Arabidopsis thaliana P4-type ATPases ALA4 and ALA5 are Necessary for Normal Cellular Elongation in Developing Vegetative Tissues and Rescue the Pollen Tube Deficiencies in Knockout ala6/7 Mutants

Members of the P4 subfamily of P-type ATPases (lipid flippases) have been shown to incorporate specific lipids into cellular membranes, as well as flip lipids from one leaflet to another to create an asymmetric distribution of lipid head groups as well as membrane curvature. In Arabidopsis thaliana, there are twelve members referred to as Aminophospholipid ATPases (ALAs). Here we show that ALA4 and ALA5 provide redundant activities that are critical for normal growth and development of vegetative tissues. A double loss of function mutant for ala4/5 results in dwarfism for all vegetative tissues, including roots, leaves and hypocotyls. Selected cell types in these tissues were shown to be smaller. While a restoration of growth to near wild type levels was observed for dark grown ala4/5 hypocotyls in the presence of a growth hormone Gibberellin (GA), a similar growth enhancement was not seen for GA treated roots or leaves. The mutant also failed to show any growth promotion in response to applications of an auxin, cytokinin, or brasinolide. An RFP-tagged ALA4 was expressed in pollen to test its ability to rescue the fertility defect associated with a combined loss of ala6/7, the two most closely related ALAs that were previously shown to be important for normal pollen tube tip growth. The RFP-ALA4 was found to fully restore pollen fertility in the ala6/7 background, as shown by the restoration of full seed set. This supports a model in which ALA4/5 can function in a cell autonomous pathway that is important for cell growth, potentially through a contribution to one or more membrane trafficking process involved in cell expansion