The cell wall defines plant cell shape; focused or specialized cell wall synthesis or remodeling leads to the development of different cell shapes that are important for plant function. For example, when germinated in the dark (i.e. under a layer of soil), dicot seedlings rapidly expand their hypocotyls to grow out of the soil and to reach light. Once the seedlings reach light, they undergo de-etiolation: hypocotyl growth slows, the apical hook opens, and cotyledons undergo greening. Both etiolated growth and the de-etiolation process involve highly coordinated hormone signaling and cell wall extension. In particular, apical hook opening requires rapid rates of cell elongation on one side of the hook and relatively slow rates on the other. Changes to soluble cell wall polysaccharides, such as pectins, are among the first detectable alterations to cell wall composition on the underside of the apical hook. These soluble cell wall polysaccharides are made in the Golgi and must be secreted to the cell wall. We have identified a mutant with defects in etiolated growth and de-etiolation, referred to as nks1. Quantitative co-localization of an NKS1-fluorescent protein fusion in complemented mutant lines demonstrated that NKS1 is localized to the Golgi apparatus. Interestingly, nks1 mutants displayed defects in protein trafficking to the plasma membrane (e.g. PIN2 secretion & recycling) and soluble polysaccharide accumulation in the cell wall, but not in endocytosis. Loss of NKS1 also resulted in dramatic alterations to Golgi morphology, documented by live cell imaging and electron microscopy. We propose a model in which loss of NKS1, a Golgi-localized domain of unknown function protein, results in changes to Golgi structure and function including inappropriate cell wall synthesis, secretion, and/or remodelling. These cellular phenotypes ultimately result in defects in coordinated cell growth and loss of tissue integrity during etiolated growth and de-etiolation.
Coauthors: Rahul Lathe – Max Planck Institute for Molecular Plant Physiology;Ghazanfar Khan – University of Melbourne;Berit Ebert – University of Melbourne;Staffan Persson – University of Melbourne