Physiological Responses To Low-light Stress And Molecular Basis Of Different Maize (Zea Mays L) Genotypes

Huang-Huai-Hai region is one of the three main maize production areas. The rainfall and accumulated temperature in this region during maize growth is plentiful. However, excessive rainfalls always bring detrimental influence on maize production because maize is fond of light. Previous study indicated that there are significant differences of maize genotypes in response to low light stress. But the physiological and molecular mechanism of the different response to low light stress is unclear. Explore the responses of different maize genotypes under low light stress at physiological and molecular level and screen low-light stress response genes are very important for the low light stress tolerance maize cultivar breeding. Low light stress tolerance and low light stress sensitive maize genotypes were used in the field and laboratory experiment. Maize root and shoot morphogenesis, ear development and kernel set of different maize genotypes under low light treatment were investigated, maize leaf volatile emission pattern and senescence progress under low light treatment were explored. And cDNA libraries of different maize genotypes grown under low light treatment and control conditions were constructed. Low light responsive genes were screened.The main results were as follows:(1) Low light treatment at seedling stress leads a reduction of maize stem diameter, leaf area, specific leaf weight, root volume, root number, shoot biomass, root biomass, total biomass and root:shoot ratio. While the plant height under low light treatment at seedling stage was similar to the control. Low light treatment at seedling stage also altered leaf morphology, led a higher leaf length:width ratio. The extent of variation between low light treated and the control was larger in the low light sensitive maize genotypes than the low light tolerance maize genotypes.(2) Low light treatment at ear differentiation stage repressed maize ear development. Ear length, ear diameter and ear fresh weight were decreased under low light treatment. The ear length, ear diameter and ear fresh weight of the low light tolerance maize genotype XD20 decreased more than the low light sensitive maize genotype YY22. Silks of XD20 emerged outside of the husk under low light treatment decreased slightly; the silks of YY22 emerged outside of the husk under low light treatment decreased significantly. Ear traits of YY22 decreased more than XD20 under low light treatment at the maturity stage. This indicates that, although ear biomass accumulation of low light tolerance maize genotype XD20 was lower than low light sensitive maize genotype YY22 under low light treatment at ear differentiation stage, the development coordination of XD20 was better than YY22. Low light treatment at ear differentiation stage resulted in a reduction of kernel number per ear. The thousand kernel weight increased after the light recovery. At a certain extent, the increase of a thousand kernel weight can make up the yield reduction caused by kernel number per ear reduction.(3) Fingerprints of maize leaf volatile changed significantly under low light treatment. Insufficient light would affect maize leaf volatile emission. High planting population density leads insufficient irradiance for the leaves grown in the mid-and lower-canopy level. The light transmittance of light in the mid-and lower-canopy level of the high population density was significantly lower than the low population density. Maize leaf volatile emission of leaves grew at the upper-and mid-canopy levels were significantly higher than the leaves grew at the lower-canopy level. The leaf volatile emission of the mid- and lower-canopy level grew leaves of the high population density were significantly lower than that of the low population density. While the leaf volatile emission of the upper canopy level grown leaves was similar both in the high and low population densities. Leaf volatile emission decreased significantly under low light treatment. Leaf volatile emission of maize leaves grown in the upper- and mid-canopy level first slightly increased and then decrease with leaf aging. The lower-canopy level grown leaves volatile emission decreased dramatically along with leaf aging. In contrast, MDA accumulation increased with leaf aging. This indicates that leaf volatile emission may negatively correlate with leaf senescence. (4) By using RNA-Seq, we constructed four cDNA libraries of a low light tolerance (Z72) and a low light sensitive (502) maize genotype grown under low light treatment and control conditions. Low light responsive genes or tags were screened. There were 571 and 1,539 genes significantly up-regulated in Z72 and 502, respectively. There were 697 and 1,657 genes significantly down-regulated in Z72 and 502, respectively. Among these significantly regulated genes,189 genes were common up-regulated, and 220 genes were common down-regulated in both Z72 and 502. Pathway enrichment analysis resulted three and four significantly changed metabolism pathway under low light treatment of Z72 and 502, respectively. The significantly changed metabolism pathways for Z72 were metabolism, secondary metabolism and a-Linolenic acid metabolism. And pathways for 502 were photosynthesis, Arginine and proline metabolism, glutathione metabolism and ribosome. Besides the seven significantly changed metabolism pathways, Porphyrin and chlorophyll metabolism in Z72 and Spliceosome metabolism in 502 were significantly up-regulated.In conclusion, low light treatment not only changes maize morphological traits and physiological metabolism, but also alters maize transcriptome. The responses of different maize genotypes to low light stress are different. The gene expression is more stable of the low light tolerance maize genotype than the low light sensitive maize genotype. And low light tolerance maize genotype performes better and could produce more yields under low light growth conditions than low light sensitive maize genotype.