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ABSCISIC ACID INSENSITIVE5 (ABI5), a key regulator of the abscisic acid (ABA) signalling pathway, plays a fundamental role in seed germination and post-germinative development. However, the molecular mechanism underlying the repression function of ABI5 remains to be elucidated. In this study, we demonstrate that the conserved eukaryotic WD40 repeat protein Receptor for Activated C Kinase 1 (RACK1) is a novel negative regulator of ABI5 in Arabidopsis. The RACK1 loss-of-function mutant is hypersensitive to ABA, while this phenotype is rescued by a mutation in ABI5. Moreover, overexpression of RACK1 suppresses ABI5 transcriptional activation activity for ABI5-targeted genes. RACK1 may also physically interact with ABI5 and facilitate its degradation. Furthermore, we found that RACK1 and the two substrate receptors CUL4-based E3 ligases (DWA1 and DWA2) function together to mediate the turnover of ABI5, thereby efficiently reducing ABA signalling in seed germination and post-germinative growth. In addition, molecular analyses demonstrated that ABI5 may bind to the promoter of RACK1 to repress its expression. Collectively, our findings suggest that RACK1 and ABI5 might form a feedback loop to regulate the homeostasis of ABA signalling in acute seed germination and early plant development. [ABSTRACT FROM AUTHOR]

AT14A has a small domain that has sequence similarities to integrins from animals. Integrins serve as a transmembrane linker between the extracellular matrix and the cytoskeleton, which play critical roles in a variety of biological processes. Because the function of AT14A is unknown, Arabidopsis thaliana AT14A, which is a transmembrane receptor for cell adhesion molecules and a middle member of the cell wall–plasma membrane–cytoskeleton continuum in plants, has been described. AT14A, co-expressed with green fluorescent protein (GFP), was found to localize mainly to the plasma membrane. The mutant Arabidopsis at14a-1 cells exhibit various phenotypes with cell shape, cell cluster size, thickness, and cellulose content of cell wall, the adhesion between cells, and the adhesion of plasma membrane to cell wall varied by plasmolysis. Using direct staining of filamentous actin and indirect immunofluorescence staining of microtubules, cortical actin filaments and microtubules arrays were significantly altered in cells, either where AT14A was absent or over-expressed. It is concluded that AT14A may be a substantial middle member of the cell wall–plasma membrane–cytoskeleton continuum and play an important role in the continuum by regulating cell wall and cortical cytoskeleton organization. [ABSTRACT FROM AUTHOR]

The scaffold protein RACK1 (Receptor for Activated C Kinase 1) serves as an integrative point for diverse signal transduction pathways. The Arabidopsis genome contains three RACK1 orthologues, however, little is known about their functions. It is reported here that one member of this gene family, RACK1A, previously identified as the Arabidopsis homologue of the tobacco arcA gene, mediates hormone responses and plays a regulatory role in multiple developmental processes. RACK1A expresses ubiquitously in Arabidopsis. Loss-of-function mutations in RACK1A confer defects in multiple developmental processes including seed germination, leaf production, and flowering. rack1a mutants displayed reduced sensitivity to gibberellin and brassinosteroid in seed germination, hypersensitivity to abscisic acid in seed germination and early seedling development, and hyposensitivity to auxin in adventitious and lateral root formation. These results provide the first genetic evidence that RACK1A is involved in multiple signal transduction pathways. [ABSTRACT FROM PUBLISHER]

We have explored a method to collect xylem sap using a Scholander pressure chamber for potted plants. Intact root system in pots which fitted the pressure chamber was pressurised at a pneumatic pressure numerically equal to the absolute value of shoot water potential. The rate of xylem flow obtained from the stem stump under such pressure was found similar to the rate of transpiration before detopping. The rate of pressurised flow from detop-ped roots was linearly related to the pressure applied in both well-watered and soil-dried plants. The osmotic concentration of the xylem sap was negatively related to the rate of volume flow, suggesting the necessity to collect xylem sap at in vivo flow rate if original solute concentration is to be evaluated. The concentration of ABA in the xylem sap, however, did not show such a relationship with water flux. Both well-watered and soil-dried plants showed the concentration of ABA in xylem sap largely stable with a range of volume flow rate, indicating a linear relationship between the rate of ABA delivery through xylem and that of volume flow. We also compared the concentrations of ABA in xylem sap sequentially collected from pressurised roots with that from detached shoots of the same plants. The concentration of ABA in the initial saps from shoots showed to be similar to that from roots. However, a decrease in the concentration of ABA in the xylem sap collected from detached leaf or twig was observed when more volume of sap was collected, which might also be dependent on the plant species and the volume of xylem vessels concerned. [ABSTRACT FROM AUTHOR]