Study On Alleviative Effects Of Exogenous Silicon On Water Stress-induced Injury And The Underlying Mechanisms In Tomato Seedlings

Water deficiency is one of the abiotic stress factors limiting sustainable development ofagriculture. With the continuous global climate warming, water shortage is becoming moreand more serious, which causes great losses in agricultural production. Thus, research relatedto the drought resistance mechanisms has been paid great attention. Silicon (Si) is a beneficialelement for plant growth and previous research has shown that exogenous Si could to someextent mitigate the inhibiting effect of drought stress on plant growth and development.Therefore, application of Si fertilizer is expected to be an important pathway to improve cropproduction in arid or semiarid areas. However, the mechanism for Si-mediated droughtresistance still remains not very clear. In this study, the effects of Si (as0.5mMNa2SiO39H2O) on seed germination were investigated in four tomato cultivars(‘Jinpengchaoguan’,‘Zhongza No.9’,‘Oubao318’and ‘Houpi L402’) under water stress,which was simulated by10%polyethylene glycol6000(PEG-6000). The possible mechanismfor Si-enhanced seed germination was also investigated. Meanwhile, the effects of Si (2.5mM)on plant growth, photosynthetic characteristics, water metabolism, accumulation ofintracellular compatible substances, oxidative damage and antioxidant defense wereinvestigated in two tomato cultivars (‘Jinpengchaoguan’ and ‘Zhongza No.9’), the aims beingto clarify the physiological regulatory mechanisms of Si in improving drought resistance oftomato. Highthrough sequencing technology was also used to investigate the influence of Sion gene expression patern (in ‘Zhongza No.9’) under water stress, and the expressions ofsome photosynthesis-and antioxidant defense-related genes were analysed by quantitativeReal-time PCR. In this study, partial pot experiemnts were also conducted to confirm the rolesof exogenous Si in improving drought resistance of tomato. The main results are as follows:1. Under water stress, the germination percentage (GP), germination energy (GE),germination index (GI), vigor index (VI), germination rate (GV), shoot length and freshweight of four tomato cultivars were all decreased significantly. The contents of superoxideanion (O2·ˉ), hydrogen peroxide (H2O2) and malondialdehyde (MDA) and activities ofsuperoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) were significantlyincreased in the radicle. The radicle total phenol content based on dry weight was significantly decreased in ‘Jinpengchaoguan’,‘Zhongza No.9’, and while it was significantlyincreased in ‘Houpi L402’. However, the radicle total phenol concentration based on watercontent was significantly increased in ‘Jinpengchaoguan’,‘Houpi L402’ and ‘Oubao318’, itwas not changed in ‘Zhongza No.9’. Under water stress, exogenous Si significantly alleviatedthe decreases in GP, GE, GI, VI and GV and improved the growth of bud seedlings of the fourcultivars. Addition of Si further increased the activities of SOD and CAT while reduced thecontents of O2·ˉ, H2O2and MDA as well as activity of POD. Exogenous Si significantlydecreased the total phenolic content based on dry weight in ‘Jinpengchaoguan’,‘Zhongza No.9’ and ‘Oubao318’, but did not affect the phenolic content in ‘Houpi L402’. However, whenexpressed on water content basis, the total phenolic content of radical was significantlydecreased in the four cultivars. The results suggest Si-regulated phenolic metabolism andformation of Si-phenol complex were benefical to suppress the generation of reactive oxygenspecies. Exogenous Si could alleviate the inhibitory effect of water stress on seed germinationby promoting antioxidant defense and reducing excessive production of ROS in tomato.2. Under water stress, the growth of shoot and root was inhibited and dry matteraccumulation was decreased in the seedlings of tomato cultivars ‘Jinpengchaoguan’ and‘Zhongza No.9’. Leaf photosynthetic parameters including net photosynthetic rate (Pn),stomatal conductance (Gs), transpiration rate (Tr), chlorophyll content, PSII maximumphotochemical efficiency (Fv/Fm), PSII potential activity (Fv/Fo), PSII effective quantumefficiency (Fv’/Fm’), PSII actual photochemical quantum efficiency (ФPSII), photosyntheticelectron transport rate (ETR), photochemical quenching coefficient (qP) and the energy ofphotochemical reactions (P) were all significantly decreased; while the photosyntheticfunction limit value L (PFD), heat dissipation rate (HDR) and antenna pigment energydissipation (D) were significantly increased. Exogenous Si promoted the accumulation of drymatter, increased root length and surface area, inhibited the degradation of chlorophyll andimproved water use efficiency of tomato seedlings under water stress. Meanwhile, Si additionalleviated the stress-induced damage to leaf PSII, enhanced energy redistribution andphotochemical activity in the PSII reaction center, and improved the photochemical efficiencyof tomato leaves under water stress. Si-mediated enhancement in root growth andphotosynthetic efficiency would help improve the drought resistance of tomato under waterstress.3. Under water stress, the leaf relative water content, total water content and waterpotential, root hydraulic conductance and root active absorption area were significantlydecreased in the seedlings of tomato cultivars ‘Jinpengchaoguan’ and ‘Zhongza No.9’. Theexpressions of plasma membrane aquaporins in the leaves and roots were down-regulated under water stress. Under water stress, addition of Si could alleviate the decreases in leafwater contents, water potential, root hydraulic conductance and root active absorption area,and it also enhanced the expressions of plasma membrane aquaporins. Analyses of osmoticadjustment substances and osmotic potential indicated that Si addition did not obviouslydecrease the osmotic potential of plants leaves. These results suggest that under water stress,Si-mediated increase in root surface area and enhancement in the expressions of plasmamembrane aquaporins in roots increased the hydraulic conducantance and water absorptionability of roots, therefore improving the water status of tomato plants.4. The relative electrolyte leakage, levels of O2·ˉ, H2O2and MDA (a membrane lipidperoxidation product) were significantly increased, and the plasma membrane integrity wasdestroyed in the leaves and roots of tomato cultivars ‘Jinpengchaoguan’ and ‘Zhongza No.9’under water stress. The activities of SOD and CAT were up-regulated at early stage of stress(before3days), while they were down-regulated later (after5days). The POD activities in thetwo tomato cultivars were by large maintained at relatively high levels under stress. Thelevels of ascorbic acid and glutathione were basically decreased in the stressed plants, exceptin the roots of ‘Jinpengchaoguan’, where the ascorbic acid level was increased to some extentunder water stress. Under water stress, exogenous Si increased the activities of SOD and CATas well as levels of ascorbic acid and glutathione, but it basically inhibited the increase inPOD activity during the stress period. Meanwhile, Si addition also decreased the contents ofO2·ˉand H2O2, alleviated lipid peroxidation and maintain plasma membrane integrity of theleaves and roots, therefore increased drought resistance of tomato seedlings under waterstress.5. Transcriptome analysis (RNA Seq) showed that there were3012differential expressedgenes between water stress treatment alone and the control in tomato leaves, including1751up-regulated genes and1261down-regulated genes. Compared with the control, in Sitreatment under normal growth conditions,244genes showed different expressions, with149genes up-regulated and95down-regulated. Compared with water stress treatment alone,exogenous Si under water stress conditions showed271differently expressed genes, with123up-regulated and148down-regulated. Compared with Si treatment in normal growthconditions, Si treatment in water stress conditions showed1373diffrently expressed genes,with940up-regulated and433down-regulated. In the roots, compared with the control, inwater stress treatment alone, there were5009differentially expressed genes, including1684up-regulated genes and3325down-regulated genes. There were237different expressiongenes with123genes up-regulated and114down-regulated between the control and Sitreatment alone. Compared with water stress treatment alone, Si treatment under water stress condition showed758different expression genes, among which528were up-regulated and230were down-regulated. Compared with Si treatment in normal growth conditions, the Sitreatment in water stressed conditions showed5512genes which were differently expressed,with1764genes up-regulated and3748genes down-regulated. The funtional analysis ondifferential genes after Si application under water stress condition indicated that themechanism of exogenous Si-mediated drought resistance in tomato under water stress wasquite complicated, and it involved the coexpression of relevant genes in stress response,defense reaction, oxidation-reduction process, metabolic regulation, ion transportation, singaltransduction, etc. Quantitative Real-time PCR was used to detect the differential expressionsof9candidate genes that were involved in photosynthesis and antioxidant defense, and theresult was consistant with the sequencing result. This verified the reliability of transcriptomedata and provided a basis for cloning drought-resistance-related genes and further study onthe molecular mechanism for Si-mediated drought resistance in tomato plants.In summary, Si could increase drought tolerance of tomato seedlings through adjustmentof multiple physiological processes such as enhancement of root water uptake and increase inantioxidant defense ability and photosynthesis.