Plasma Membrane Channels And Responses To Osmotic Stress In Plant

Detecting and responding to environmental stimulations are very important forall living organisms. One of the distinguishing features of plants is that they aresessile and thus have to endure environmental challenges. High salinity, drought,and low temperature are common environmental stresses which impeding plantgrowth and severely reducing crop yields. My research work is mainly focused onthe mechanisms of salt stress tolerance in alkali grass, bioinformatic and expressionpattern analysis on aquaporin family in rice.Most monocotyledonous crop species are glycophytes, which are sensitive tosalinity. The deficiency of good model plants is a big problem on study salttolerance mechanisms in monocotyledonous crops. In order to understand the salttolerance mechanism of the alkali grass (Puccinellia tenuiflora) compared withwheat (Triticum aestivum L.), we examined [K~+] and [Na~+] in roots and shoots inresponse to salt treatments with ion element analysis and X-ray microanalysis. Both,the rapid K~+ and Na~+ influx in response to different NaCl and KCl treatments, andthe accumulation of K and Na as the plants acclimated long term stress werestudied in culture solution experiments. We observed higher K+ uptake in the alkaligrass under normal and saline conditions to compare with that in wheat, andelectrophysiological analyses indicated that the different uptake probably resultedfrom higher K+/Na+ selectivity of the plasma membrane. When external [K+] washigh, K+ uptake and transport from root to shoot were inhibited by exogenous Cs+,while TEA (tetraethylammonium) only inhibited K+ transport from root to shoot.Cs+ did not influence K+ uptake during K+ starvation. It was shown by X-raymicroanalysis that high [K+] and low [Na+] existed in the cells of root andendodermis of alkali grass suggesting that these are the sites of Cs+ inhibition.These results show that the K+/Na+ selectivity of potassium channels and theexistence of an apoplastic barrier, the Casparian bands of the endodermis, lead tothe lateral gradient of K+ and Na+ across root tissue resulting in not only high levelsof retained [K+] in shoot but also a large [Na+] gradient between root and shoot.Aquaporins, which show the ability to facilitate passive exchange of wateracross membranes, belong to a high conserved membrane protein family MIP(Major Intrinsic protein). In plants, the activities of aquaporins can be controlled attranscription, translation and post-translation modification levels. The discovery ofaquaporins in plants has resulted in a paradigm shift in plant physiology. Comparedwith animals, the abundance of aquaporins may reflect the complexity of anadequate water relationship in plants. Analyzing all the members of a gene family isnecessary in order to obtain an accurate view of its overall function. Therefore, weidentified a potential complete set of rice aquaporins by scanning sequenceinformation available in public databases including the rice genome and ESTdatabase. There are at least 35 members in rice aquaporin family, which could bedivided into four subfamilies (12 PIPs, 11 TIPs, 11 NIPs and 1 SIP). The genestructure and protein sequence of OsAQPs were analyzed then classified based ontheir intron position and the construct of amino acid residues in aromatic/Arg (ar/R)filter region. As a step toward understanding the aquaporin function in plants undervarious environmental stimuli, the expressions of a gene family encoding 12 PIPsin rice under drought, cold, and high salinity, or abscisic acid (ABA) treatment wereinvestigated by a quantitative real-time reverse transcription-PCR analysis withgene specific TaqMan-MGB (minor groove binder) probes. Several PIP genes werepredominantly expressed either in the roots or in the shoots. The expressions ofboth the highly expressed aquaporins included PIP1;1, PIP1;2, and PIP2;3 while theweakly expressed aquaporins included PIP2;1, PIP3;1, PIP4;1, and PIP5;1. Theanalyses of our data revealed that most of the PIP genes were down regulated undercold stress. Marked up-or down-regulation in PIP expression was observed underdrought and salt stresses. The responses of each PIPs to ABA were different,implying that the regulation of aquaporin expression involves both ABA-dependentand ABA-independent signaling pathways. Taken together, our comprehensiveexpression profile of the 12 members of the OsPIP gene family provides novelbasis to allocate the stress-related biological function to each OsPIP gene.