Studies On Physiological Mechanisms Of Over-accumulating Glycinebetaine In Improving The Salt Tolerance In Wheat

Under environmental stress such as salinity and drought, plant cell usually accumulates some compatible solutes (also regarded as osmolytes) in order to alleviate the injury derived from environmental stress. Glycinebetaine (GB) is regarded as an extremely effective compatible solute, therefore researchers pay much attention to the bio-engineering of GB. The BADH gene encoding BADH which catalyzes betaine aldehyde (BA) into GB stands the essential position in the synthesis of GB, and gene engineering of it has become an important means to raise the GB content in vivo of plants. One wild-type wheat cultivar (Triticum aestivum L.) Shi4185 and three transgenic lines with BADH gene from Atriplex hortensis L were used to study the improvement of over accumulating GB on salt tolerance of wheat and the underlying mechanisms involved in were discussed. Salt stress was imposed by Hoagland solution containing 200mmol/L NaCl for 2 and 4 days. Indexes determined included content of GB, relative water content, osmotic adjustment ability, content of compatible solutes, ions, and chlorophyll content, the activity of ATPase, Hill reaction, anti-oxidant enzymes activities, and the components of thylakoid membrane lipids. The main results are as follows:1. Over accumulated GB enhanced the water-absorbing from salt stress environment and also maintained well water status in wheat seedlings, which may due to amelioration of osmotic adjustment and ion homeostasis.(1)Introducing of foreign BADH gene visibly induced the accumulation of GB. Besides osmolyte itself, accumulated GB could sustain a higher level of osmotic adjustment (OA) via accumulating some other compatible solutes such as proline and soluble sugar, which enhances water-absorbing and consequently maintains a better water status. (2)Salt stress caused accumulation of toxic Na+ and Cl-, and destroyed the ion homeostasis of wheat plants. Over accumulating GB restrained the transport of Na+ and Cl- from roots to sheaths and leaves, maintained a higher level of content of K+, Ca2+, Mg2+ and rate of K+/Na+ in leaves, mitigated the toxic effect of ions, and ultimately ameliorated the salt tolerance of transgenic lines.2. Over accumulating GB maintained higher activity of the main antioxidant enzymes and effectively protected the membrane integrity and stability from salt stress.Salt stress caused increase in superoxide radical (O2.-) production and destroyed the membrane integrity and stability, and ultimately resulted in evident increase in malondialdehyde (MDA) content and electrolyte leakage. Some main antioxidant enzymes such as SOD, CAT, POD and APX of transgenic lines with over accumulating GB maintained higher activity under salt stress, which then inhibited increase of O2.- production, MDA content and electrolyte leakage caused by salt stress. And ultimately, over accumulating GB effectively decreased the peroxidant level and protected the membrane integrity and stability of transgenic lines from salt stress.3. Over accumulating GB maintained higher level of photosynthesis through protecting membrane lipid components and function of protein complexes from salt stress.(1)Compared with wild-type Shi4185, transgenic lines with over accumulating GB were ameliorated in many photosynthesis indexes, such as decrease in CO2 assimilation, photochemical efficiency of PSâ…¡and stomatal conductance alleviated. And what's more, degradation of pigments, decrease in Hill-reaction and ATPase activity were all mitigated.(2)Reactive oxygen species induced by salt stress might attack the unsaturated fatty acid, caused peroxidantion of membrane lipid, and also induced changes in molecular structure and component of membrane lipids. Transgenic lines with over accumulating GB had a relatively stable status under salt stress in components and levels of thylakoid membrane lipids. And this is advantageous not only to moderate the pigment degradation, Hill-reaction and ATPase activity decline, destruction in photosynthetic apparatus, but also maintain a higher level of photosynthetic ability.From all the results above, we propose that, the enhancement in photosynthetic ability and salt tolerance in transgenic lines under salt stress might be relative to these aspects. Firstly, GB ameliorates water status of cell by enhancing OA, which functioning by GB itself and other osmoyltes induced by GB. This is favorable for stomatal and non-stamatal factors of photosynthesis. Secondly, through maintaining or enhancing the activity of antioxidant enzymes, the accumulation of ROS and peroxidant level of membrane lipids are mitigated, integrity and fluidity of thylakoid membrane are sustained. Functional proteins embed in thylakoid membrane exist in a relatively stable environment and their functions are also protected indirectly. Thirdly, GB can help to maintain higher photosynthetic ability by directly protecting the structure and function of protein complexes embed in thylakoid membrane.