Research Interests:
Plant nutrition involves the acquisition of the essential chemical elements, and their utilization for growth and development. Research in my laboratory is focussed upon membrane transport processes responsible for transferring inorganic ions (particularly K+, NH4+ and NO3-) across the plasma membrane, from external (soil) solution, into the roots of higher plants.

We are studying these transport processes, with emphasis on NO3- and NH4+ transport, at the molecular level (cloning genes and examining the results of gene deletions), through the physiological to ecological levels. In the case of NO3-, substantial quantities of this ion are transported to the leaves of many plants where it is reduced to NH4+ and converted to amino acids and other N compounds. Thus, ion uptake is not exclusively a function of root cells only, but leaves too must reabsorb nitrate from the xylem sap.

The basic approach that my group is undertaking is to integrate physiological, biochemical and molecular studies of the transport of NO3- and NH4+ across the plasma membranes of root cells. Genes encoding high- and low-affinity NO3- transporters and high-affinity NH4+ transporters have been cloned by my group, and we are attempting to resolve the manner in which the proteins encoded by these genes participate in transport processes, and the manner in which they are regulated.

In Arabidopsis thaliana there are a total of eleven genes encoding NO3- transporters and 5 encoding NH4+ transporters. We are attempting by use of T-DNA insertional mutants and other methods to determine the function of each of these transporters. Given the complexity of this large number of genes we are also working with a relatively much simpler fungal system, Aspergillus nidulans, where there are only two NO3- transporters. We have cloned both of the genes encoding these transporters and are trying to work out the functional significance of the two transporters.

Britto, DT, MY Siddiqi, ADM Glass, & HJ Kronzucker. 2001. Futile membrane ion cycling: a new cellular hypothesis to explain ammonium toxicity in plants. PNAS 98: 4255-4258.

Glass, ADM, DT Britto, J Kaiser, HJ Kronzucker, A Kumar, M Okamoto, MY Siddiqi & JJ Vidmar. 2001. The regulation of nitrate and ammonium transport systems in plants. J. Exp. Bot. In press.

Unkles, SE, D Zhou, MY Siddiqi, JR Kinghorn & ADM Glass. 2001. Apparent genetic redundancy facilitates ecological plasticity for nitrate transport. EMBO J. 20: 6246-6255.

Vidmar, J, D Zhuo, MY Siddiqi, JK Schjoerring, B Touraine, & ADM Glass. 2000. Regulation of HvNRT2 expression and high-affinity nitrate influx in roots of Hordeum vulgare by ammonium and amino acids. Plant Physiology 123: 307-318.