Study On The Tolerance To Cd Stress Of A Rolled Leaf Mutant X173WG In Rice

Rice is the most important cereal food crop in China. Leaf shape is an important agronomic trait. Moderate leaf rolling could keep leaf erect, raise the light energy utilization, reduce plant transpiration, improve the photosynthesis efficiency of population and thereby enhance grain yield and quality. With the rapid development of industry and agriculture, the cadmium pollution of cultivated land in China has become an increasingly serious concern. It negatively influences the growth and development of rice seedling, and affects rice yield and quality. Therefore, screening and breeding the rice varieties which accumulate less Cd and be more tolerant to Cd has important guiding meaning to guarantee the safety of agricultural products. In this paper, the comparative analysis had been carried on the alterations of phenotypes, agronomic and physiological characters, and also on the accumulation of Cd and mineral nutrients related with photosynthesis. And the genome sequencing of X173 WG and W0 was been taken via RNA-Seq. On this basis, we studied the responses to Cd stress of the rice mutant X173 WG during seed germination and seedling growth in depth. The main finding are as follow:1. Characteristics of a rolled leaf mutant in riceVia the survey and analysis the growth of X173 WG in all growth period, it was found that the leaf rolling lasts a lifetime. The leaf blades are narrow and rolled inward. And the Leaf Rolled Index(LRI) is far above the wild type W0. The main agronomic characters has also changing in X173 WG mutant. The panicle length, branch distribution, number of spikelets, number of grain per panicle and the setting percentage all decreases significantly.And the length-width ratio of grains is also different. The leaves of X173 WG are more green than W0, which causes high value of SPAD. From the cadmium uptake and accumulation in rice plants, Cd are mainly enriched in roots, followed by stems, grains and glumes, the least in leaves. Compared with W0, the Cd content in X173 WG of leaves and brown rice is much less, and the absorption of the minerals is much more. Via RNA-Seq,the differently expressed genes(DEGs) were selected out. And the pathway analysis byMap Man on the main DEGs related to leaf rolling indicated that the unigenes might play a regulatory role when under stress.2. Effects of Cd stress on seed germination of X173WGIn our study, 1, 10, 50, 100, and 1000 μM Cd was added at seed germination stage of X173 WG and W0. The results showed that cadmium stress(1~100 μM) had little effect on the germination rate, only inhibited remarkablely by the highest concentration(1000 μM).Different reactions to Cd was found in the plumule and radicle. W0 and X173 WG also showed different. 1-100 μM could obviously promote the growth of plumule, and it promoted more in W0. However, the plumele growth was restrained seriously when treated by 1000 μM Cd, and the tolerance was higher in X173 WG than W0 in this case. The roots of W0 were extremely sensitive to Cd. With the Cd concentration increasing, the roots became shorter and thicker. And roots of the mutant X173 WG was stimulated less compared with W0.3. Effects of Cd stress on seedling growth of X173WGUnder solution culture condition, the active toxicity on rice seedling was showed more clearly than seed germination. The growth of rice seedlings was very slow and leaves turned yellow when treated by high Cd. Then the plant height, root length and aboveground biomass was suppressed evidently. With the increase of the Cd level, the leaf chlorophyll content, net photosynthetic rate, stomatal conductance and transpiration rate all decreased gradually in these two rice materials. As the variation of rice leaf intercellular CO2 concentration was contrary to that of net photosynthetic rate, it revealed that the non-stomas factors were could be the main reason for the photosynthesis reduction when under cadmium stress. Furthermore, the photosynthetic parameters of X173 WG varied less than W0. The content of malondialdehyde(MDA) stayed at low levels when treated with low concentration of Cd(1~50 μM). Then it added rapidly when treated with 100 μM. And it still keep low levels when at 1000 μM. On the contrary, the activities of SOD, POD and CAT first increased and decreased and then increased with the rising of the concentration of Cd. Besides, Cd stress could obviously increase the Cd accumulation of rice seedling.However, The contents of Cd were different between different materials and organs. The roots of X173 WG absorbed more amount of Cd than W0. And the contents in leaves was less than W0. And therefore, Cd stress hurt less on the shoots of X173 WG. The Cd stress drived the contents of minerals like Cu, Zn, Mn, Fe, Mg down and up in Ca in rice leaves.The contents of Cu、Zn、Mg in roots decreased, and the accumulation of Fe, Mn, Ca was increasing obviously by the enhancement of Cd stress. Besides, the uptake andaccumulation of Fe was positively correlated with the contents of Cd in roots of X173 WG.Above all, the possible complex relationship between heavy metal Cd and minerals Fe, Mn, Ca in roots, might establish a special protective mechanism which could prevent the absorption of Cd, raise the activity of protective enzymes, guard against lipid peroxidation of cellular membrane and so mitigate the damage in rice plants. As a whole,the Cd tolerance of X173 WG seedlings is higher than W0 significantly.