| Heavy metal pollution is a serious environmental problem facing mankind today.Excessive heavy metals in soil severely affect plant growth and agricultural product safety.Plants have a certain tolerance to heavy metals,which is influenced and regulated by a variety of factors.For example,abscisic acid(ABA)has been well known to strongly improve plant tolerance to heavy metals.Most researches have focused on the physiological and biochemical events regarding the roles of ABA on plant tolerance to heavy metal,however,the molecular mechanisms are still limited.Moreover,the function of ABA on regulating different plant tissues under heavy metal stress is poorly understood.In this study,cadimium(Cd),a heavy metal with wide pollution and high toxicity,was used as a heavy metal stress to explore the physiological and molecular mechanisms underlying ABA regulating Cd tolerance in mung bean seedlings.The experiments were set as the controls,ABA treatment,Cd treatment,and ABA+Cd treatment groups.The contents of Cd and other mineral elements in leaf,stem and root tissues of mung bean seedlings were detected using flame atomic absorption spectrometry,and the contents of endogenous hormones including ABA,IAA,GA,and SA were measured by using ELISA method.The changes of gene expression profiles and differential expression genes at transcriptome level were analyzed using RNA-Seq technology.The main results are as follows:1.ABA affected plant growth and mineral concentrations in mung bean seedlings under Cd stress.A concentration of 10 μM ABA significantly reduced Cd accumulation in leaves,stems,and roots of mung bean seedlings,which decreased by 49.6%,46.2% and 16.7%,respectively.ABA alleviated the damage caused by Cd stress,and the plant height,root length,lateral root number,dry weight of aboveground and dry weight of roots were increased by15.0%,7.4%,3.0%,11.5%,and 12.4%,respectively.Cd stress significantly decreased K,Zn,and Fe contents in leaves,stems,and roots of mung bean seedlings,and decreased Ca contents in leaves and stems and Mg contents in roots,while increased Ca content in roots and Mg content in leaves and stems.ABA reversed the decreases of mineral nutrient content caused by Cd stress in all three tissues of seedlings,resulting in an increase of 16.5%,17.1%,and 26.2% in K,Zn,and Fe,respectively;and reduced the increases of mineral nutrients caused by Cd stress,resulting in a decrease of 8.9% in Ca in roots,16.5% and 16.5% in Mg in leaves and stems of seedlings,respectively.In conclusion,exogenous ABA reversed the content changes of mineral elements in mung bean seedlings caused by Cd stress.Our results showed that the regulation of the mineral element concentration and ion homeostasis was one of the important mechanisms for ABA to effectively alleviate the toxic effect of Cd on plants.2.ABA affected plant hormone contents in mung bean seedlings under Cd stress.Cd stress decreased the contents of IAA and GA3,whereas increased the contents of ABA and SA in the three tissues.ABA treatment reversed the decreases of IAA and GA3 contents in the three tissues of seedlings caused by Cd stress(increased by 15.6% and 7.7% respectively),and reversed the increases of SA contents caused by Cd stress(decreased by 6.4% on average),i.e.,ABA reversed the change of hormone contents caused by Cd stress in mung bean seedlings.ABA can balance the plant growth by regulating the contents and ratios of endogenous hormones to alleviate the inhibitory effect of Cd stress on mung bean seedlings.These results showed that regulating the contents and ratios of endogenous hormones was another important mechanism for ABA to effectively alleviate the toxic effect of Cd on plants.3.Under normal growth conditions,ABA affected the gene expression of mung bean seedlings.ABA predominantly regulated MAPK signaling,plant hormone signal transduction,phenylpropanoid biosynthesis,flavonoid biosynthesis,and plant-pathogen interaction pathways,and regulated the differential expression of the genes related to the function of plant stress response.Furthermore,ABA caused significant differences in spatio-temporal expression profiles of genes in leaves,stems,and roots,i.e.,the expression of the genes involved in stress response showed an ascending trend in leaves,while a descending trend in stems with the prolongation ABA treatment time.4.Under Cd stress,ABA also affected the gene expression of mung bean seedlings,which was different to that under normal growth.At the metabolic pathway level,ABA predominantly regulated lipid metabolism,secondary metabolism,amino acid metabolism,energy metabolism and signaling pathways.At the gene expression level,ABA regulated the expression of genes related to the function of plant lipid metabolism,cell wall processes,secondary metabolism,defense and stress responses,signal transduction,photosynthesis,cell division,transporters,protein synthesis and modification,and transcription factors.Furthermore,ABA also affected the spatio-temporal expression profiles of genes in leaves,stems,and roots under Cd stress.The expression of genes involved in stress response,such as sulfur compounds and glycosidases,showed an ascending trend in leaves;the expression of protein phosphorylation genes involved in stress response showed an ascending trend while the lipid metabolism genes showed a descending trend;the expression levels of stress,detoxification,and antioxidant genes involved in the stress process in roots showed a decreasing firstly and then ascending trend with the prolongation of ABA+Cd treatment time.5.RT-qPCR analysis verified the transcriptome data,which also demonstrated that the expression and ABA responsive-genes were different in the various tissues or at different stress times.For example,the probable aquaporin TIP-type-coding gene was significantly expressed in three organs with a trend root > leaf > stem.The expression of non-specific lipid-transfer protein 1-coding gene was higher in the stems than in roots and leaves.The repetitive proline-rich cell wall protein 1-coding gene was highly expressed in roots after ABA treatment.The expression of pathogenesis-related protein 2-like gene and repetitive proline-rich cell wall protein 1-coding gene were higher in the roots than in stems and leaves,while the expression of plasma membrane-associated cation-binding protein 1 gene was higher in the leaves than in stems and roots.The abscisic acid receptor PYL4 gene was highly expressed in stems after ABA+Cd treatment. |
