The water permeability of Arabidopsis plasma membrane is regulated by divalent cations and pH

Mechanisms that regulate water channels in the plant plasma membrane (PM) were investigated in Arabidopsis suspension cells. Cell hydraulic conductivity was measured with a cell pressure probe and was reduced 4-fold as compared to control values when calcium was added in the pipette and in bathing solution. To assess the significance of these effects in vitro, PM vesicles were isolated by aqueous two-phase partitioning and their water transport properties were characterized by stopped-flow spectrophotometry. Membrane vesicles isolated in standard conditions exhibited reduced water permeability (P(f)) together with a lack of active water channels. In contrast, when prepared in the presence of chelators of divalent cations, PM vesicles showed a 2.3-fold higher P(f) and active water channels. Furthermore, equilibration of purified PM vesicles with divalent cations reduced their P(f ) and water channel activity down to the basal level of membranes isolated in standard conditions. Ca2+ was the most efficient with a half-inhibition of P(f) at 50-100 microM free Ca2+. Water transport in purified PM vesicles was also reversibly blocked by H+, with a half-inhibition of P(f )at pH 7.2-7.5. Thus, both Ca2+ and H+ contribute to a membrane-delimited switch from active to inactive water channels that may allow coupling of water transport to cell signalling and metabolism.     

The irregular xylem 2 mutant is an allele of korrigan that affects the secondary cell wall of Arabidopsis thaliana

The irregular xylem 2 (irx2) mutant of Arabidopsis thaliana exhibits a cellulose deficiency in the secondary cell wall, which is brought about by a point mutation in the KORRIGAN (KOR) beta,1-4 endoglucanase (beta,1-4 EGase) gene. Measurement of the total crystalline cellulose in the inflorescence stem indicates that the irx2 mutant contains approximately 30% of the level present in the wild type (WT). Fourier-Transform Infra Red (FTIR) analysis, however, indicates that there is no decrease in cellulose in primary cell walls of the cortical and epidermal cells of the stem. KOR expression is correlated with cellulose synthesis and is highly expressed in cells synthesising a secondary cell wall. Co-precipitation experiments, using either an epitope-tagged form of KOR or IRX3 (AtCesA7), suggest that KOR is not an integral part of the cellulose synthase complex. These data are supported by immunolocalisation of KOR that suggests that KOR does not localise to sites of secondary cell wall deposition in the developing xylem. The defect in irx2 plant is consistent with a role for KOR in the later stages of secondary cell wall formation, suggesting a role in processing of the growing microfibrils or release of the cellulose synthase complex.