Growth and Development of higher plants

Mineral nutrients are one of the prime limiting factors for plant growth and development.
Deficiencies of essentials macronutrients are know to elicit various molecular responses, resulting in modification of the root system architecture and the shoot to root biomass ratio. We will analyze in more detail two cases that are studied by P3 and P4A.

It is not clearly identified what systems mediate Mg transport into the root or how Mg is loaded into and unloaded from the vascular system. The overall goal is to understand how plants acquire Mg through the roots, distribute and regulate their internal Mg level. Two complementary approaches will be used to optimise our chances to elucidate these mechanisms: (1). to clone new genes in Arabidopsis mutants that contain abnormal levels of Mg. (2). to identify transcriptome changes in roots in response to Mg deficiency.

The focus of the first objective will be the study of Mg hypo-accumulators and hyper-accumulators listed in the Arabidopsis Ionomics Database (NSF Plant Functional Genomics). These mutants are a novel tool for the studies in the IAP project. Our analysis will include a physiological characterisation consisting of photosynthesis measurements using fluorescence imaging and infrared gas analysis (collaboration with P1D), mineral and sugar profiles of root and shoot, phloem and xylem sap composition analyses (interaction with P2). P3 will use both positional cloning and microarray-based cloning to identify the mutations in genes responsible for Mg-profile changes. Anticipated results of this forward genetic approach are to discover novel genes involved in Mg transport and/or homeostasis.

The goal of the second proposed study is to provide a better understanding of Mg uptake in roots, allocation and homeostasis in plants. P3 recently reported the first physiological analysis of Mg deficient Arabidopsis. This study provides a concrete basis for further genetic analysis, using a genomic approach. The roots, which first undergo external Mg limitation, will constitute the material for transcriptomic analysis using Arabidopsis microarrays (in collaboration with P1).

Fe deficiency has already been characterized in more detail by many laboratories. However, P4A recently discovered that PDR3, an ATP-binding cassette (ABC) transporter belonging to the Pleiotropic Drug Resistant subfamily, was strongly induced under iron deficiency conditions. This expression was recently localized to the root (unpublished data). As iron transporters have already been identified, we favour the hypothesis that the ABC transporter might be involved in transporting out of the epidermal cell molecules such as organic acids that might improve iron solubility in the soil. Alternatively, PDR3 might play a role in parenchyma cells associated with conducting vessels by transporting outside the cell metabolites that keep iron soluble. This hypothesis will be tested using different tools. PDR3 expression will be determined at the cell level using the GUS reporter. Functional aspects will be analyzed by examining transgenic plants in which PDR3 expression has been prevented by gene knock-out or RNA interference. This analysis will be performed in collaboration with P1.