The Mechanism Of Silicon On Enhancing Sorghum Resistance To Drought, Salt, Cadmium, K-deficiency

Silicon is the second most abundant element in the soil after oxygen. Althoughsilicon is not currently considered an essential element for higher plants, siliconuptake has frequently been found to be beneficial in increasing plant resistance tomultiple stresses. Moreover, previous experiments were conducted to investigate themechanism of silicon on enhancing plant resistance to stress. There are accumulatingevidence demonstrated that the physical barrier formed by silicon deposited theorganism was responsible for silicon enhancing plant resistance to stress. However,with the further extended investigation, the molecular and physiological mechanismwas also involved in silicon-induce resistance to stress. The objective of this paperwas to investigated the the effect and mechanism of silicon on enhancing theresistance to water deficient, salt, cadmium and K-deficiency stress.1: The influence and mechanism of silicon on enhancing sorghum seedlingsresistance to drought. The fact that silicon application alleviates water deficitstress has been widely reported, but the underlying mechanism remains unclear. Herewe investigated the effects of silicon on water uptake and transport of sorghumseedlings (Sorghum bicolor L.) growing under water deficit stress in both hydroponicand sand culture. Water deficit dramatically decreased dry weight, photosynthetic rate,transpiration rate, stomatal conductance and leaf water content, but silicon applicationreduced these stress-induced decreases. Meanwhile, although silicon application hadno effect on stem water transport capacity, but whole plant hydraulic conductance(Kplant) and root hydraulic conductance (Lp) were higher in silicon-treated seedlingsthan those without silicon under water deficit stress. Furthermore, the changes intranspiration rate were consistent with the changes in Kplantand Lp. The contributionof aquaporin to Lp was characterized using the aquaporin inhibitor mercury. Underwater deficit stress, the exogenous application of HgCl2decreased the transpiration rates of seedlings with and without silicon to the same level; after recovery inducedby dithiothreitol (DTT), however, the transpiration rate was higher in silicon-treatedseedlings than in untreated seedlings. In addition, transcription levels of several rootaquaporin genes were increased by silicon application under water deficit stress.These results indicate that the silicon-induced up-regulation of aquaporin, which wassupposed to increase Lp, was involved in increasing root water uptake under waterdeficit stress.2: The influence and mechanism of silicon on enhancing sorghum seedlingsresistance to salt. Silicon improved plant salt resistance has been widely reported.Most previous researches were focused on how silicon prevents Na+uptake andaccumulation, but the performance and underlying mechanism of silicon onalleviating salt-induced osmotic stress has been largely ignored. In the present study,the mechanism of silicon on alleviating salt-induced osmotic stress was investigatedby sorghum in a hydroponic system. Silicon had no effect on seedlings growth undernormal condition. Under salt stress, photosynthesis and transpiration rate weredecreased, but their decrease were alleviated by silicon application. Meanwhile, leafwater content and leaf elongation rate were kept at a higher level than that withoutsilicon. Root hydraulic conductance (Lp) of seedlings were inhibited by salt, butsilicon application alleviated the inhibition. Under salt stress, the transpiration rates ofseedlings both with and without silicon were decreased to the same level by HgCl2treatment, and partially recovered by-Mercaptoethanol treatment, suggesting thataquaporin was responsible for the alleviation the decrease in Lp. Moreover, parts ofaquaporin genes transcription levels were up-regulated by silicon. Under salt stress,silicon inhibits the increase in root H2O2contents and enhanced antioxidant enzymeactivities. Meanwhile, similar as silicon, pre-treatment by catalase resulted inalleviation decreased in transpiration rate, indicating that silicon mediated H2O2accumulation was benefit to enhance aquaporin activity. These results indicate thatunder short-term salt stress, silicon application could alleviate the decrease of Lp bymediating aquaporin activity, which lead to increase the water uptake and resist to salt-induced osmotic stress.3: The influence and mechanism of silicon on enhancing sorghum seedlingsresistance to cadmium. Although the effect of silicon on cadmium tolerance iswildly reported, the underlying mechanism is still unclear. In the present study, weinvestigated the effect of silicon on sorghum seedling growth, anti-oxidant enzymeactivity and organic acids content. The result indicated silicon application alleviatedcadmium-induced the decrease in fresh weight and dry weight both in shoot and root.The results of anti-oxidant enzyme activity indicated that under control condition, theanti-oxidant enzyme activity was not affected by silicon. However under cadmiumstress, the CAT and POD activity were significantly increased by silicon. The SODactivity was not influenced by silicon. Corresponding to these results, under cadmiumstress, MDA was increased, while it was decreased by silicon. The results of organicacid (OA) indicated that the species, content and tendency of OA were different inleaf and root. In leaf,3types OA were detected including Oxalic acid; Propanedioicacid; Succinic acid. All of them were increased by silicon under cadmium stress. Inthe shoot, there are4types OA were detected: Oxalic acid; Malic acid; Propanedioicacid; Citric Acid. Oxalic acid and Propanedioic acid content were increased by silicon.Citric Acid content was not influenced by silicon. However, the Malic acid wasdecrease by silicon. All the results indicated that: the enhancement of anti-oxidantenzyme activity and OA content were involved in the silicon-induced the sorghumseedlings cadmium tolerance.4: The influence and mechanism of silicon on enhancing sorghum seedlingsresistance to K-deficiency. It has been reported that silicon application couldalleviate K-deficiency stress, but the mechanism was unclear. In the present study, weinvestigated the mechanism of silicon on alleviating K-deficiency in hydroponicculture. K-deficiency significantly decreased the fresh and dry weight of both shootand root. However silicon application alleviated the decrease in biomass. Based onthis result, we investigated the old and new leaf’s photosynthetic rates respectively.The results indicated that K-deficiency decreased the photosynthetic rate no matter in both old and new leaf. It is worth noting that silicon application significantlyalleviated the decrease in old leaf’s photosynthetic rate. Which is mainly due tosilicon application could maintain the stomatal conductance and transpiration rate at arelative high level compared with that without silicon application under K-deficiencycondition.The influence and mechanism of silicon on enhancing sorghum seedlingsresistance to drought, salt, cadmium and K-deficiency were investigated in ourexperiment. Summarized the results, we find that the mechanisms of silicon onenhancing plant resistance to different stresses were not the same except the physicalbarrier. That is to say, under different stress conditions silicon could regulated thedifferent function of molecular and physiology to enhance resistance to the stresses.Combined with the previous result, we speculated that silicon act as the “modulator”which could sense the stress firstly and then regulated relevant genes expression toenhance resistance to environmental stress.