Silicon improves the growth of Glycyrrhiza uralensis Fisch. under salt and/or drought stress by regulating respiration metabolism

Abstract

Drought and salt stress severely threaten the growth and productivity of Glycyrrhiza uralensis Fisch. (G.uralensis), and usually, they occur concomitantly. Silicon (Si) addition is well known to enhance the tolerance of plants. Here, we analyzed the changes in parameters related to growth and respiration, including glycolysis pathway (EMP), tricarboxylic acid (TCA) cycle, and pentose phosphate pathway (PPP) by Si in G. uralensis seedlings under salt and/or drought stresses. Results showed that Si effectively alleviated the effect of salt, drought and combined stress on the growth and respiration metabolism of G. uralensis seedlings. Under salt stress, Si increased leaf number by 15.13%, upregulated the TCA pathway at the transcriptional level, and increased PFK, PK, and content of acetyl CoA in both aerial and underground, ICDHm, α-KGDH in aerial part and NAD-MDH in underground part, thus alleviating the inhibitory effect of salt stress on the EMP–TCA process, but weakening the PPP by declining G6PDH and 6-PGDH in aerial part by 34.10% and 55.54%, and G6PDH in underground part by 83.86%. Under drought stress, Si increased underground biomass by 11.40%, slowed the EMP by decreasing PFK in aerial part by 21.76%, PK and PFK in underground part by 38.11% and 64.71%, weakened PPP by reducing 6-PGDH in aerial part by 72.9%, G6PDH and 6-PGDH in underground part by 77.00% and 64.71%. Si increased SDH and phosphoenolpyruvate carboxylase (PEPC) by 28.66% and 145.48% with the TCA cycle and GABA shunt as the main respiratory pathways in the underground part of drought-stressed seedlings. Under combined stresses, Si addition increased leaf number and aboveground biomass by 11.96% and 23.88%, upregulated the EMP and TCA pathways at the transcriptional level, and alleviated the effects of combined stress on EMP–TCA metabolic processes at the physiological and biochemical levels by increasing PDH, α-KGDH and NAD-MDH in aerial part by 27.22%, 18.85% and 140.37%, HK, PK and PEPC in underground part by 93.97%, 25.81% and 31.19%, initiated the replenishment mechanism and inhibited GABA shunt in underground part of G. uralensis to provide energy for vital activities. In conclusion, Si can effectively regulate respiration metabolism process to provide more substrates for subsequent physiological processes, thus improving tolerance to salt and drought stress, and thereby promote the growth of G. uralensis seedlings under stress conditions.