Corrigendum: Expression of Pumpkin CmbHLH87 Gene Improves Powdery Mildew Resistance in Tobacco.

[This corrects the article DOI: 10.3389/fpls.2020.00163.].

Expression of Pumpkin CmbHLH87 Gene Improves Powdery Mildew Resistance in Tobacco.

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.

Improved Powdery Mildew Resistance of Transgenic Nicotiana benthamiana Overexpressing the Cucurbita moschata CmSGT1 Gene.

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.

[Effects of NaCl stress on free polyamines content and reactive oxygen species level in pumpkin roots].

Taking Cucurbita moschata Duch hybrid 360-3 x 112-2 and C. ficifolia Bouche as test materials, the effects of NaCl stress on their plant growth and the O2*- production rate and H2O2 and free polyamines (PAs) contents in their roots were studied with hydroponic culture. The results showed that after 10 d NaCl stress, the plant growth of the two pumpkin varieties was strongly inhibited, compared with the control, and C. ficifolia was more injured than hybrid 360-3 x 112-2. Under NaCl stress, the root O2*- production rate and H2O2 content of the two pumpkin varieties were increased, but their absolute values were lower in hybrid 360-3 x 112-2 than in C. ficifolia. The contents of PAs, putrescine (Put), spermidine (Spd) and spermine (Spm) and Put/PAs ratio in the roots of the two pumpkin varieties were always higher than the control and had a trend of increased first and decreased then; while the (Spd + Spm)/Put ratio was lower than the control and decreased first and increased then. Compared with C. ficifolia Bouche, hybrid 360-3 x 112-2 always had a lower Put/PAs ratio and a lower Put content in its roots, but the (Spd + Spm)/Put ratio and Spd and Spm contents were always higher. It was concluded that under NaCl stress, the increasing PAs content in the roots of test materials played an active role in decreasing or scavenging reactive oxygen species (ROS). The conversion of Put to Spd and Spm was advantageous to the increase of plant salt tolerance. The higher salt tolerance of hybrid 360-3 x 112-2 was closely related to the lower Put/PAs ratio and the higher (Spd + Spm)/Put ratio and PAs content in its roots, and thus, the stronger capacity to scavenge ROS.

Species-specific transcriptional activity of synthetic flavonoids in guinea pig and mouse cells as a result of differential activation of the aryl hydrocarbon receptor to interact with dioxin-responsive elements.

To investigate possible species-specificity of aryl hydrocarbon receptor (AhR)-mediated signal transduction pathways, activities of 2,3,7,8-tetrochlorodibenzo-p-dioxin (TCDD) and six synthetic flavonoids were evaluated in mouse hepatoma and guinea pig adenocarcinoma cells transfected with an AhR-responsive luciferase reporter. Rank order potency in these two cell lines was similar for the ability of these flavonoids to antagonize TCDD-induced reporter gene expression. However, in the presence of flavone alone, a species-specific difference in agonist activity was observed. In guinea pig cells, several flavonoids demonstrated agonist activity up to 50% of the maximum TCDD response. In mouse cells, however, no significant agonist activity was observed at the same concentrations based on luciferase enzyme activity, protein expression, and mRNA analysis. Moreover, competitive ligand-binding assays, using [(3)H]TCDD in cytosolic fractions, demonstrated that 3'-methoxy-4'-nitroflavone had a similar IC(50) in both recombinant cell lines, suggesting that the flavone has similar binding affinity to receptors from both species. However, electrophoretic mobility shift assay using the cytosolic fractions demonstrated that this flavone elicited binding to the DRE by guinea pig but not mouse AhR complex. The dependence of the AhR in this differential interaction was further demonstrated using in vitro synthesized guinea pig and mouse Ah receptors and mouse Arnt. Together, these data suggest that the differential agonist/antagonist activity of these flavone derivatives is caused by the efficacy of these flavonoids in eliciting an AhR conformation that recognizes regulatory response elements in a species-specific manner.