P. cubensis, downy mildew continues to be a major threat to cucumber production in the United States. Cucumber accessions with high levels of resistance have been identified. Development of cultivars with high levels of resistance remains an important objective of cucumber breeding programs. We tested a set of cucumber cultigens, including highly resistant PI accessions and susceptible control lines, to observe the effect of plant age on resistance. Cultigens responded differently to disease across plant developmental stages. In general, older plants had more disease symptoms, even those classified as resistant, such as PI 197088. However, PI 330628 and PI 605996 held their resistance even at late developmental stages. It is possible that these lines were resistant at late stages due to other factors, such as their rapid, indeterminate growth, that allows them to outgrow the disease. However, although PI 197088 appears to have a rapid, indeterminate growth habit, it did not have more resistance at later stages of plant maturity. Regardless of the mechanism involved, plant breeders should use the genetic resistance in PI 330628 and PI 605996 over PI 197088.

Host resistance in WI7120 cucumber to prevailing downy mildew pathogen field populations is conferred by two major-effect, one moderate-effect and two minor-effect QTL. Downy mildew (DM) caused by the obligate oomycete Pseudoperonospora cubensis is the most devastating fungal disease of cucumber worldwide. The molecular mechanism of DM resistance in cucumber is poorly understood, and use of marker-assisted breeding for DM resistance is not widely available. Here, we reported QTL mapping results for DM resistance with 243 F2:3 families from the cross between DM-resistant inbred line WI7120 (PI 330628) and susceptible ‘9930’. A linkage map was developed with 348 SSR and SNP markers. Phenotyping of DM inoculation responses were conducted in four field trails in 2 years at three locations. Four QTL, dm2.1, dm4.1, dm5.1, and dm6.1 were consistently and reliably detected across at least three of the four environments which together could explain 62–76 % phenotypic variations (R 2). Among them, dm4.1 and dm5.1 were major-effect QTL (R 2 = 15–30 %) with only additive effects; dm2.1 (R 2 = 5–15 %) and dm6.1 (R 2 = 4–8 %) had moderate and minor effects, respectively. Epistatic effects were detected for dm2.1 and dm6.1 with both dm4.1 and dm5.1. One additional minor-effect QTL, dm6.2 (R 2 = 3–5 %) was only detectable with the chlorosis rating criterion. All alleles contributing to DM resistance were from WI7120. This study revealed two novel QTL for DM resistance and the unique genetic architecture of DM resistance in WI7120 conferring high level resistance to prevailing DM populations in multiple countries. The effects of disease rating scales, rating time and criteria, population size in phenotyping DM resistance on the power of QTL detection, and the use of DM resistance in WI7120 in cucumber breeding were discussed.

S-adenosyl-L-methionine (SAM) synthetase is the key enzyme involved in the biosynthesis of SAM, which serves as a common precursor for polyamines (PAs) and ethylene. A SAM synthetase cDNA (SlSAMS1) was introduced into the tomato genome using the Agrobacterium tumefaciens transformation method. Transgenic plants overexpressing SlSAMS1 exhibited a significant increase in tolerance to alkali stress and maintained nutrient balance, higher photosynthetic capacity and lower oxidative stress compared with WT lines. Both in vivo and in vitro experiments indicated that the function of SlSAMS1 mainly depended on the accumulation of Spd and Spm in the transgenic lines. A grafting experiment showed that rootstocks from SlSAMS1-overexpressing plants provided a stronger root system, increased PAs accumulation, essential elements absorption, and decreased Na(+) absorption in the scions under alkali stress. As a result, fruit set and yield were significantly enhanced. To our knowledge, this is the first report to provide evidence that SlSAMS1 positively regulates tomato tolerance to alkali stress and plays a major role in modulating polyamine metabolism, resulting in maintainability of nutrient and ROS balance.

Detrimental effects of NaCl and NaHCO3 stress on tomato plants were compared. With increasing stress intensity, accumulation of biomass in tomato shoots and roots significantly decreased in both NaCl and NaHCO3 treatments. Compared to NaCl stress, NaHCO3 stress showed greater reduction of tomato plant growth, which was in accordance with stronger inhibition of photosynthesis by NaHCO3 stress. Chlorophyll (Chl) contents, the ratio of the variable fluorescence to the maximum fluorescence (Fv/Fm) and the actual quantum efficiency of photosynthetic system II (ФPSII) declined with increasing concentration of NaCl and NaHCO3, and their decreasing degree was more significant in NaHCO3 treatment. On the contrary, higher non-photochemical quenching (NPQ) was observed with increasing concentration of NaCl and NaHCO3, especially in NaHCO3 treatment. Na content in tomato leaves was positively correlated with NaCl and NaHCO3 stress intensity, while contents of K, P, and N as well as nitrogen metabolism-related enzymes including nitrate reductase (NR), glutamine synthetase (GS), glutamate dehydrogenase (GDH) and glutamate synthase (GOGAT) showed significant negative correlation with stress intensity, and their changes were much higher in NaHCO3 treatment than in NaCl treatment. Both NaCl and NaHCO3 stress induced excess reactive oxygen species (ROS) accumulation and obvious lipid peroxidation in tomato leaves, though was much higher in the NaHCO3 treatment. Compared to catalase (CAT) and guaiacol peroxidase (GPX), superoxide dismutase (SOD) and the ascorbate–glutathione cycle components including ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR) and glutathione reductase (GR) showed higher levels, which could play an important role in scavenging ROS in tomato leaves under salt and alkali stress.

Sugars, including fructose, glucose, and sucrose, contribute signifi cantly to the fl avor and consumer acceptance of snap beans (Phaseolus vulgaris L.). Differences between dry and snap bean cultivars and among snap bean cultivars in the patterns of accumulation of sugars have been observed. In ‘Eagle’, a white-seeded, largesieve snap bean cultivar, fructose and glucose concentrations in developing pods decreased while sucrose concentration increased with increasing pod size. In contrast, fructose and glucose concentrations increased while sucrose remained unchanged with increasing pod size in Puebla 152, a black-seeded dry bean landrace from Mexico. A population derived from the cross Eagle × Puebla 152 consisting of 75 F 9:10 recombinant inbred lines (RILs) was developed by single seed descent. Signifi cant differences in fructose, glucose, and sucrose concentrations of sieve size 4 (8.33–9.52 mm) pods were observed among RILs. No signifi cant genotype × year interactions were observed. Heritability estimates for fructose, glucose, and sucrose were 0.85 ± 0.16, 0.81 ± 0.16, and 0.85 ± 0.16, respectively. A single quantitative trait locus (QTL) on linkage group B1 was identifi ed that is closely linked to random amplifi ed polymorphic DNA (RAPD) marker W9.1050 and explains 28.8 and 26.6% of the variation in pod fructose and sucrose concentration, respectively. A two-QTL model, including W9.1050 and RAPD marker F8.500 on linkage group B6, explained 36.4% of the variation in glucose concentration in pods.

Sugars, including fructose, glucose, and sucrose, contribute significantly to the flavor and consumer acceptance of snap beans (Phaseolus vulgaris L.). Little is known regarding differences in sugar content among snap bean and dry bean cultivars and the patterns of sugar accumulation with increasing pod size. Alcohol–soluble sugar concentration of five snap bean cultivars and one dry bean cultivar planted in field trials was assayed throughout pod development over 2 years using high-performance liquid chromatography. Significant differences in sugar accumulation patterns and quantity were observed among cultivars. In general, fructose and glucose content decreased, whereas sucrose increased with increasing pod size in snap beans. In contrast, fructose and glucose amounts increased, whereas sucrose concentration remained unchanged with increasing pod size in the dry bean cultivar. No year-by-genotype interactions were observed for sugar accumulation patterns or sugar amount. Results indicate that sieve size No. 3 (7.34 to 8.33 mm) or No. 4 (8.33 to 9.52 mm) pods are suitable for detecting differences in sugar concentration among genotypes.