| Drought and salinity are two major environmental factors determining crop/forage productivity and plant distribution. Drought and salinity affect more than 10 percent of arable land worldwide and this situation is rapidly increasing, becoming a primary obstacle to the sustainable development of agriculture and animal husbandry. Environmental degradation as well as unreasonable farming management results in the occurrence of soil nutrient imbalance, such as worsening potassium shortage. Therefore, it is an effective way to improve the environment that select plant species with strong resistance against stresses from nature and use them in genetic breeding and vegetation restoration. Apocynum venetum, a perennial herb or undershrub, is distributed widely in saline, semi-arid and arid regions of northern China. This species is proved to be promising multipurpose material in ecological, economical and pharmaceutical use because of its great resistance to abiotic stresses and superior fiber. However, little studies have been conducted on resistant mechanism of A. venetum. In the present study, the physiological mechanisms of seedlings responding to low potassium, salinity and drought were comprehensively investigated based on growth, water regulation, photosynthetic characteristic, chlorophyll fluorescence, ion absorption and distribution. We also cloned the full-length cDNA of inward-rectifying K+ channel AvAKT1 gene from A. venetum, and analyzed the expression patterns of AvAKT1 under low potassium, salt and osmotic stress. The main results as follows:1. No differences on growth, water status, photosynthetic characteristic, K net uptake rate(KNUR), K accumulation amount(KAA), K efficiency ratio(KER) and K use efficiency(KUE) were observed between K+-starved plants and K+-sufficient plants, showing prominent K+ uptake and utilization efficiency in seedlings grown potassium deficiency. We propose that A. venetum should be considered as a potassium-efficient species, which was noticeably tolerant to low potassium availability in medium.2. A significant reduction of stomatal conductance(Gs) was noted in seedlings subjected to 50 mM NaCl, resulting in the increase of stomotal limitation(Ls) as well as the decrease of intercellular CO2 concentration(Ci), net photosynthetic rate(Pn) and transpiration rate(Tr). These changes are beneficial to reducing water loss by regulating stomatal closure and sacrificing the accumulation of photoassimilates. The severe stress of 150 mM NaCl significantly inhibited the growth of A. venetum seedlings, a significant decrease of mass but enhanced root/shoot ratio were found. The detrimental effects were strengthened by potassium deficiency under 150 mM NaCl, the maximal photochemical efficiency of PSⅡ(Fv/Fm), potential activities of PSⅡ(Fv/Fo), excitation capture efficiency of open centers(Fv’/Fm’), actual photochemical efficiency(ΦPSII), photosynthetic electron transport rate(ETR) and photochemical quenching(qP) significantly declined, parameters of the fraction of antenna heat dissipation(D) and excess energy(E), conversely. These results showed that A. venetum has evolved effective strategies in order to cope with drought stress through enhancing the root/shoot ratio and protecting the photosynthetic apparatus by increasing heat dissipation.3. Na+ contents in different parts of A. venetum significantly rised with the increase of external NaCl concentrations, while shoot K+ notably decreased but still remained at high level. Moreover, root K+ always remained stable under salinity. This suggested that by enhancing the capacity of selective absorption for K+ and its transport, to maitain high K+ contents and K+/Na+ ratio in leaf, are the key adaptive mechanisms in A. venetum for salt-stress tolerance.4. Osmotic stress significantly inhibited the growth of A. venetum: the plant height, relative growth rate(RGR), biomass, chla/b, Pn, Fv/Fo and ETR significantly reduced of seedlings exposed to-0.2 MPa, compared with control. The addition of 25 mM Na Cl under osmotic stress visibly stimulated plant growth, the parameters related to water status, photosynthesis, chlorophyll fluorescence of seedlings under D+S treatments all reached to the levels as control, such behavior indicated that moderate sodium chloride alleviates sorbitol-induced osmotic stress in A. venetum by improving photosynthetic capacity and water status.5. Under osmotic stress, K+ contents in shoot changed little even increased, it was over 10 times higher than Na+ from the quantitative point of view. In addition, our data show that the contribution of K+ to leaf total osmotic potential exceeded 37% under any condition. These results indicated that K+ acts as the uppermost osmolyte playing a crucial role in osmoregulation for water-stress resistance in A. venetum.6. A potassium channel gene was isolated from A. venetum using RT-PCR and RACE, and named as AvAKT1. The cDNA is 2906 bp in length encoding 899 amino acid residues. Multiple sequence alignment revealed that AvAKT1 shares high homology with the known AKT1-type channels(over 58%). Furthermore, the deduced polypeptide exhibited all of the structural features that are shared by plants inwardly rectifying K+ channels. The phylogenetic analysis showed that AvAKT1 belongs to GroupⅠ(AKT1-subfamily) in the Shaker-like K+ channel family, and forms a clade with the closest relation to the dicotyledons AKT1 homologue NtAKT1 from Nicotiana tabacum. Real-time fluorescent quantitative PCR demonstrated that AvAKT1 was mainly located in root, and was not sensitive to low potassium, but it was induced strongly by supplying of 5 mM K+ in the medium. This indicated that AvAKT1 is probably involved in the process of low affinity K+ absorption. The expression level of AvAKT1 was significantly increased in short time such as 6 h under-0.2 MPa osmotic stress or 25 mM NaCl treatment. The results showed that Av AKT1 likely plays an effective role in response to salinity or drought, as well as maintaining the absorption or transport for K+ and Na+. |
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