The Role Of The Mitochondrial Alternative Oxidase Pathway In Photoprotection In Malus Hupehensis (Pamp.) Rehd. Leaves Under Drought Stress

There are two respiratory electron transport chains in higher plant, namely the cytochrome respiration oxidase pathways and the alternative oxidase pathway. The alternative pathway of mitochondrial respiration branches from the cytochrome pathway in the inner mitochondrial membrane at the ubiquinone pool and passes electrons to a single terminal oxidase, the alternative oxidase?AOX?. Because it is not restricted by the proton gradient across the membrane and the limitation of ATP/ADP ratio, The AOX pathway can bypass the oxidative phosphorylation process, directly consume NAD?P? H, so it can consume excessive electrons on the electron transport chain, and prevent respiratory chain from peroxide. In recent years, research has shown that the AOX pathway plays an important role in the process of plants resistance to adversity, because it can reduce the generation of reactive oxygen species. Under natural conditions, the high light is often an adversity faced by the plant. Excess light energy is harmful to plants and leads to photoinhibition. The study founds that the AOX pathway plays an important role in the photoprotection in arabidopsis and other plants. Recetly, the AOX pathway has become a hot topic in study of plant defencing adversity and photoinhibition.So far, all the studies in the photoprotection of the AOX pathway were limited in the condition of well wated plants, and whether the AOX pathway plays a role in photoprotection under drought stress is an unsolved problem. Under natural conditions, Drought stress is often an adversity faced by the plant. Drought causes leaf stomatal closure, limits the supply of raw material for photosynthesis, resulting in more severe photoinhibition. At the same time, drought stress may also inhibits the expression of enzymes involved in respiratory metabolism. In this case, wether the activity of AOX paythway is affected or not by the drought stress, or wether the photoprotection of the AOX pathway is affected by drought stress? It is a scientific question remained to be clarified. It will be theoretical significant to clarify the effect of drought on the activity of AOX and the photoprotection of the AOX pathway under drought stress.In addition, narly all the studies about the AOX photoprotection were performed using herb plants such as arabidopsis, wheat, tobacco and so on. No studies have been done using woody plants so far. In this study, the role of mitochondrial AOX pathway in photoprotection in Malus hupehensis?Pamp.? Rehd. leaves under drought stress was studied. The main results obtained are as follows:?1? Drought increased the amount of leaf AOX protein and also enhanced the activity of AOX pathway?2? Under high light intensity, after inhibiting the AOX pathway with salicylhydroxamic acid?SHAM?, the maximal photochemical efficiency of PSII?F_v/F_m?, the J steps at the relative variable fluorescence transient?VJ?, the PSII excitation pressure?1-qP?, and electron transport rate?ETR? were not affected in water-replete plants, while the ETR and F_v/F_m significantly decreased and the 1-qP and VJ significantly increased in drought-stressed plants, indicating that under drought-stress condition, the inhibition of AOX pathway caused more severe photoinhibition. The results demonstrated that the AOX pathway played an important role in the photoprotection in Malus hupehensis?Pamp.? Rehd. leaves under drought stress.?3? In water-replete plants, after inhibiting the AOX pathway with SHAM, the content of H₂O₂ was not affected. While under drought, after treated with SHAM to inhibit the AOX pathway, the content of H₂O₂ was increased.?4? The role of mitochondrial AOX pathway in photoprotection in detached Malus hupehensis leaves under water stress was explored. After inhibiting the AOX pathway with SHAM, the effects of AOX pathway on photoprotection in detached Malus hupehensis?Pamp.? Rehd. leaves under water stress were studied by simultaneously analyzing chlorophyll a fluorescence transient and light absorbance at 820 nm. The results indicated that water stress induced the up-regulation of AOX activity. The inhibition of AOX pathway caused more severe photoinhibition. Under water stress, maximum quantum yield of primary PSII photochemistry?TRo/ABS? and PSII trapped electron being transferred from QA to QB?ETo/TRo? decreased, average absorbed photon flux per PSII reaction center?ABS/RC? increased, while the maximum PSI redox acitity??I/Io? was not affected. After inhibiting the AOX pathway, ABS/RC markedly increased, TRo/ABS, ETo/TRo as well as ?I/Io significantly decreased. It indicated that under water stress, the inhibition of AOX pathway caused more severe photoinhibition, especially to the PSI. Generally, the results demonstrate that the AOX pathway played an important role in the photoprotection in Malus hupehensis?Pamp.? Rehd. leaves under water stress, particularly in the photoprotection of PSI.?5? To eliminate the influence of signals produced by the root material?such as ABA? which may influence photosynthetic mechanism, we studied the dehydrarted leaves in vitro to find that under drought stress, the AOX pathway played a direct role of photoprotection.