Short blue root (sbr), an arabidopsis mutant that ectopically over-expresses and ABA- and auxin-inducible transgene Dc3-GUS and has defects in the cell wall

Plant growth and development is regulated by complex interactions among different hormonal, developmental, and environmental signaling pathways. Isolation of mutants in these processes is a powerful approach to dissect unknown mechanisms in regulatory networks. The plant hormones abscisic acid (ABA) and auxin are involved in vegetative, developmental and environmental growth responses, including cell division and elongation, and vascular tissue differentiation and regulation. The uidA (β-glucuronidase; GUS) reporter gene driven by the carrot (Daucus carota) Late Embryogenesis-Abundant Dc3 promoter in the roots of transgenic Arabidopsis thaliana seedlings is ABA-, drought- and auxin-inducible in a tissue specific manner and is shown to efficiently distinguish between these signals. An ethyl methane sulfonate induced M2 mutant population of abi2/abi2 homozygous plants of a line that carries two independent Dc3-GUS reporter genes was screened for mutants with altered expression of the Dc3-GUS transgene upon induction with ABA. Of these, one mutant class had very short roots that ectopically over-express Dc3-GUS and hence was named short blue root (sbr). The physiological ABA responses of the sbr mutant tested in both abi2 and Ler backgrounds were normal and the mutant phenotype does not require the presence of abi2-1 mutant allele. The sbr mutant seedlings had normal levels of ABA suggesting that ectopic Dc3 expression is not due to higher endogenous levels of ABA. Further morphological characterization revealed that the sbr mutant is a seedling-lethal dwarf and has abnormal cell shape in the epidermal layer of all parts; epidermal cells in the root and hypocotyl are short and radially swollen. The sbr mutant has supernumerary cell number in the root cortex and epidermis, discontinuous root vasculature and impaired venation in cotyledons and true leaves. The mutant also exhibits a semi-dominant root phenotype of reduced growth and lateral root development. These defects were suggestive of impaired responses to auxin, ethylene or gibberellic acid or defects in cell expansion due to defects in the wall. However, responses of the sbr mutant to auxin, ethylene and GA were found to be unaffected per se. The mutant phenotype was also not rescued by the exogenous application of various plant growth regulators. The sbr mutant also had ectopic lignification and accumulation of callose in the roots, characteristic of other cell wall and expansion mutants in addition to radial swelling and epidermal cell bulging. Biochemical analysis of the sbr cell wall monosaccharide composition revealed deficiencies in xylose, mannose and galactose compared to wild-type suggesting defects in hemicellulose component of the sbr cell wall. Additive phenotypes in double mutants of sbr and the cellulose deficient prc1-1 mutant as well suggested defects in the cell wall. Ectopic expression of Dc3-GUS was observed in cell wall related mutants, prc1-1 and bot1-1 (impaired in microtubule organization) as well. It is possible that loss of turgor pressure due to cell wall defects can mimic drought responses and hence induce Dc3-GUS. The sbr lous has been a mapped to a 340kb interval on chromosome I. Further cloning and characterization of the sbr gene may enhance our knowledge on wall biogenesis and provide insights into the relationship of cell wall signaling and hormone-regulated gene expression in plant growth and development