With untransformed rice cv. Kitaake as control, the characteristics of carbon assimilation and photoprotection of a transgenic rice line over-expressing maize phosphoenolpyruvate carboxylase (PEPC) were investigated. The PEPC activity in untransformed rice was low, but the activity was stimulated under high irradiance or photoinhibitory condition. PEPC in untransformed rice contributed by about 5-10 % to photosynthesis, as shown by the application of the specific inhibitor 3,3-dichloro-2-(dihydroxyphosphinoylmethyl)propenoate (DCDP). When maize PEPC gene was introduced into rice, transgenic rice expressed high amount of maize PEPC protein and had high PEPC activity. Simultaneously, the activity of carbonic anhydrase (CA) transporting CO 2 increased significantly. Thus the photosynthetic capacity increased greatly (50 %) under high CO 2 supply. In CO 2 -free air, CO 2 release in the leaf was less. In addition, PEPC transgenic rice was more tolerant to photoinhibition. Treating by NaF, an inhibitor of phosphatase, showed that in transgenic rice more phosphorylated light-harvesting chlorophyll a/b-binding complexes (LHC) moved to photosystem 1 (PS1) protecting thus PS2 from photo-damage. Simultaneously, the introduction of maize PEPC gene could activate or induce activities of the key enzymes scavenging active oxygen, such as superoxide dismutase (SOD) and peroxidase (POD). Hence higher PS2 photochemical efficiency and lower superoxygen anion (O 2˙-) generation and malonyldiadehyde (MDA) content under photoinhibition could improve protection from photo-oxidation.
Photoinhibitory performance and its physiological basis have been studied in Oryza sativa L. subspecies indica and japonica, and their reciprocal F1 hybrids. The results demonstrate that the japonica ssp. was usually more tolerant to photoinhibition, indicated by higher maintaining capacity of D1 protein (less degradation), higher induced superoxide dismutase (SOD) activity and longer duration of the activity. Compared with japonica, the indica ssp. was more sensitive to photoinhibition, and exhibited more degradation of D1 protein and a much larger xanthophyll pool. A statistically significant positive correlation exists between D1 protein content and Fv/Fm, PSII activity and Pn during photoinhibition (r2 = 0.98, 0.93, 0.95, respectively, P < 0.01). This result further supported the hypothesis that D1 protein encoded by plastid genes might play an important physiological role in the mechanism of photoinhibition. This hypothesis is also enhanced by the fact that the capacity of D1 protein synthesis mediated the components and cycle of xanthophyll and non-photochemical quenching in treatment with streptomycin, a D1 protein synthesis inhibitor, while the xanthophyll cycle had a photoprotective role for D1 protein and Fv/Fm in treatment with DDT, a xanthophyll cycle inhibitor. The photoinhibition of reciprocal F1 hybrids between japonica and indica is mainly intermediate, but somewhat inclined to the maternal line. This demonstrates that the basic feature of photoinhibition was controlled by the interaction of an intrinsic factor, D1 protein encoded by plastid genes, with the xanthophyll cycle, as well as SOD, controlled by nuclear genes. Since the response to photoinhibitory treatment of indica–japonica F1 hybrids seems to depend on female parents, we propose to select and use photoinhibition-tolerant varieties as female parents for generation of photoinhibition-tolerant hybrids.
Changes in actual efficiency of PS II photochemistry, non-photochemical quenching (NPQ), content of xanthophylls and kinetics of de-epoxidation were studied in ABA-fed and non-ABA-fed leaves of rice and cabbage under NaCl stress. Salt stress induced more progressive decrease in actual efficiency of PS II photochemistry (ФPS II), higher reduction state of PS II, and a small significant increase in NPQ in NaCl-sensitive rice plants as compared with NaCl-tolerant cabbage plants, whereas exogenously supplied ABA alleviated the decrease in actual efficiency of PS II photochemistry (ФPS II), induced a lower reduction state of PS II, and caused higher capacity of NPQ in ABA-fed plants than in non-ABA-fed plants. As a result, there were higher activities of photosynthetic electron transport, higher capacity of energy dissipation, and lower cumulation of excess light in cabbage than in rice plants, and in ABA-fed leaves than in non-ABA-fed leaves. The effect of ABA was more efficient in cabbage than in rice plants. Addition of exogenous ABA resulted in enhancement of the size of the xanthophyll cycle pool, promotion of de-epoxidation of the xanthophyll cycle components, and a rise in the level of NPQ by altering the kinetics of de-epoxidation of the xanthophyll cycle. Protection from photodamage appears to be achieved by coordinated contributions by exogenous ABA and xanthophyll cycle-mediated NPQ. This variety of photoprotective mechanisms may be essential for conferring photodamage tolerance under NaCl stress.
In an attempt to explore the relationships between phosphatidylglycerol (PG) molecular species of thylakoid membrane lipids and sensitivities to chilling-induced photoinhibition, PG molecular species, D1 protein, electron transport activities of thylakoid membrane and the potential quantum yield (F(v)/F(m)) in rice treated under middle and low photon flux density (PFD) at 11 degrees C were analyzed by high performance liquid chromatography, enzyme hydrolysis, gas phase chromatography (GC) and so on. Results showed that the major molecular species of PGs in rice thylakoid membrane were 18:3/16:0, 18:3/16:1(3t), 18:2/16:0, 18:2/16:1(3t), 18:1/16:0, 18:1/16:1(3t), 16:0/16:0, 16:0/16:1(3t). There were large differences in the contents of unsaturated PG molecular species such as 18:1 approximately 3/16:0 approximately 16:1(3t) and saturated PG molecular species like 16:0/16:0 approximately 16:1(3t) among japonica cv 9516 (j-9516), japonica-indica hybrid F1 j-9516/i-SY63 (ji-95SY) and indica cv Shanyou 63 (i-SY63). J-9516 containing higher contents of unsaturated PG molecular species was manifest in stable D1 protein contents under chill and tolerant to chill-induced photoinhibition. In contrast to j-9516, i-SY63 with lower contents of unsaturated PG molecular species, exhibited unstable D1 protein contents under chill and was sensitive to chill-induced photoinhibition. ji-95SY containing middle contents of unsaturated PG molecular species between those of j-9516 and i-SY63, exhibited mid extent of sensitivity to chill-induced photoinhibition. The losses in D1 protein also account for the inhibition in electron transport activity of thylakoid membrane and the observed decline in F(v)/F(m). The PG molecular species that is efficient in raising chilling-resistant capacity were those containing unsaturated fatty acids, namely, unsaturated PG molecular species. These results implied that the substrate selectivity of the glycerol-3-phosphate acyltransferase in chloroplasts towards 16:0 or 18:1 displayed greatly the difference between japonica and indica rice. It was possible to enhance the capacity of resistance to chilling-induced photoinhibition by improving or modifying the GPAT gene.
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