Global warming poses a huge challenge for plants to withstand increasing ambient temperatures. Cucumber is a kind of worldwide vegetable crop which is very sensitive to high temperature (HT). The mechanism of auxin affecting plant resistance to high temperature is still unclear. This study showed the relationship between auxin and heat resistance of cucumber. Here, we found that exogenous auxin improved cucumber heat tolerance by promoting root activity and mitigating the accumulation of ROS at the cotyledon stage. Additionally, exogenous auxin delayed internode shortening and leaf senescence under HT at three true-leaf stage. Exogenous auxin maintains photosystem Ⅱ (PS Ⅱ) stability in the thermotolerant cucumber plants. Transcriptome results indicated that photosynthesis and DNA repair processes were activated by exogenous 2,4-dichlorophenoxyacetic acid (2,4-D). Moreover, glutathione metabolism and carotenoid biosynthesis process were involved in ROS scavenging after the 2,4-D treatment under HT. In addition, overexpression of auxin biosynthesis gene CsYUC10b elevated cucumber resistance to high temperatures and enhanced the photosystem activity and DNA repair ability. Thus, auxin promoted heat resistance in cucumber by enhancing PS Ⅱ and activating DNA repair pathways.

The transcriptional cascade and regulatory loop play crucial roles in regulating plant-specialized metabolite biosynthesis. Capsaicinoids are unique to the genus Capsicum and confer a pungent flavor to its fruits. However, the transcriptional regulation of capsaicinoid biosynthesis remains largely unknown. In this study, two AP2/ERF transcription factors (TFs), CaERF102 and CaERF111, were characterized for their role in the capsaicinoid biosynthesis process. Expression analysis of two ERFs and capsaicinoid biosynthetic genes (CBGs) suggested that they were associated with capsaicinoid biosynthesis. Both ERFs encode nuclear-localized proteins and function as transcriptional activators through their C-terminal activation motifs. The two ERF TFs participated in capsaicinoid biosynthesis by directly activating the promoters of key CBGs, and this activation was significantly enhanced when CaMYC2 was co-expressed. Moreover, CaERF102 and CaERF111 were found to interact with CaMYC2. This study helps elucidate the AP2/ERF TF regulatory network that governs capsaicinoid biosynthesis in Capsicum species.

High-temperature stress threatens the growth and yield of crops. Basic helix-loop-helix (bHLH) transcription factors (TFs) have been shown to play important roles in regulating high-temperature resistance in plants. However, the bHLH TFs responsible for high-temperature tolerance in cucumbers have not been identified. We used transcriptome profiling to screen the high temperature-responsive candidate bHLH TFs in cucumber. Here, we found that the expression of 75 CsbHLH genes was altered under high-temperature stress. The expression of the CsSPT gene was induced by high temperatures in TT (Thermotolerant) cucumber plants. However, the Csspt mutant plants obtained by the CRISPR-Cas9 system showed severe thermosensitive symptoms, including wilted leaves with brown margins and reduced root density and cell activity. The Csspt mutant plants also exhibited elevated H₂O₂ levels and down-regulated photosystem-related genes under normal conditions. Furthermore, there were high relative electrolytic leakage (REC), malondialdehyde (MDA), glutathione (GSH), and superoxide radical (O₂^·−) levels in the Csspt mutant plants, with decreased Proline content after the high-temperature treatment. Transcriptome analysis showed that the photosystem and chloroplast activities in Csspt mutant plants were extremely disrupted by the high-temperature stress compared with wild-type (WT) plants. Moreover, the plant hormone signal transduction, as well as MAPK and calcium signaling pathways were activated in Csspt mutant plants under high-temperature stress. The HSF and HSP family genes shared the same upregulated expression patterns in Csspt and WT plants under high-temperature conditions. However, most bHLH, NAC, and bZIP family genes were significantly down-regulated by heat in Csspt mutant plants. Thus, these results demonstrated that CsSPT regulated the high-temperature response by recruiting photosynthesis components, signaling pathway molecules, and transcription factors. Our results provide important insights into the heat response mechanism of CsSPT in cucumber and its potential as a target for breeding heat-resistant crops.