Research On The Expression Of Plastid Terminal Oxidase And Related Genes In Soybean Under Different Light Intensities

In recent years, the maize-soybean relay strip systems have acquired a great development and became an effective way to promote soybean industry in China. But in the maize-soybean relay strip intercropping systems, during co-growth period high shade of maize and enhanced light intensity after maize harvest affect the photosynthetic ability of soybean, which are also the two key factors of limiting yield. Therefore, to better develope the maize-soybean relay strip systems, we'd tend to further understand of soybean's photosynthetic characters under different light intensities, stabilize and increase soybean's photosynthetic ability. In the present study, two soybean cultivars (Nandou 12 and Nan 032-4) with different yields and shade tolerance were respectively selected to study the effects of light intensity on plastid terminal oxidase and related functional genes under three different light intensities (low light, moderate light and high light) in the phytotron chamber and two cultivation patterns in the field condition like soybean monoculture and intercropping with maize. The two pathways of cycle electron transportation were regulated by NAD(P)H dehydrogenase (NDH) complex and proton gradient regulation 5 (PGR5), respectively. The phytoene desaturase (PDS) and ?-carotene desaturase (ZDS) are two important enzymes in carotenoid biosynthesis. RbcS is the small subunit of 1,5-ribulose bisphosphate carboxylase oxygenase in photosynthesis and is encoded by nuclear gene. AOX1, AOX2a and AOX2b are three types of alternative oxidase (AOX), which show homology with PTOX. Besides, the photosynthetic parameters, chlorophyll fluorescence measurements, photosynthetic pigments, antioxidase activity and malate dehydrogenase activity under three different light intensities were determined in two soybean cultivars. The main results were as follows:1. Under different light intensities in the phytotron chamber, the transcriptional levels of PTOX, PGR5, PDS, ZDS, AOX1 and AOX2a were up-regulated gradually with the increase of light intensity in both cultivars, of which the transcript levels of PTOX, PGR5, PDS and ZDS in Nandou 12 were higher than in Nan 032-4, while the AOX1 and AOX2a were just the reverse. But the transcript levels of NDH-H?RbcS and AOX2b remained almost the same under different light intensities, in both cultivars.2. Under field conditions, before the maize harvest, the transcriptional levels of PTOX and PGR5 in monoculture were higher than in intercrop, and in upper leaves were higher than in lower leaves, in both cultivars; the AOX1 and AOX2a transcriptional levels of Nandou 12 in monoculture were lower than in intercropping, while in Nan 032-4 were just opposite, and the AOX1 and AOX2a transcript levels in upper leaves were higher than in lower leaves, in both cultivars; The transcript levels of PDS and ZDS in intercropping were higher than in monoculture, and in upper leaves were higher than in lower leaves, in both cultivars. After the maize harvest, the transcript levels of PTOX, PGR5, AOX1 and AOX2a in monoculture were higher than in intercropping, and in upper leaves were higher than in lower leaves, in both cultivars. Besides, compared with the cultivars before the maize harvest, the expression levels of PTOX, PGR5 and PDS in Nandou 12 were higher than in Nan 032-4, at the same cultivation pattern and the leaves of same location, while the AOX1? AOX2a and ZDS were just the reverse. There was no difference of PTOX and ZDS between two cultivars at the same cultivation pattern and leaves of the same location after the maize harvest. The transcriptional levels of NDH-H?RbcS and AOX2b were unchangeable under any conditions, in both cultivars.3. Under different light intensities, the net photosynthetic rate in Nandou 12 was higher than in Nan 032-4, and its intercellular CO2 concentration was lower comparing with Nan 032-4, while there was no significant difference in stomatal conductance between the two cultivars. This suggested that the difference in PN between cultivars should be owing to non-stomatal factors. The contents of chlorophylls and carotenoids were all decreased with the increase of light intensity, and the contents of these pigments in Nandou 12 were higher than in Nan 032-4 under low light intensity, moderate light intensity and field conditions, but this difference was not significant under high light intensity. This suggested that the difference in photosynthetic ability have a close relationship with the ability of capturing light in both cultivars, but there were still some other reasons remained unclear under high light intensity. In my further study, the maximal quantum yield of PS ? photochemical quenching and quantum efficiency of PS ? in both cultivars were all declined with the increase of light intensity. Their nonphotochemical quenching had a higher level under high light intensity, of which Nandou 12 was significantly higher than Nan 032-4. These results suggested the happening of photo-inhibition in both cultivars, and Nandou 12 had stronger ability to dissipate excess excitation energy than Nan 032-4 under high light intensity. Besides, The activities of superoxide oxidase, ascorbate oxidase and two malate dehydrogenases in both cultivars were all rising with the increase of light intensity, in detail the activities of superoxide oxidase and ascorbate oxidase in Nandou 12 were higher than Nan 032-4, while the two malate dehydrogenases were just the reverse. This indicated that Nandou 12 might have a higher activity of antioxidase, but its ability of exporting reducing substances in chloroplast was weaker than NanO32-4's.In conclusion, the gene expression of terminal oxidases, including PTOX and AOX, were up-regulated in both soybean cultivars when light intensity increased. Meanwhile, the related functional genes (PGR5, PDS, ZDS) were also up-regulated. These results indicated potential roles of chlororespiration and alternative respiration, as well as CET in soybean's photo-protective mechanisms. Under high light intensities, the Nandou 12 had higher activity of antioxidase, PN and NPQ, as well as a higher expression level of PTOX, PGR5, PDS, ZDS than Nan 032-4. This implied that Nandou 12, which has stronger chlororespiration and CET, might regulate the light distribution better than Nan 032-4 which has stronger alternative respiration in mitochondria. That is the reason why two cultivars have different photosynthetic abilities.