A Summary of Current Research Interests

by Mark Tester

Coordinated responses by the whole plant to environmental stimuli are essential to allow plants to respond appropriately to changes in their environment, yet our understanding at the molecular and cellular levels of high level co-ordination processes remains minimal. One system in which such research is potentially tractable is that of the control of accumulation of inorganic nutrients and toxins by the plant. The measurable output, shoot nutrient concentration, is easily obtained, yet this is clearly the outcome of a complex series of events which take place at all levels (molecule, cell, tissue, organ and organism). Furthermore, variations in the supply of these nutrients can be manipulated easily, so whole plant responses to this challenge can readily be probed. The ultimate intellectual aim of my research programme is to understand mechanisms of long-distance communication within plants by studying genes which co-ordinate whole plant responses that are induced to correct inappropriate nutrient concentrations in the shoot.

The immediate aim of research in my laboratory is to elucidate the molecular mechanisms that enable certain plants to thrive in sub-optimal soil conditions, such as high salinity or acidity – where excess accumulation in the shoot of toxic elements such as Na and Al limit growth. The applied outputs of this programme are to genetically modify crop plants in order to increase their productivity on such soils; and to increase the mineral content of their seeds. The current focus is on salinity tolerance, so the solutes being studied are Na+, Cl- and boric acid, complemented by work with Ca2+, increasing concentrations of which usually ameliorate sodium toxicity.

Many components of salinity tolerance of a whole plant require particular characteristics of specific cells, rather than generic properties of all cells within a plant. Most notably, to facilitate Na+ exclusion from the shoot (commonly associated with salinity tolerance), Na+ would need to be pumped out of cells in the outer part of the root, but into cells in the inner part of the root, adjacent to the xylem (to maintain low Na+ in the xylem and thus low delivery to the shoot). This cell specificity of Na+ transport, essential for Na+ tolerance, is likely to be the main reason why previous attempts to use standard biotechnology for generating salt tolerant plants have mostly failed. The novel approach being employed in my laboratory is the study and manipulation of ion transport in specific cell types within the plant, particularly in the root.

Cell specific transport has been studied in my laboratory using electrophysiological techniques for several years. Recently, we have started to exploit a novel system for controlling gene expression to generate transgenic plants with altered levels of expression of Na+ transporters in specific cell types. This new approach to the study of salinity tolerance is expected to provide insights into the molecular basis for salinity tolerance, and to provide the technology for the generation of salt tolerant crops in the future. To this end, we are continuing to characterise the transport pathways for the entry and intra-plant distribution of the target solutes, with a view to future cell-specific misexpression of appropriate genes.

Such work is now being complemented by more genomic level approaches, namely the random activation of genes in specific cell types; and the screening of mutants with randomly activated genes for abnormal accumulations of solutes in the shoot using ICP-MS. This programme has the added value of providing material of interest to human nutritionists, as plants with increased accumulation of valuable elements such as Fe, I and Zn will be identified. Deficiencies in these elements generate some of the most significant health problems globally. This will give me the chance to access new funding sources, such as the Rockefeller Foundation and the Wellcome Trust, as well as providing the chance to elucidate fundamental molecular principles underlying the control of nutrient accumulation in higher plants.

The study of cell-specific processes catalysing and controlling transport and accumulation has been boosted greatly by the recent award of a BBSRC Research Development Fellowship. This award, which lasts for three to five years, will enable me to focus on the development and application of techniques to study quantitatively entire profiles of expression in single cells. The aim of this work is to amplify quantitatively all poly-A RNA from single cells by employing TPEA-PCR on single cell samples for a small number of cycles, providing a template for linear amplification of sense or antisense RNA. Using microarrays will enable quantitative comparison of expression profiles in samples obtained from different cell types in plants grown in different conditions, thus providing information on the modulation of gene expression in specific cell types in response to stress. This should indicate genes involved in specific, adaptive responses to stress, unlike current studies of gene responses that use whole tissue samples. Given the importance of cell-specific processes in stress responses, these latter analyses are more likely to reveal genes involved in damage response, rather than damage limitation. Genes that limit damage will clearly be of greater interest in mis-expression studies.

Most research in my laboratory employs the classic model plant, Arabidopsis thaliana, whose small size and sequenced genome pre-disposes it to molecular and genetic studies. However, about two years ago I made a strategic decision to begin using rice, as an important crop in its own right, but also as a powerful model cereal and monocotyledon which is not as extensively studied as Arabidopsis. This move is timely, as in the past year transformation technologies for rice have improved vastly and the public genome sequence is expected to become available within the next year or so.

Work is funded primarily by the BBSRC, but support has also been obtained from the EU, The Royal Society, The Leverhulme Foundation, NIREX and Monsanto - in the past year, I have obtained over £1 million pounds in grant income. This currently supports a group with one technician, five graduate students, two postdoctoral research associates, two research fellows and an overseas visiting professor. Three more PDRAs, a graduate student and a technician will be appointed in the coming months.

To summarise, we are performing international standard research that is providing insights into the processes underlying salt toxicity and tolerance in higher plants. Furthermore, we anticipate being able to broaden our remit to study the control of shoot accumulation of many nutrients, with wider implications for plant biology and agronomy.