Studies On The Functions Of Chloroplast Lon4 Protease In Arabidopsis Thaliana

Due to their sessile nature, plants are constantly faced with various adverse environmental factors, including drought, high salt, heavy metals, cold, heat shock, and ozone, during their whole life span. In the long-term evolution of higher plants, it has developed a series of sophisticated self-regulating system to adapt to the natural environment. Proteases are widely distributed in bacteria, archaea and eukaryotic cells and participate in the regulation of numerous physiological processes.Lon is an ATP-dependent serine protease, there are at least four Lon proteases in Arabidopsis. Lonl and Lon4 are shown to be located in chloroplast, while Lon2 and Lon3 are targeted to mitochondria by GFP signal detected analyses. To study the in vivo function of LON4, we obtained the T-DNA insertion mutant lines from the SALK collections. In order to have better studies in the mutant response to drought stress, many physiological and biochemical data involved drought stress had been measured, which suggested that Lon4 protease can protect plant from drought stress.Under normal growth conditions, the lon4 mutant displays a phenotype similar to that of the wild-type. The Fv/Fm ratios (indicating the maximum potential capacity of the photochemical reactions of PSII), chlorophyll and photosynthetic proteins contents, and photosynthetic protein complexes of lon4 mutant were also similar to that of wild-type plants. However, under drought stress, the lon4 mutant displayed increased sensitive to drought stress, compared to wild-type. When re-watering, plants can grow, but the mutant lon4 were grown much slower than wild type. Analysis of the water content and water loss rate of wild-type and lon4 mutant w showed that the water contents of lon4 mutant leaves were unaffected, while the reduced rate of water content in lon4 mutant was much higher than that of wild type with increasing time, these results indicated that the reduced tolerance of mutant to drought is mainly due to low capacity with holding water.Under drought stress, the proline and soluble sugar contents of the wild-type and mutant were increase, but the increasing extent in wild-type plant is significantly higher than that of wild-type mutant. Drought-induced lipid peroxidation is key factor to cause the injury of plant cell membrane, and membrane damage will lead to plant tissue damage and senescence. Drought stress leading to increased H2O2 content in lon4 mutant plants which maybe for the increased lipid peroxidation of mutant. The activity of SOD and CAT in wild-type and mutant were increased in response to drought stress; SOD activity of lon4 mutant was significantly higher than that of the wild type. However the CAT and APX activity of lon4 mutant was less affected. Both endogenous hormone cytokinin (ZR) and gibberellin (GA) levels were decreased in wild type and lon4 mutant under drought stress conditions.Electron micrographs of ultrathin sections from the leaves of wild-type and mutant plants under drought stress conditions were analyzed, unlike the wild type stomatal; the atlon4 mutant stomatal was not closed normally to decrease the water loss from leaves. Meanwhile, the ABA level in the atlon4 mutant was significantly decreased compared with that in the wile type plants under the drought stress condition, which suggested that the LON4 may be involved in the degradation of the proteins that are involved in ABA signal pathway. In our study, the Arabidopsis lon4 mutant was sensitivity to drought stress, which is due to substantial water loss, decreased water utilization efficiency and impaired stomatal closure in lon4 mutant plants under drought stress conditions.