PLASMA-MEMBRANE TRANSPORT-SYSTEMS IN HIGHER-PLANTS - FROM BLACK-BOXESTO MOLECULAR PHYSIOLOGY

Considerable progress in identifying transport systems of the plant pl asma membrane has been made recently. The putative systems cloned to d ate comprise H+-ATPases, potassium, chloride and water channels, and c arriers involved in the transport of glucose, sucrose, amino acids, pe ptides, potassium, nitrate, ammonium, phosphate, sulfate, iron and cop per. Most of these systems were identified first in Arabidopsis thalia na. Successful cloning strategies have involved the following variety of techniques: complementation of yeast mutants, screening of Arabidop sis mutants, immunoscreening of a cDNA expression library expressed in mammalian cells, screening of genomic and cDNA libraries with probes (or degenerate oligonucleotides) derived from yeast and/or animal gene s, or database screening for sequence similarity to eukaryotic counter parts. Many related transport systems have subsequently been identifie d either by screening Libraries directly, or by systematic cDNA sequen cing programs. Surprisingly large gene families have been revealed. He terologous expression systems, such as yeast, Xenopus oocytes or insec t cells, provide tools for studying the transport activities, biochemi cal properties and structure-function relationships of these systems. Their expression and functions in planta are investigated using northe rn blot analysis, in situ hybridization, and transgenic approaches. In dividual systems encoded by the same gene family can differ in their t ransport properties and have distinct tissue expression patterns. Such diversity might be central to the integration of solute transport at the whole plant level, allowing the differential expression of sets of transport systems specifically tailored to the requirements of each t issue.