Growth inhibition of sourgrass as a function of period of darkness after diquat application

Photosystem-inhibiting herbicides, such as diquat, act by inducing oxidative stress. However, oxidative damage impairs translocation, resulting in regrowth of the plants. The aim was to evaluate the effectiveness of diquat in controlling the growth of sourgrass exposed to different periods of darkness after application of the herbicide, as well as to evaluate the photosynthetic activity and the production of reactive oxygen species. Two experiments (field and greenhouse) were conducted by applying diquat (200 g a.i. ha-1) on sourgrass plants at the 3 to 4 tiller stage. The treated plants were subjected to different periods of darkness after diquat application (0, 1, 2, 3, 4, 5, and 6 h), in addition to the control treatment without any application. Growth inhibition and mass evaluations of the sourgrass plants were performed in both experiments, whereas photosynthetic activity and H2O2 accumulation in the leaves were evaluated in the greenhouse experiment. The results showed an increase in the sourgrass growth inhibition with an increase in the period of darkness after application. There was a need for a minimum of 6 h of darkness after diquat application to fully inhibit growth (100%) of the sourgrass, whereas plants that remained in the sun since application exhibited less than 50% inhibition. The increase in the period of darkness after diquat application resulted in a reduction in photosynthetic activity and, consequently, lower accumulation of H2O2. Thus, the maintenance of sourgrass in the dark for at least 6 h enables total control of the growth of the plants, preventing regrowth.

Functional Analysis of SI10875 in the Regulation of PS I Activity in Synechocystis sp.PCC 6803 / 中国生物化学与分子生物学报

Phylloquinone is a unique cofactor of photosystem I (PS I ) , made up of a redox-active naphthoquinone ring attached to a partially saturated C-20 phytyl side chain.At present, the research on the biosynthesis of phylloquinone in cyanobacteria is mainly focused on the formation of naphthoquinone ring, while there was a shortage of reports in the biosynthesis of phytyl side chain.In this study, a highly homologous protein S110875 was found in Synechocystis sp.PCC 6803 by homologous sequence alignment with VTE6, a kinase involved in phylloquinone biosynthesis by converting phytyl-phosphate into phytyl- diphosphate in Arabidopsis thaliana.The resulting S110875 mutant, called As/Z0875, accumulates none phylloquinone and tocopherol, as well as low amounts of chlorophyll content (P<0.05).The mutant had retarded growth in the absence of added glucose.Chlorophyll fluorescence, P700 absorbance changes, 77 K fluorescence emission spectra and Western blot analyses showed that in As/Z0875, PS I function was impaired and accumulation of the PS I complex was reduced remarkably (P<0.01), indicating that phvlloquinone deficiency affected PS I function, thus hindering the normal growth of cyanobacteria.Our results provide the evidence that the phytol phosphorylation pathway is essential for phylloquinone biosynthesis in cyanobacteria for the first time, and a basis for further investigate the protein synthesis, assembly and stability of PS I complex in cyanobacteria.

Mechanism of reductive photoactivation of enzymes of C4 pathway.

Light, besides initiating primary photochemical processes, alters the redox state of soluble components in chloroplast. The present review attempts to cover the mechanism of reductive photoactivation of enzymes of photosynthetic carbon reduction cycle using key enzymes as examples. The reduced soluble components — ferredoxin, thioredoxin and NADPH, in turn, cause the reduction of disulphides to dithiols of chloroplastic enzymes. NADP-malate dehydrogenase is subject to activation by light through changes in NADPH/NADP. The key enzyme of C4 photosynthesis-PEP carboxylase, though cytosolic, has been shown to be activated by disulphide/sulphhydryl interconversion by reductants generated in light through chloroplast electron transport flow. Pyruvate Pi dikinase activity is controlled by the adenylate energy charge. It remains unclear how light controls the activation of cytosolic enzymes.