Submitted to: Plant Physiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/12/2004
Publication Date: 9/1/2004
Citation: Sze, H., Padmanaban, S., Cellier, F., Honys, D., Cheng, N., Bock, K.W., Conejero, G., Li, X., Twell, D., Ward, J.M., Hirschi, K. 2004. Expression patterns of a novel atchx gene family highlight potential roles in osmotic adjustment and k+ homeostasis in pollen development1[w]. Plant Physiology. 136:2532-2547. Interpretive Summary: Plant contain a multitude of transporters with varying biological properties. The majority of transporters have not been ascribed any biological function. Here we analyze the expression pattern of several different transporters in an attempt to infer their function. Using computer modeling, bioinformatics and reporter constructs we demonstrate that a majority of these transporters are expressed in flowering tissues. We then speculate to the function of these transporters and discuss future experiments to test our preliminary conclusions. These studies represent an important first step in determining the biological properties of transporters.
Technical Abstract: A combined bioinformatic and experimental approach is being used to uncover the functions of a novel family of cation/H+ exchanger (CHX) genes in plants using Arabidopsis as a model. The predicted protein (85-95 kD) of 28 AtCHX genes after revision consists of an amino-terminal domain with 10 to 12 transmembrane spans (approximately 440 residues) and a hydrophilic domain of approximately 360 residues at the carboxyl end, which is proposed to have regulatory roles. The hydrophobic, but not the hydrophilic, domain of plant CHX is remarkably similar to monovalent cation/proton antiporter-2 (CPA2) proteins, especially yeast (Saccharomyces cerevisiae) KHA1 and Synechocystis NhaS4. Reports of characterized fungal and prokaryotic CPA2 indicate that they have various transport modes, including K+/H+ (KHA1), Na+/H+-K+ (GerN) antiport, and ligand-gated ion channel (KefC). The expression pattern of AtCHX genes was determined by reverse transcription PCR, promoter-driven beta-glucuronidase expression in transgenic plants, and Affymetrix ATH1 genome arrays. Results show that 18 genes are specifically or preferentially expressed in the male gametophyte, and six genes are highly expressed in sporophytic tissues. Microarray data revealed that several AtCHX genes were developmentally regulated during microgametogenesis. An exciting idea is that CHX proteins allow osmotic adjustment and K+ homeostasis as mature pollen desiccates and then rehydrates at germination. The multiplicity of CHX-like genes is conserved in higher plants but is not found in animals. Only 17 genes, OsCHX01 to OsCHX17, were identified in rice (Oryza sativa) subsp. japonica, suggesting diversification of CHX in Arabidopsis. These results reveal a novel CHX gene family in flowering plants with potential functions in pollen development, germination, and tube growth.