Mechanism Of Silicon Induced UV-B Resistance In Soybean Seedlings

Increased solar UV-B (280-320 nm) radiation caused by stratospheric ozone depletion has influence on plant morphology, physiology, growth and development. Silicon is known to induce plant UV-B resistance, although its mechanism still remains to be known. In the present study, a hydroponic study was conducted to examine effects of silicon (1.70 mM) on morphology, primary and second metabolites content, level of endogenous hormones, expressions of leaf proteins and DNA damage of soybean seedlings under enhanced UV-B radiation (5.4 kJ m-2d-1). This study aimed to further illuminate biological efficiency of silicon in enhancing UV-B radiation stress resistance and repair of soybean seedlings based on former researches. Results obtained are summarized as follows:1. Impacts of UV-B radiation stress on soybean seedlin morphology included increase in cell death and curling of the leaf surface and decrease in leaf area; inhibiting growth of roots, resulting in smaller root length, volume and surface area;, thus resulting in dwarf plants and less dry matter accumulation, and changing biomass partitioning between overground and underground part. Silicon notably mitigated UV-B-induced increase in cell death and curling of the leaf surface and decrease in leaf area, ameliorated the detriment influence on growth of roots, reduced the diminution of root morphology index like root length, volume and surface area, and mitigated UV-B-caused inhibition on plant growth, resulting in higher seedlings, more total biomass and better biomass partitioning between overground and underground part compared with UV-B-treated plants.2. UV-B radiation stress caused an increase in ABA and ETH concentration and decreased IAA content and IAA/ABA ratio; exogenous silicon could significantly alleviate the increase in ABA and ETH level, the decrease in IAA content and IAA/ABA, thus the inhibiting effect of UV-B radiation stress on growth of soybean seedlings. Meanwhile, exogenous silicon enhanced the content of soluble sugar and protein which were decreased by UV-B radiation stress, while decreased the content of free amino acid which was increased under UV-B radiation stress, promoting regular growth of seedlings. In addition, UV-B radiation induced higher PAL activity, secondary metabolites content like cuticular wax, UV-B absorbing substance, phenolic and fiavonoid, while exogenous silicon could further increase PAL activity and secondary metabolites content.3. Exogenous silicon could regulate expressions of relevant proteins under UV-B stress to strengthen the resistance and adaptation of soybean to UV-B radiation stress. The proteins were mainly involved in energy/metabolism, secondary metabolism, plant defense, antioxidant system and phytohormone synthesis. And we might conclude that UV-B stress can be an inducer of system acquired resistance via molecular signals, ethylene and ROS to make plants better adapt to the altered environment. Silicon could significantly enhance broad spectrum disease resistance and improve the photosynthesis and secondary metabolism.4. UV-B radiation stress did induce formation and accumulation of CPDs in leaves of soybean seedlings, while direct oxidative stress caused no such damage. That is to say, the formation of DNA photoproduct cyclobutane pyrimidine dimers was exclusively due to UV-B through direct radiation and indirect UV-B-induced ROS involvement. Photo-repair pathway was more efficient than dark-repair in soybean seedlings and was further investigated. In summary, exogenous silicon could prevent the formation of CPDs, reduce accumulation of CPDs by promoting photo-repair activity, and enhance the repair ability of DNA damage. As an effective UV-B protectant, silicon provided protection against UV-B-induced CPD formation in three ways:1) repair mechanisms, DNA damage repaired by photolyase of cyclobutane pyrimidine dimers (CPDs); 2) avoidance mechanisms, epidermal screening of UV-B radiation by accumulation of UV-B absorbing compounds like secondary compounds; 3) antioxidation mechanism, formation of antioxidants, like ascorbate.