Screening, Cloning And Function Analysis Of Drought-tolerant Mutants Lew2 And Abo1 In Arabidopsis

Drought stress is one of the environmental factor that greatly limit crop production and plant distribution in the world. The study of the cellular and molecular responses of plants to drought stress will help to broaden the possibilities for genetically engineering plants. By means of a genetic screen for different drought responses, we identified two drought-tolerated mutants that showed drought resistant phenotype under water stress conditions in Arabidopsis. One is the lew2 (leaf wilting), the other is the abol (ABA overly-sensitive).Although water transported in the xylem from root to shoot is physiologically studied in higher plants in great detail, little is known of the molecular and genetic control mechanisms. Recessive mutations in several loci of Arabidopsis were previously shown to lead to a collapsed xylem phenotype but water-transport deficiency was not observed. We found that the leaves of lew2 plants wilted easily under high transpiration that did not have any harmful effect on wild-type plants. Map based cloning revealed that the lew2 mutant is a new allele of the AtCesA8/IRX1 gene which encodes a subunit of a cellulose synthesis complex. Cross-sections of stems, roots and leaves showed severely collapsed xylem walls. The flow rate of the transpiration stream of lew2 is slower than that of the wild type, which suggests that water transport is impeded, probably because of its collapsed xylem. In contrast to their leaf-wilting phenotypes, lew2 plants accumulate more ABA, proline and soluble sugars than the wild type and are more tolerant to drought stress as well as to NaCl, mannitol and other osmotic stresses. The expression of one stress-inducible marker gene RD29A, one proline synthesis-related gene P5CS (pyrroline-5-carboxylate synthase) and one ABA synthesis-related gene SDR1 (alcohol dehydrogenase/reductase) were higher in lew2 than in the wild type. Our results suggest that cellulose synthesis is crucial for drought and osmotic stress responses including drought-induction of gene expression.Plant hormone abscisic acid (ABA) plays vital roles in modulating plant growth, development as well as stress responses. Stomata formed by. pairs of guard cells are important accesses for uptake of CO2 for photosynthesis and the evaporation of water during transpiration. The abo1 mutation augments ABA-hypersensitive stomatal closing and increases ABA sensitivity in inhibiting the seedling growth. Fascinatingly, ABO1also modulates development and growth of guard cells originated from satellite meristemoid mother cells, but not from meristemoid mother cells. Mapping based cloning identified ABO1 gene which encodes a homolog of animal IKAP (IkB Kinase-Associated Protein) protein. IKAP in animals or its homolog IKI3/ELP1 in yeast is the largest subunit of Elongator complex that controls gene expression at the elongating stage of mRNA processing. Our results uncover crucial roles for ABO1 in regulating mRNA processing for modulating the growth of guard cells and controlling stomatal movement in responding to ABA.