Unveiling Mechanisms of Silicon-Induced Salt or/and Drought Tolerance in Glycyrrhiza uralensis Fisch by Physiological and Transcriptomic Analysis

The study aims to assess the role of Si on growth and photosynthesis of Glycyrrhiza uralensis Fisch. (G. uralensis) seedlings under salt or/and drought stresses. G. uralensis seedlings were employed to study the change of plant growth parameters, chlorophyll (Chl) contents, photosynthesis parameters and fluorescence parameters, key enzyme activities in Calvin cycle and photorespiration, and transcriptome sequencing analysis by stresses and silicon treatment. Si increased Chl a, energy dissipation (Y(NPQ)), and unregulated energy dissipation (Y(NO)) but reduced the maximum quantum efficiency and actual photosynthetic efficiency of PS II (F-v/F-m and Y(II)) in salt-stressed plants, indicating that increased Chl a may absorb excess light energy thus damaging PSII; the downregulation of "photosynthesis" and "photosynthesis-antenna proteins" further confirmed this inference. Besides, Si reduced Rubisco, GAPDH, and downregulated "carbon fixation in photosynthetic organisms" pathway; caused a lower CO2 assimilation; and finally reduced photosynthesis in S + Si-treated plants. Under drought stress, Si decreased content and proportion of carotenoid (Car), leading to a great decrease in Y(NPQ); however, Si increased GO and intensified photorespiration for photoprotection, thus improving the utilization of light energy, and increases in F-v/F-m and Y(II) were observed; meanwhile, Si enhanced GAPDH and FBPase to increase synthesis and utilization of triose phosphate, and finally improved photosynthesis. Under combined stress, Si remarkably reversed the inhibition of combined stress on decrease in photosynthesis parameters (P-n, g(s), C-i, and T-r), F-v/F-m, Y(II), GAPDH, and FDA, thus improving photosynthesis. Furthermore, Si regulated the expression levels of core genes CAB215, CAB6A, and LHCB4.3 to reconstruct the photosynthetic apparatus, thus balancing the absorption and utilization of light energy. Si can effectively alleviate the inhibitory effect caused by drought and combined stresses to photosynthesis of G. uralensis.