Abstract
Powdery mildew (PM), which is mainly caused by Podosphaera xanthii, is a serious biotrophic pathogen disease affecting field-grown and greenhouse-grown cucurbit crops worldwide. Because fungicides poorly control PM, the development and cultivation of PM-resistant varieties is critical. A homolog of SGT1 (suppressor of the G2 allele of skp1), which encodes a key component of the plant disease-associated signal transduction pathway, was previously identified through a transcriptomic analysis of a PM-resistant pumpkin (Cucurbita moschata) inbred line infected with PM. In this study, we have characterized this SGT1 homolog in C. moschata, and investigated its effects on biotic stress resistance. Subcellular localization results revealed that CmSGT1 is present in the nucleus. Additionally, CmSGT1 expression levels in the PM-resistant material was strongly induced by PM, salicylic acid (SA) and hydrogen peroxide (H2O2). In contrast, SA and H2O2 downregulated CmSGT1 expression in the PM-susceptible material. The ethephon (Eth) and methyl jasmonate (MeJA) treatments upregulated CmSGT1 expression in both plant materials. The constitutive overexpression of CmSGT1 in Nicotiana benthamiana (N. benthamiana) minimized the PM symptoms on the leaves of PM-infected seedlings, accelerated the onset of cell necrosis, and enhanced the accumulation of H2O2. Furthermore, the expression levels of PR1a and PR5, which are SA signaling transduction markers, were higher in the transgenic plants than in wild-type plants. Thus, the transgenic N. benthamiana plants were significantly more resistant to Erysiphe cichoracearum than the wild-type plants. This increased resistance was correlated with cell death, H2O2 accumulation, and upregulated expression of SA-dependent defense genes. However, the chlorosis and yellowing of plant materials and the concentration of bacteria at infection sites were greater in the transgenic N. benthamiana plants than in the wild-type plants in response to infections by the pathogens responsible for bacterial wilt and scab. Therefore, CmSGT1-overexpressing N. benthamiana plants were hypersensitive to these two diseases. The results of this study may represent valuable genetic information for the breeding of disease-resistant pumpkin varieties, and may also help to reveal the molecular mechanism underlying CmSGT1 functions.
Highlights
The genus Cucurbita is composed of several species, including the cultivated Cucurbita moschata Duch., Cucurbita pepo L., Cucurbita maxima Duch., and several wild species
At the amino acid level, CmSGT1 was highly similar to the SGT1 from the following plant species: Cucumis melo (CmSGT1, 84.4% identity), and Cucumis sativus (CsSGT1, 82.5% identity), N. benthamiana (NbSGT1.1 and NbSGT1.2, 89.5% identity), and Arabidopsis thaliana (AtSGT1a and AtSGT1b, 75.2% identity)
The results indicated that the overexpression of CmSGT1 promoted the accumulation of H2O2 in transgenic plants infected with Powdery mildew (PM) (Figure 4C)
Summary
The genus Cucurbita is composed of several species, including the cultivated Cucurbita moschata Duch., Cucurbita pepo L., Cucurbita maxima Duch., and several wild species. Pumpkins (C. moschata) are valued for their fruit and seeds. They are rich in vitamins, amino acids, flavonoids, phenolics, and carbohydrates, and possess medicinal properties, including antidiabetic, anti-oxidant, anti-carcinogenic, and anti-inflammatory activities (Wang et al, 2002; Yadav et al, 2010). Cucurbit powdery mildew (PM) is a serious disease affecting field-grown and greenhousegrown cucurbit crops worldwide. The disease is mainly caused by Podosphaera xanthii (formerly known as Sphaerotheca fuliginea), which is a biotrophic plant pathogen (Perez-Garcia et al, 2009; Fukino et al, 2013). Fungicide applications poorly control PM and the long-term use of pesticides may lead to increased environmental pollution and the residual chemicals on food crops may be harmful for humans and animals. Studying the mechanism underlying PM resistance and exploiting the resistance genes to breed resistant varieties represents an effective way to control PM in pumpkin
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