Low red to far-red light ratio promotes salt tolerance by improving leaf photosynthetic capacity in cucumber

Soil salinity severely inhibits leaf photosynthesis and limits agricultural production. Red to far-red light ratio (R/FR) affects leaf photosynthesis under salt stress, however, its regulation mechanism is still largely unknown. This study investigated the effects of different R/FR on plant growth, gas exchange parameters, photosynthetic electron transport, Calvin cycle and key gene expression under salt stress. Cucumber seedlings were exposed to four treatments including 0 mM NaCl and R/FR=7 (L7, control), 0 mM NaCl and R/FR=0.7 (L0.7), 80 mM NaCl and R/FR=7 (H7) and 80 mM NaCl and R/FR=0.7 (H0.7) for 9 days in an artificial climate chamber. The results showed that compared to L7 treatment, H7 treatment significantly reduced relative growth rate (RGR), CO2 assimilation rate (Pn), maximum photochemical efficiency PSII (Fv/Fm), most JIP-test parameters and total Rubisco activity, indicating that salt stress severely inhibited photosynthetic electron transport from PSII to PSI and blocked Calvin cycle in cucumber leaves. However, these suppressions were effectively alleviated by low R/FR addition (H0.7 treatment). Compared to H7 treatment, H0.7 treatment significantly increased RGR and Pn by 209.09% and 7.59%, respectively, enhanced Fv/Fm, maximum quantum yield for primary photochemistry (φPo), quantum yield for electron transport (φEo) and total Rubisco activity by 192.31%, 17.6%, 36.84% and 37.08%, respectively, and largely up-regulated expressions of most key genes involved in electron transport and Calvin cycle. In conclusion, low R/FR effectively alleviated the negative effects of salt stress on leaf photosynthesis by accelerating photosynthetic electron transport from PSII to PQ pool and promoting Calvin cycle in cucumber plants. It provides a novel environmentally friendly light-quality regulation technology for high efficiency salt-resistant vegetable production

Dynamic Expression of miRNAs and Their Targets in the Response to Drought Stress of Grafted Cucumber Seedlings

Grafting of cucumber is widely used to improve growth and tolerance to biotic and abiotic stresses. MicroRNAs (miRNAs) regulate plant growth and development, and respond to various stresses through negative, post-transcriptional regulation of the expression of their target genes. We grafted cucumber (Cucumis sativus) scions onto pumpkin (Cucurbita moschata) rootstocks to study the molecular mechanisms of miRNA-mediated grafting-induced responses to drought stress. The relative expressions of 17 selected miRNAs and their predicted target mRNAs were detected by quantitative real-time PCR (qRT-PCR) in leaves and roots of hetero-grafted cucumber seedlings (cucumber as scion and pumpkin as rootstock) and auto-grafted cucumber seedlings (cucumber as scion and rootstock) after 24 hours of 15% PEG6000 treatment. Compared with the expression in leaves of auto-grafted cucumber seedlings, the expressions of most miRNAs in the leaves of hetero-grafted seedlings changed dynamically: induced under normal conditions, and reduced after 3 h of drought stress, and then induced after 8 h and 24 h of drought stress. Similarly, compared with the expression in roots of auto-grafted cucumber seedlings, the expressions of most miRNAs in the roots of hetero-grafted cucumber seedlings changed dynamically: reduced under normal condition, and still reduced after 1 h, but induced after 3 h and 8 h, then reduced significantly after 24 h of drought stress. These results are useful for the functional analysis of miRNAs in the mediation of grafting-dependent drought tolerance

The physiological and molecular mechanism of brassinosteroid in response to stress: a review

Abstract The negative effects of environmental stresses, such as low temperature, high temperature, salinity, drought, heavy metal stress, and biotic stress significantly decrease crop productivity. Plant hormones are currently being used to induce stress tolerance in a variety of plants. Brassinosteroids (commonly known as BR) are a group of phytohormones that regulate a wide range of biological processes that lead to tolerance of various stresses in plants. BR stimulate BRASSINAZOLE RESISTANCE 1 (BZR1)/BRI1-EMS SUPPRESSOR 1 (BES1), transcription factors that activate thousands of BR-targeted genes. BR regulate antioxidant enzyme activities, chlorophyll contents, photosynthetic capacity, and carbohydrate metabolism to increase plant growth under stress. Mutants with BR defects have shortened root and shoot developments. Exogenous BR application increases the biosynthesis of endogenous hormones such as indole-3-acetic acid, abscisic acid, jasmonic acid, zeatin riboside, brassinosteroids (BR), and isopentenyl adenosine, and gibberellin (GA) and regulates signal transduction pathways to stimulate stress tolerance. This review will describe advancements in knowledge of BR and their roles in response to different stress conditions in plants

24-Epibrassinolide Ameliorates Endogenous Hormone Levels to Enhance Low-Temperature Stress Tolerance in Cucumber Seedlings

Phytohormone biosynthesis and accumulation are essential for plant growth and development and stress responses. Here, we investigated the effects of 24-epibrassinolide (EBR) on physiological and biochemical mechanisms in cucumber leaves under low-temperature stress. The cucumber seedlings were exposed to treatments as follows: NT (normal temperature, 26 °C/18 °C day/night), and three low-temperature (12 °C/8 °C day/night) treatments: CK (low-temperature stress); EBR (low-temperature and 0.1 μM EBR); and BZR (low-temperature and 4 μM BZR, a specific EBR biosynthesis inhibitor). The results indicated that low-temperature stress proportionately decreased cucumber seedling growth and the strong seedling index, chlorophyll (Chl) content, photosynthetic capacity, and antioxidant enzyme activities, while increasing reactive oxygen species (ROS) and malondialdehyde (MDA) contents, hormone levels, and EBR biosynthesis gene expression level. However, EBR treatments significantly enhanced cucumber seedling growth and the strong seedling index, chlorophyll content, photosynthetic capacity, activities of antioxidant enzymes, the cell membrane stability, and endogenous hormones, and upregulated EBR biosynthesis gene expression level, while decreasing ROS and the MDA content. Based on these results, it can be concluded that exogenous EBR regulates endogenous hormones by activating at the transcript level EBR biosynthetic genes, which increases antioxidant enzyme capacity levels and reduces the overproduction of ROS and MDA, protecting chlorophyll and photosynthetic machinery, thus improving cucumber seedling growth

Gibberellin Is Involved in Inhibition of Cucumber Growth and Nitrogen Uptake at Suboptimal Root-Zone Temperatures

<div><p>Suboptimal temperature stress often causes heavy yield losses of vegetables by suppressing plant growth during winter and early spring. Gibberellin acid (GA) has been reported to be involved in plant growth and acquisition of mineral nutrients. However, no studies have evaluated the role of GA in the regulation of growth and nutrient acquisition by vegetables under conditions of suboptimal temperatures in greenhouse. Here, we investigated the roles of GA in the regulation of growth and nitrate acquisition of cucumber (<i>Cucumis sativus</i> L.) plants under conditions of short-term suboptimal root-zone temperatures (T_r). Exposure of cucumber seedlings to a T_r of 16°C led to a significant reduction in root growth, and this inhibitory effect was reversed by exogenous application of GA. Expression patterns of several genes encoding key enzymes in GA metabolism were altered by suboptimal T_r treatment, and endogenous GA concentrations in cucumber roots were significantly reduced by exposure of cucumber plants to 16°C T_r, suggesting that inhibition of root growth by suboptimal T_r may result from disruption of endogenous GA homeostasis. To further explore the mechanism underlying the GA-dependent cucumber growth under suboptimal T_r, we studied the effect of suboptimal T_r and GA on nitrate uptake, and found that exposure of cucumber seedlings to 16°C T_r led to a significant reduction in nitrate uptake rate, and exogenous application GA can alleviate the down-regulation by up regulating the expression of genes associated with nitrate uptake. Finally, we demonstrated that N accumulation in cucumber seedlings under suboptimal T_r conditions was improved by exogenous application of GA due probably to both enhanced root growth and nitrate absorption activity. These results indicate that a reduction in endogenous GA concentrations in roots due to down-regulation of GA biosynthesis at transcriptional level may be a key event to underpin the suboptimal T_r-induced inhibition of root growth and nitrate uptake. These findings may have important practical implications in effective mitigation of suboptimal temperature-induced vegetable loss under greenhouse conditions.</p></div

Effects of inhibitors of key enzymes in N assimilation on ¹⁵NO₃^- influx of cucumber seedlings.

<p>20-day-old cucumber seedlings were exposed for 6 h to 16°C T_r (16°C), 16°C T_r in the presence of 5 μM GA (16°C+GA), 5 μM GA plus 0.5 mM tungstate (W), 5 μM GA plus 0.25 mM L-methionine sulphoximine (MSX), 5 μM GA plus 0.5 mM azaserine (AZA), and 5 μM GA plus 1 mM aminooxyacetate (AOA). Data are means±SE. Different letters on the top of column indicate significant differences (<i>P <0</i>.<i>05</i>, n = 3).</p

Effect of suboptimal T_r and GA on ¹⁵NO₃^- influx of cucumber.

<p>15-day-old cucumber seedlings were transferred to 22°C T_r and 16°C T_r conditions in the presence or absence of GA 5 μM GA for 8d. Data are means±SE. Different letters on the top of column indicate significant differences (<i>P <0</i>.<i>05</i>, n = 3).</p

Antisense Overexpression of Gγ Subunit CsGG3.1-2 Reduces Soluble Sugar Content and Chilling Tolerance in Cucumber

Plant G protein γ subunits have multiple functions in growth and development processes and in abiotic stress responses. Few functions of Gγ in horticultural crops have been revealed thus far. In this study, the potential function of CsGG3.1-2, one of the two alternative splice variants of Gγ gene CsGG3.1 in cucumber (Cucumis sativus L.), was investigated using transgenic plants overexpressing antisense CsGG3.1-2 under the control of the 35S promoter. The tolerance to chilling stress in transgenic plants was significantly decreased. Cold stress-related physiological parameters and the expression of CBFs and their downstream target genes were then measured. Compared with WT, the maximum efficiency of photosystem II (Fv/Fm), antioxidative enzymes activities, soluble protein, and proline accumulation decreased significantly in transgenic plants treated with cold stress, whereas the malonaldehyde (MDA) content increased. However, the overexpression of antisense CsGG3.1-2 did not affect the induction of cold-inducible genes. Quantitative real-time PCR (qPCR) analysis showed the increased expression of CBF genes and their downstream target genes in transgenic plants, suggesting that CsGG3.1-2 affects cold responses via CBF-independent pathways in cucumber. At the same time, the sucrose and fructose contents decreased in transgenic plants under both normal and cold conditions. These findings suggest that soluble sugar deficiency is associated with chilling sensitivity in transgenic plants, and CsGG3.1-2 may have a role in regulating carbohydrate metabolism in cucumber

Effects of suboptimal T_r and GA on root morphological parameters of cucumber seedlings.

<p>(A) Total root length of cucumber seedlings. (B) Average diameter of roots of cucumber seedlings. (C) Number of root tips of cucumber seedlings. (D) Root surface area of cucumber seedlings. 10-day-old seedlings were transferred to 16°C T_r conditions in the presence or absence of exogenous 5 μM GA for 5 d. Data are means±se. Different letters on the top of column indicate significant differences (<i>P <0</i>.<i>05</i>, n = 6).</p

Effect of suboptimal T_r and GA on the growth of cucumber seedlings.

<p>(A) Phenotypes of cucumber seedlings. (B) Root dry mass (DM) of cucumber seedlings. (C) Shoot DM of cucumber seedlings. (D) Leaf area of cucumber seedlings. (E) Root to shoot ratio of cucumber seedlings. 15-day-old cucumber seedlings were transferred to 22°C T_r and 16°C T_r conditions in the presence or absence of GA 5 μM GA for 8d. Data are means±SE. Different letters on the top of column indicate significant differences (<i>P <0</i>.<i>05</i>, n = 6). Bar = 10 cm.</p