Studies On The Alleviation Of Heavy Metal Stress And The Regulatory Mechanisms In Plants

In this paper,we investigated the physiological and biochemical mechanisms about calcium(Ca),silicon(Si)and plant growth regulating substances(auxin,cytokinins,putrescine(Put))in responses to heavy metal stress,especially cadmium(Cd)and aluminum(Al)in plants(Arabidopsis,Soybean,Poa annua).1 Ca alleviates Cd-induced inhibition on root growth by maintaining auxin homeostasis in Arabidopsis seedlingsCa is involved in the regulation of plant cell metabolism and signal transduction and plays an important role in biotic and abiotic stresses.Our results showed that 50?M Cd significantly inhibited the seedling growth and decreased the chlorophyll content in Arabidopsis.Specifically,the primary root(PR)and lateral root(LR)length were decreased but LR number was increased under Cd stress.Furthermore,Cd enhanced the hydrogen peroxide(H2O2)content,lipid peroxidation as indicated by malondialdehyde(MDA)accumulation and dead cell numbers.The activities of antioxidant enzymes were enhanced.Cd also altered the level and the distribution of auxin in PR tips(as evidenced by the expression of DR5::GUS and PINs::GFP reporters)and the expression of several putative auxin biosynthetic-,catabolic-and transport-related genes.Application of 3 mM Ca alleviated the inhibition of Cd on the root growth.Ca application not only led to reduced oxidative injuries but also restored the normal auxin transport and distribution in Arabidopsis roots under Cd stress.Taken together,these results suggest that Ca alleviates the root growth inhibition caused by Cd through maintaining auxin homeostasis in Arabidopsis seedlings.The information from this study provides new insights in understanding the mechanisms involved in the defense of plant cells against Cd stress.2 Exogenous auxin and cytokinin alleviates Cd toxicity in Arabidopsis thaliana shootsPhytohormones are involved in not only plant growth and developmental processes but also plant responses to abiotic stresses,including the heavy metal stress.In this study,Cd stress decreased the endogenous auxin and cytokinin level,whereas exogenous auxin(a-naphthaleneacetic acid,NAA,a permeable auxin analog)and 6-BA reduced the shoot Cd concentration and rescued the Cd-induced chlorosis in Arabidopsis thaliana.Exogenous NAA and 6-BA significantly altered not only the level and distribution of auxin/cytokinin,but also the transport of auxin in the roots under Cd-stress.Meanwhile,NAA/6-BA increased the Cd retention in roots and most Cd in roots under Cd stress conditions.Taken together,these results demonstrate that auxin/cytokinin-induced alleviation of Cd toxicity in the shoots of Arabidopsis is mediated through rescuing auxin and cytokinin homeostasis in Arabidopsis seedlings,thus increasing the accumulation of Cd in roots.3 The alleviative effects and regulatory mechanisms of Si on Cd stress in Poa annuaSi could enhance plant tolerance to heavy metals;however,the mechanism of Si-mediated alleviation of Cd toxicity in Poa annua was not clear.In this study,100 ?M Cd significantly inhibited the growth of Poa annua seedlings.Furthermore,Cd enhanced the H2O2 and MDA content.The activities of SOD and APX were enhanced,but the CAT and POD activities were reduced.Cd also altered the activity of G6PDH and the expression of G6PDH in roots.Application Na3PO4,an inhibitor of G6PDH,the activity and expression of G6PDH decreased and the Cd toxicity was also increased,suggesting that G6PDH is involved in the regulation of oxidative stress induced by Cd.Application of 1 mM Si alleviated the inhibition of Cd on the growth of Poa annua seedlings.Si application not only led to reduced oxidative injuries but also decreased the accumulation of Cd in Poa annua seedlings under Cd stress.Furthermore,Si restored the normal level of the activity and expression of G6PDH under Cd stress,which demonstrated that Si maybe attenuates the Cd toxicity in Poa annua by inducing the expression of G6PDH.When G6PDH was inhibited,the alleviation impact of Si in Cd stress was abolished.Taken together,these results demonstrated that the Si-enhanced Cd tolerance in Poa annua is mainly due to the decrease of Cd uptake in roots and Cd distribution in roots and leaves,as well as lowering oxidative stress induced by Cd in plants,and increasing the acitivity and expression of G6PDH to reduce the Cd toxicity.The information from this study provides new evidence for understanding the mechanisms involved in the alleviation impact of Si in plant against metal stresses.4 Put aggravates Al toxicity by affecting cell wall polysaccharides in soybean rootsPolyamines are important molecules in modulating numerous environmental stresses in plants,especially Al stress.In this study,we investigated the role of endogenous Put accumulation in modulating A1 tolerance of two soybean cultivars Jindou19(JD19)and Zhonghuang30(ZH30)with different tolerance.The results showed that when exposed to A1 stress ZH30 was more sensitive than JD19 not only with more inhibition of the primary root growth and higher AI accumulation in root tips,but also with more MDA and H2O2 accumulation and higher contents of pectin and hemicellulose and lower degree of methylation of pectin.Meanwhile,we discovered that endogenous Put content was higher in the roots of ZH30 than in JD19.To explore the role of Put contribution to modulate the Al toxicity,roots of soybean seedlings co-treated with exogenous Put or D-arginine(Darg,an inhibitor of Put biosynthesis)were used to investigate the effect of A1 in this study.Results indicated that Put aggravates the Al toxicity and inhibits the root elongation by increasing contents of pectin and hemicellulose,and decreasing the degree of methylation of pectin,which determines the Al-binding capacity in cell walls and leads to more A1 acciumulation in cell walls.To alleviate the A1 toxicity,plants need to increase the activity of antioxidant enzymes(e.g.superoxide dismutase,catalase,peroxidase,ascorbate peroxidase)and the content of antioxidant substances(e.g.glutathione and ascorbate)to resist damages of lipid peroxidation of Put accumulation induced by A1 and promote root elongation.The results of this study provide new evidence to further explore the role and regulation mechanism of Put in A1 stress.