Abstract
Powdery mildew (PM), caused by Podosphaera xanthii, is a major threat to the global cucurbit yield. The molecular mechanisms underlying the PM resistance of pumpkin (Cucurbita moschata Duch.) are largely unknown. A homolog of the basic helix-loop-helix (bHLH) transcription factor was previously identified through a transcriptomic analysis of a PM-resistant pumpkin. In this study, this bHLH homolog in pumpkin has been functionally characterized. CmbHLH87 is present in the nucleus. CmbHLH87 expression in the PM-resistant material was considerably downregulated by PM; and abscisic acid, methyl jasmonate, ethephon, and NaCl treatments induced CmbHLH87 expression. Ectopic expression of CmbHLH87 in tobacco plants alleviated the PM symptoms on the leaves, accelerated cell necrosis, and enhanced H2O2 accumulation. The expression levels of PR1a, PR5, and NPR1 were higher in the PM-infected transgenic plants than in PM-infected wild-type plants. Additionally, the chlorosis and yellowing of plant materials were less extensive and the concentration of bacteria at infection sites was lower in the transgenic tobacco plants than in the wild-type plants in response to bacterial wilt and scab pathogens. CmbHLH87 may be useful for genetic engineering of novel pumpkin cultivars in the future.
Highlights
Pumpkin (Cucurbita moschata Duch.) is an important vegetable crop and is widely cultivated in China, with total harvested area of 438,466 hectares in 2017 (i.e., 17.42% of the global area) (Food and Agriculture Organization, 2017)
At the amino acid level, CmbHLH87 was highly similar to the bHLH87 transcription factors from C. moschata (99.1% identical), Cucurbita pepo (98.8% identical), and Cucurbita maxima (71.0% identical), but was relatively dissimilar to Nicotiana tabacum bHLH87 (38.3% identical) and Arabidopsis AtbHLH087 (41.7% identical)
The CmbHLH87 protein was localized to the nucleus in Arabidopsis protoplasts, which is consistent with the observations of earlier studies involving the basic helix-loop-helix (bHLH) transcription factors from other plant species (Wang et al, 2015)
Summary
Pumpkin (Cucurbita moschata Duch.) is an important vegetable crop and is widely cultivated in China, with total harvested area of 438,466 hectares in 2017 (i.e., 17.42% of the global area) (Food and Agriculture Organization, 2017). The basic helix-loop-helix (bHLH) transcription factors constitute one of the largest transcription factor families in plants, wherein they help regulate developmental processes and responses to environmental stresses. These proteins have a 60amino-acid conserved domain, which contains the following two functionally distinct regions: an N-terminal basic region (13–17 amino acids) that functions as a DNA-binding domain and a Cterminal HLH region that contributes to the formation of homodimers or heterodimers (Heim et al, 2003; Toledo-Ortiz et al, 2003). Recent studies have indicated that a number of bHLH transcription factor genes are involved in responses to abiotic stresses including drought, salt, and cold. BHLH functions related to biotic stress resistance remain poorly characterized in plants
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