Regulatory Mechanisms Of Silicon On Physiological And Biochemical Characteristics,Ultrastructure And Related Gene Expression Of Rice Under Water Stress

During the vegetative growth phase of rice, it is not beneficial to the formation of strong seedlings and the subsequent reproductive growth, if rice is subjected to water deficit. Silicon, which is actively absorbed and accumulated by rice, plays a unique role in the growth and development, stress tolerance, and formation of high yield and good quality of rice. Under water stress condition, the study on the effect of silicon application on rice seedlings at vegetative growth stage (seedling and jointing stage) could contribute to the understanding of regulatory mechanism of rice growth and development, and provide theoretical support for rice cultivation under drought stress and the promotion of silicon application. However, the relevant research is very lacking. Therefore, it is very important to study the regulatory mechanism of silicon on the rice growth under water stress.In this paper, Xiushui11(lowland rice) and Brazil upland rice (upland rice) were used as experimental materials. Water stress environment was induced by polyethylene glycol (PEG-6000) or soil drought by withholding irrigation. The purpose of this study was to study the influence of silicon on the physiological and biochemical characteristics by means of determination of physiological and biochemical indexes, microscopic observation and gene expression analysis techniques. The main results are as follows:1. Effects of silicon on the physiological and biochemical characteristics of rice at seedling stage under PEG-induced water stressPEG stress inhibited the absorption of silicon of rice seedlings. Application of silicon significantly alleviated leaf wilting, and improved the dry weight, free water content, total moisture content and relative water content in the roots and leaves of rice under PEG stress. The effect of improving water content in Brazilian upland rice was better than that in Xiushui11. Silicon application enhanced the osmotic adjustment ability, maintained higher cell turgor pressure, and improved the water status of rice. Under PEG stress, the contribution of soluble sugar to the osmotic adjustment in the leaves was the largest among the osmolytes in the cells, followed by K+. In the roots, K+was the largest contributor to the osmotic adjustment, followed by soluble sugar; while proline and soluble protein played a minor role to the osmotic adjustment. Application of silicon significantly reduced the relative electrolyte permeability, malondialdehyde (MDA) content, superoxide radical anion (O2-) generation rate and hydrogen peroxide (H2O2) content of rice under PEG stress. Silicon decreased the activity peaks of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APX); however, it was also advantageous to maintaining higher antioxidant enzyme activities at late stage of PEG treatment. In addition, silicon could significantly retard the decline of reduced glutathione (GHS), ascorbic acid (AsA) and carotenoid (Car) contents of rice seedlings under PEG stress. Application of silicon could alleviate the decline in leaf chlorophyll content, photosynthetic rate, stomatal conductance and transpiration rate, improve water use efficiency in PEG-stressed rice. Analysis of the chlorophyll fluorescence kinetics parameters in the leaves showed that silicon application significantly retarded dark-adapted maximum fluorescence yield (Fm), dark-adapted PS Ⅱ maximum quantum yield (Fv/Fm), steady-state fluorescence yield (Fs), light-adapted maximum fluorescence yield (Fm’), light-adapted PS Ⅱ maximum quantum yield (Fv’/Fm’), PS Ⅱ actual photochemical efficiency (ΦPS Ⅱ), photosynthetic electron transport rate (ETR) and photochemical quenching coefficient (qP) under PEG stress. Silicon could slow down the decline of respiration rate under PEG stress, and the retarding effect was more obvious in leaves than that in roots. Under PEG stress, silicon could enhance the vigor and the active absorbing area of roots. The application of silicon could retard the degradation of total protein, RNA, DNA and abscisic acid (ABA) in rice seedlings.2. Effects of silicon on the physiological and biochemical characteristics of rice at jointing stage under soil drought stressThe osmotic adjustment capacity of Xiushui11was slightly stronger than Brazilian upland rice, and therefore the tolerance performance to drought stress of Xiushui11was better than Brazilian upland rice. Soil water deficit significantly inhibited the absorption of silicon by rice seedlings. Under drought stress, silicon could enhance the osmotic adjustment ability, increase dry weight and water content, slow down the drop of cell turgor pressure of rice seedlings. Under drought stress, K+, NO3-and soluble sugar were the main osmolytes in osmotic adjustment; however, proline and soluble proteins contributed little to osmotic adjustment. The application of silicon could obviously decrease the formation of MDA and rupture of plasma membrane under drought stress, reduce the production rate of O2-and H2O2, slow down the decline of antioxidant enzymes activities in rice leaves, and improve non-enzymatic antioxidant contents under drought stress. Silicon could inhibit the degradation of chlorophyll, slow down the drop of the net photosynthetic rate and transpiration rate, increase water use efficiency in rice leaves under water deficit stress. Under drought stress, silicon application enhanced the vigor and the water flow rate of rice root systems. The physiological and biochemical characteristics of rice under soil drought stress were consistent with PEG stress treatment.3. Effects of silicon on the ultrastructure of rice seedlings at seedling stage under PEG stressUnder PEG stress, rice chloroplasts were obvious deformed and disintegrated, and most of mesophyll cells were deformed seriously because of dehydration. However, under PEG treatment supplied with silicon, the shape of many chloroplasts was more regular, and the deformation of most mesophyll cells was less serious than the treatment which was treated with PEG only, In-Si/+PEG treatment, nuclear disintegration occurred more frequently in root cells than that in+Si/+PEG treatment. The root cell wall was thickened after the application of silicon, and perhaps this thickening effect was related with enhanced root lignification by silicon application.4. Effects of silicon on the differential gene expression of the silicon-binding proteins, silicon transporter proteins, aquaporins and late embryogenesis-abundant proteins of rice seedlings at seedling stage under PEG stressThe gene expression level of the silicon-binding proteins and silicon transporter proteins could be enhanced by the application of silicon, but their expression levels were significantly inhibited by PEG. The expression levels were higher at the initial stage after silicon application, however, with stress progressing, the expression decreased gradually. The mRNA expression relative abundance of the silicon-binding proteins and silicon transporter proteins of Xiushui11were significantly positively correlated with the total silicon content in corresponding organs (leaves and roots). Application of silicon further strengthened the inhibition of gene expression of rice aquaporins under PEG treatment. Under PEG stress, compared with the control, the gene expression relative abundance of late embryogenesis-abundant proteins (LEA) was significantly increased, but it was significantly reduced by silicon application. The gene expression level of LEA protein was extremely significantly positively correlated with the ABA content, significantly negatively correlated with the MDA content, relative water content and cell turgor pressure in leaves of Xiushui11.In conclusion, under water-deficit stress, silicon application could increase water content and improve organizational water status, thereby enhancing the drought-resistance ability of plant, and manifested as improvement of photosynthesis and other physiological characters. The enhanced drought-resistance ability might be mainly based on the improved osmotic adjustment ability, plasma membrane stability and root activity in the stressed rice after treated with silicon.