BackgroundAbiotic stresses due to environmental factors could adversely affect the growth and development of crops. Among the abiotic stresses, drought and heat stress are two critical threats to crop growth and sustainable agriculture worldwide. Considering global climate change, incidence of combined drought and heat stress is likely to increase. The aim of this study was to shed light on plant growth performance and leaf physiology of three tomatoes cultivars (‘Arvento’, ‘LA1994’ and ‘LA2093’) under control, drought, heat and combined stress.ResultsShoot fresh and dry weight, leaf area and relative water content of all cultivars significantly decreased under drought and combined stress as compared to control. The net photosynthesis and starch content were significantly lower under drought and combined stress than control in the three cultivars. Stomata and pore length of the three cultivars significantly decreased under drought and combined stress as compared to control. The tomato ‘Arvento’ was more affected by heat stress than ‘LA1994’ and ‘LA2093’ due to significant decreases in shoot dry weight, chlorophyll a and carotenoid content, starch content and NPQ (non-photochemical quenching) only in ‘Arvento’ under heat treatment. By comparison, the two heat-tolerant tomatoes were more affected by drought stress compared to ‘Arvento’ as shown by small stomatal and pore area, decreased sucrose content, ΦPSII (quantum yield of photosystem II), ETR (electron transport rate) and qL (fraction of open PSII centers) in ‘LA1994’ and ‘LA2093’. The three cultivars showed similar response when subjected to the combination of drought and heat stress as shown by most physiological parameters, even though only ‘LA1994’ and ‘LA2093’ showed decreased Fv/Fm (maximum potential quantum efficiency of photosystem II), ΦPSII, ETR and qL under combined stress.ConclusionsThe cultivars differing in heat sensitivity did not show difference in the combined stress sensitivity, indicating that selection for tomatoes with combined stress tolerance might not be correlated with the single stress tolerance. In this study, drought stress had a predominant effect on tomato over heat stress, which explained why simultaneous application of heat and drought revealed similar physiological responses to the drought stress. These results will uncover the difference and linkage between the physiological response of tomatoes to drought, heat and combined stress and be important for the selection and breeding of tolerant tomato cultivars under single and combine stress.

BackgroundThe basic helix-loop-helix (bHLH) proteins are a superfamily of transcription factors that can bind to specific DNA target sites. They have been well characterized in model plants such as Arabidopsis and rice and have been shown to be important regulatory components in many different biological processes. However, no systemic analysis of the bHLH transcription factor family has yet been reported in tomatoes. Tomato yellow leaf curl virus (TYLCV) threatens tomato production worldwide by causing leaf yellowing, leaf curling, plant stunting and flower abscission.ResultsA total of 152 bHLH transcription factors were identified from the entire tomato genome. Phylogenetic analysis of bHLH domain sequences from Arabidopsis and tomato facilitated classification of these genes into 26 subfamilies. The evolutionary and possible functional relationships revealed during this analysis are supported by other criteria, including the chromosomal distribution of these genes, the conservation of motifs and exon/intron structural patterns, and the predicted DNA binding activities within subfamilies. Distribution mapping results showed bHLH genes were localized on the 12 tomato chromosomes. Among the 152 bHLH genes from the tomato genome, 96 bHLH genes were detected in the TYLCV-susceptible and resistant tomato breeding line before (0 dpi) and after TYLCV (357 dpi) infection. As anticipated, gene ontology (GO) analysis indicated that most bHLH genes are related to the regulation of macromolecule metabolic processes and gene expression. Only four bHLH genes were differentially expressed between 0 and 357 dpi. Virus-induced gene silencing (VIGS) of one bHLH genes SlybHLH131 in resistant lines can lead to the cell death.ConclusionIn the present study, 152 bHLH transcription factor genes were identified. One of which bHLH genes, SlybHLH131, was found to be involved in the TYLCV infection through qRT-PCR expression analysis and VIGS validation. The isolation and identification of these bHLH transcription factors facilitated clarification of the molecular genetic basis for the genetic improvement of tomatoes and the development of functional gene resources for transgenic research. In addition, these findings may aid in uncovering an unexplored mechanism during the TYLCV infection in tomatoes.Electronic supplementary materialThe online version of this article (doi:10.1186/s12864-015-1249-2) contains supplementary material, which is available to authorized users.

In nature, crops encounter a combination of abiotic stresses that severely limit yield. Our aim was to dynamically expose the changes of tomatoes' physiological parameters to drought, heat and their combination and thereby clarify the relationship between the responses to single and combined stress. We studied the effect of single and combined drought and heat stresses on the shoot and root of two tomato cultivars (Sufen No.14 as CV1; Jinlingmeiyu as CV2). After being exposed to combined stress for 6 days, the dry weight of shoot and root significantly decreased. The F '/F ' (quantum yield of photosystem II) was significantly lower in CV1 upon drought and combined stress and in CV2 subjected to combined stress (between days 4 and 6) compared to control. The relative water content during combined stress was significantly lower than control from day 4 to recovery day 2. On days 3 and 6, the water loss rate significantly increased under heat stress and decreased at drought and combined stress, respectively. The combined stress caused severe damages on photosystem II and chloroplast ultrastructure. The root activity after stress recovered even though drought significantly increased the activity from day 2 to day 6. Combined stress result in complex responses during tomato growth. The CV1 was more heat tolerant than CV2, but there was no varietal difference at drought and combined stress. This study contributes to the understanding of the underlying physiological response mechanism of plant to combined stress and crop improvement by providing valuable information for abiotic stress-tolerant tomato breeding.

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Background Co-occurrence of cold and drought stress can alter the response of plants at morphological, physiological and molecular levels, which finally affect crop production, more than individual stress. Understanding the responses of crop to combined stress is necessary to improve tolerance and maintain crop production especially in the field where combined stress frequently occurs. We aimed to clarify the underlying leaf physiological and molecular mechanisms of tomato by imposing combining cold and drought on one popular tomato cultivar ‘Jinlingmeiyu’ as an example. Results The physiological and genetic responses were identified in tomatoes after 42 h exposure to control, cold, drought and combined treatments. As compared with control, water loss rate at the three stresses including cold, drought and combined stress significantly decreased until 40 min after taking samples from the plants. The content of H 2 O 2 , zeatin riboside (ZR) and melatonin in all stress treatments were significantly higher than the control. Drought stress alone and combined stress induced the accumulation of abscisic acid (ABA) and auxin (IAA) as compared with control. The individual cold and combined stress significantly decreased the maximum quantum efficiency of PSII (F v /F m ), quantum yield of PSII (F q ′ /F m ′ ) and electron transport rate (ETR). In total, 7141, 1850 and 7841 genes were involved in the stress response to cold, drought and their combination. Functional analysis of the stress-inducible genes provided more insights concerning the complex regulatory mechanisms that were involved in combined stress. The expression level of 12 genes were validated by quantitative real-time PCR (qRT-PCR). Conclusions We found that the expression of stress-specific genes changed with physiological variation, indicating the close crosstalk between physiological and genetic response especially under combined stress. This study provides new knowledge on the complex regulatory mechanism genes in tomato (‘Jinlingmeiyu’) leaf to abiotic stresses. Electronic supplementary material The online version of this article (10.1186/s12870-019-1982-9) contains supplementary material, which is available to authorized users.