Roles Of Osmolytes, Polyamines And Aquaporins In Adaptation Of Vetivers Grass (Vetiveria Zizanioides) To Salt And Drought Stresses

Salinity and drought stresses were the most deleterious abiotic stresses, which adversely influence plant growth, development, and were also the important factores in limiting agricultural productivity. Generally, osmolytes, polyamines and aquaporins (AQPs) were considered to be the important mechanism in plant to drought and saline environments. Vetiver grass (Vetiveria zizanioides), is a kind of sterile and perennial grass with multi-tolerance to lots of abiotic stresses, such as drought, salinity, et al., has high photosynthesis and yield. At present, vetiver grass is being used worldwidely as means of rehabilitation of heavy metal mines, soil and water conservation, and so on. However, up to date, its tolerance mechanisms were still undefined, and especially on the molecullar biological mechanisms. In this study, the basic biological characteristics, the mechanisms of osmolytes, polyamines and aquaporins (AQPs) in the adaptation of vetiver grass to saline and drought environments were investigated. These results showed as follows:Vetiver grass (Vetiveria zizanioides) has fast growth, high net photosynthesis rate and dry yield, and low content of mineral elements. The max Pn was 15.3±1.77μmol CO2 m-2 s-1, the highest growth rate was 42.1±5.1 cm month-1 in June; its dry yield up to 6634.24±889.69 Kg hm-2. Vetiver grass leaf compositions were evaluated with regard to cellulose (32.7±0.58%), hemicelluloses (37.48±0.94%), and lignin (15.08±1.22%). And their hydrolysis products were determined by HPLC, which including glucose (36.41±1.93%), xylan (22.50±2.59%). In brief, the photosynthesis, yield, stress tolerance and leaf compositions and its hydrolysis products of vetiver grass are very near to switchgrass. In addition, vetiver grass can withstand pests, pathogen and weeds; and it can not be spread to the weed, and is planted in many countries. So, we think that vetiver grass can be considered as a potential lignocellulosic energy plant.The accumulation of inorganic and organic osmolytes and their role in osmotic adjustment were investigated in roots and leaves of vetiver grass (Vetiveria zizanioides) seedlings stressed with 100,200,300 mmol L-1 NaCl for 9 days. although the contents of inorganic (K+, Na+, Ca2+, Mg2+, Cl-, NO3-, S, P) and organic (soluble sugar, organic acids and free amino acids) osmolytes all increased with NaCl concentration in vetiver grass seedlings, the contribution of inorganic ions (mainly Na+, K+ and Cl-) to osmotic adjustment was hingher (71.50%-80.56% of total) than that of organic solutes (19.43%-28.50%). The contribution of inorganic ions increased and that of organic solutes decreased in roots with the enhanced NaCl concentration, whereas the case in leaves was opposite. On the other hand, the osmotic adjustment is only effective for vetiver grass seedlings under moderate saline stress (less than 200 mmol L-1 NaCl).Under moderate salt stresses (100 and 200 mmol L-1 NaCl) for 9 days, vetiver grass grew at similar vigor when compared with seedlings under normal growth conditions. However, growth was severely arrested when plants were treated with severe salt stress (300 mmol L-1 NaCl). Under the above 3 different concentrations of NaCl stress, free Put, Cad, Spd, Spm and total free PA contents substantially decreased in roots and leaves, more severe losses of free PAs was observed under higher NaCl concentration. Conjugated Put, Cad, Spd, Spm, and total conjugated PAs remarkably increased, the extent of the increase after 300 mmol/L NaCl treatment was smaller than those after 100 or 200 mmol/L NaCl treatments. Bound Put, Cad and total bound PAs decreased in both roots and leaves under salt stress, bound Spd and Spm decreased in leaves but increased in roots in response to salt treatments, the more obvious rise was displayed under moderate salt stresses. With the exception of the significant decreases of free (Spd+Spm)/Put ratio were observed in roots and leaves after 300 mmol/L NaCl treatment, the other ratios of free, conjugated, bound and the total (Spd+Spm)/Put increased in roots and leaves of vetiver grass seedlings after different salt treatments, especially under moderate salt stresses. These results indicate that maintaining homeostasis of total PAs content and high (Spd+Spm)/Put ratios could be an adaptation mechanism in vetiver grass to moderate saline environment.When vetiver grass was exposed to the moderate (20% and 40% PEG-6000 solutions) and severe (60% PEG solution) water deficit for 6 days, the plant injury degree (expressed as the parameters of plant growth, cell membrane integrity, water relations and photosynthesis) increased and contents of free and conjugated Put decreased with the rise of PEG concentration. Under the moderate water deficit, the plants could survive by the reduced osmotic potential (ψs), increased free and conjugated Spd and Spm in leaves. After subsequent rewatering, the osmotic balance was re-established; most of the above investigated physiological parameters were fully or partly recovered to the control levels. However, it was not the case for the severely-stressed and rewatering plants. It indicates that, vetiver grass can cope well with the moderate water deficit/drought stress by using the strategies of osmotic adjustment and maintenance of total contents of free, conjugated and bound PAs in leaves.PIP1 and PIP2 gene segment sequences of aquaporins were successfully cloned from vetiver grass, and the similarity between them and cDNA sequences of other plant aquaporins was analyzed by bioinformatics software. The results showed that VzPIPl and VzPIP2 sequence have highly similarity with other known aquaporins, such as ZmPIP1;1 and ZmPIP2;1, and VzPIPl was belonging to the PIP1 class, and the VzPIP2 belonging to the PIP2 class. And when vetiver grass was exposed to the moderate water deficit (25% PEG solution) for 3 days, the expression levels of PIP1, PIP2 significantly increased in roots, which improved the hydrostatic conductance (Lph) and osmotic conductance (Lpos), and enhanced the root hydraulic conductance. The expression levels of PIP1 significantly increased in leaves, which decreased the stomatal conductance, leaf hydraulic conductance, and reduced the transpiration and loss of water. So, by the above strategies, vetiver grass can maintain the water balance, mitigate the stress injury and cope well with the drought stress.