| Cadmium,a Group I carcinogen,is readily bioavailable in soil and is transported and accumulated in the food chain,posing serious health risks.Leafy vegetables’edible parts absorb and accumulate large amounts of heavy metals more readily than fruit or root vegetables.Water spinach（Ipomoea aquatica Forsk.）,also known as hollow cabbage,is a popular leafy vegetable in Asian countries such as China.Because there are differences in Cd accumulation by absorption between varieties,it is necessary to select and breed low-Cd varieties to reduce human Cd intake.Therefore,in this study,two different Cd-accumulating water spinach varieties were used as test materials to investigate the mechanisms of Cd accumulation differences between different genotypes of water spinach from physiological and ecological characteristics,the morphology and characteristics of Cd in plants,and at the molecular level.Multi-walled carbon nanotubes（MWCNTs）,a new nanomaterial,and their impact on the two types of Cd absorption were both evaluated.This research concentrates on how Cd accumulates and how Cd tolerance differs between the two types,and it offers a foundation and empirical advice for identifying agricultural methods that can lower the Cd content of water spinach while maintaining its output and quality.The following are the key conclusions:To investigate the differences in cadmium accumulation in different parts of the high-accumulating variety T308 and the low-accumulating variety Q322 under high and low cadmium concentration pollution,and to focus on comparing the differences in the chemical morphology and subcellular distribution of cadmium in the roots,stems and leaves of the two varieties.The results showed that under the same treatment,T308had significantly higher above-ground Cd content than Q322 and also had higher Cd tolerance,while Q322 roots absorbed more Cd.Further exploring the distribution of Cd in the plant,the cell wall of the root system of the low-Cd accumulating variety Q322had a stronger ability to block Cd in the roots,and the low mobility of the Cd chemotype further reduced the translocation of Cd into the roots,which may be one of the main mechanisms for the lower above-ground Cd accumulation in the low-Cd variety.The high-Cd accumulating variety T308 had strong Cd tolerance,with high proportions of Cd-F3 and FNa Cl indicating resistance to Cd toxicity through Cd absorbed by vacuolization.The molecular mechanisms underlying the variations in Cd accumulationbetween the two genotypes of water spinach under high Cd contamination were further investigated using transcriptome sequencing.Under 50 u M Cd stress,root systems of T308 and Q322 were found to have 5242 and 3209 differentially expressed genes,respectively.According to the results of GO and KEGG enrichment analysis,the differentially expressed genes in T308 and Q322 were enriched in various pathways to varying degrees,suggesting that different varieties of water spinach may have various defense mechanisms against Cd and other heavy metals.Further analysis of differentially expressed genes,specifically Cd-induced transcriptional changes in genes or pathways involved in cell wall biosynthesis and metal transporter proteins,among others,may be an important molecular mechanism leading to differences in Cd tolerance and accumulation genotypes in water spinach.High expression of genes encoding cell wall and casparian strip biosynthesis,glutathione metabolism and metallothionein enhanced the ability of Q322 roots to chelate cadmium,thereby reducing above-ground cadmium accumulation in Q322;genes encoding metal transporter proteins were expressed in both water spinach species,but at higher levels in T308,suggesting that these genes may be involved in the mechanism of high cadmium accumulation in T308.In addition,genes encoding two heavy metal tolerance proteins（PDR and MTP）and antioxidant enzymes were expressed at higher levels in T308,which may account for the greater tolerance of T308 under high Cd stress.These differentially expressed genes could help us further understand the mechanism of Cd accumulation in water spinach under Cd stress.The effects of MWNTs with different concentrations(0-50 mg L-1)and high Cd concentration on the growth and Cd accumulation of two types of water spinach were investigated.The results showed that low concentration of MWNTS(5 mg L-1)could alleviate the oxidative stress caused by cadmium toxicity in roots by promoting root growth and increasing the activities of antioxidant enzymes in roots,thereby reducing cadmium accumulation in roots.However,low concentration MWCNTs aggravated oxidative damage and inhibited root growth of Q322 under Cd stress,leading to damage of root defense system and increase of shoot Cd content.Therefore,the effects of MWCNTs on plant growth and heavy metal accumulation were strongly dependent on MWCNTS concentration and specific plant species.Appropriate MWCNTs concentrations and corresponding crop genotypes should be used to avoid the adverse effects of MWCNTs,and in this study,the combined application of low concentration MWCNTs with high cadmium accumulation and high tolerance varieties was an effective way to reduce the risk of cadmium contamination in the food chain. |
