One of my major interests is the regulation of plant growth and cell expansion. In particular, my laboratory is focused on environmental problems affecting plant growth: soil salinity, drought and cold.
There is a very strong linkage between cell elongation, plant productivity and crop yields. The production of all plant parts is dependent upon the supply of external resources, such as water, mineral nutrients or carbon. Cell elongation is important for the capture of these resources, particularly when they are limiting growth. An increase in cell size will increase cell surface area, enabling roots to explore more of the soil for water and minerals, and leaves to capture more photosynthetic radiation. The size of a stressed plant is dependent upon cell production and cell expansion, both of which may be affected by stress. Through the use of new techniques in biotechnology, we can make better plants, which are more suitably adapted to these environmental conditions.
For the last two decades, my research has focused on salinity stress. The inhibition of plant growth by salinity involves two components. Initially, the plant experiences a water stress, but with time salts accumulate in the plant creating an additional ionic stress. In my laboratory, we are currently focusing on how salinity inhibits plant growth by these two components.
The water stress component has been investigated by studying the immediate effects of salinity on the growth parameters regulating leaf elongation. We found that salinity increases the apparent yield threshold of the cell walls in the growing region of the leaf, which results in lower growth rates. We have interesting correlations of the plant response to the plant growth regulator, abscisic acid with salinity stress, particularly the effects on growth, cell wall yield threshold and cytosolic calcium.
The identification and characterization of the mechanisms regulating plant growth under saline conditions is leading to the molecular characterization and manipulation of proteins associated with salt-tolerance in plants. Currently we are taking a functional genomic approach by screening 70,000 line of Arabidopsis mutants for salt tolerance in order to identify which genes are involved in salt tolerance
Wine Grape Studies (Drought, salinity and cold stresses)
Northern Nevada appears to have a good climate for growing grapes (Vitis vinifera). My lab has been investigating the feasibility of growing grapes in Nevada (see Nevada Grapes). Nevada's climate is very stressfull and includes drought, salinity, cold and high temperatures.
Recent advances in genomics have led to improved strategies for engineering stress tolerance in plants. Drought, salinity and cold stress alter gene expression in plants with considerable overlap among these stresses. This fact is exemplified by the observation that over-expression of a single Arabidopsis transcription factor enhances the expression of multiple gene products involved in drought, salinity and cold tolerance responses. My current research program on wine grapes will lead to a better understanding of the genetic mechanisms for cold, drought and salinity tolerance in V. vinifera. Ultimately this research will improve wine grape production efficiency in colder and drier regions of the world and a better understanding of the factors that contribute to improved wine quality under abiotic stress conditions.
Impacts of drought, salinity and cold on grapevines:
1. Water deficit
The influence of plant water status on grape productivity and fruit composition had a priority rank of 2.57 (7th most important issue) in the 1999 AVF California Viticulture Research Survey Summary. Water usage is a major concern to many growers, particularly those in NV and CA, from both an economic and quality point of view. Controlled irrigation can not only save water, but also have a positive impact on the quality of wine made from grapes grown in semi-arid regions.
At the UNR vineyard in Reno, NV, (summer of 2000) we applied water at 75 % of crop evapotranspiration, once plants reached a water potential of -10 bars. Only 9 applications of water were needed for the entire season resulting in an 80% reduction in water use from the previous year. This represents 0.25-acre feet of water &endash;a very low rate of water application. This water saving is extremely significant in arid Nevada. By comparison, Churchill County farmers produce quality alfalfa hay with an average application of 3.5 acre feet of water (14 times more water than our grapes). As water rights have become limiting in this and other areas throughout NV and CA, the use of alternative crops using less water has become a high priority.
Regulated deficit irrigation can improve grape quality, affecting wine aroma and taste by altering metabolite and glucan composition, with little, if any, loss in production. This was true in the UNR controlled irrigation study described above, where water deficit balanced vegetative to fruit growth resulting in higher quality fruit and a better quality wine.
2. Salinity stress
Irrigation can have an important impact on grapevines in semi-arid regions because of salt build-up. Saline soils make up 23% of the world's cultivated soils and are seriously threatening irrigated crops in semi-arid regions. Saline soils make up approximately 30 and 50% of the irrigated soils in CA and NV, respectively. Grapevines are considered moderately sensitive to salinity. They are particularly sensitive to chloride and different rootstocks can give important chloride exclusion properties to the scion.
3. Cold stress
Vine damage due to freezing is an important constraint and economic cost to growers in the northern regions of North America and Europe. Occasionally, entire vineyards of V. vinifera must be replaced because of extensive freezing damage at very low temperatures (-10 to -15°F). Native North American species of Vitis can survive temperatures down to -25°F. In Nevada, these temperatures are occasionally observed in Reno and Fallon. The record lows for these areas are -19°F (1890) and -27°F (1989), respectively, providing strong justification for improving cold tolerance traits if vineyard expansion in desirable arid areas like NV is to proceed.
Strategies for improving stress tolerance in grapevine:
There are several different approaches to developing more stress tolerant V. vinifera plants including: a) adapting cultural practices, b) selecting for more tolerant germplasm, c) making hybrids of V. vinifera with more tolerant native North American species, and d) using genetic engineering technology to develop more hardy genotypes.
Breeding for stress tolerance has proven difficult and has not provided desirable outcomes. Breeding specific characteristics takes considerable time for V. vinifera. Consequently, clones are vegetatively propagated to prevent loss of desirable grape and wine qualities. Hybrids that are more cold tolerant than V. vinifera have been developed at Cornell University, but wine made from these grapes is inferior to premium quality wines made from V. vinifera grapes grown in the major wine producing regions of the world. Thus, the genetic modification of specific premium quality V. vinifera clones by recombinant DNA technology is viewed as the most attractive option for improving stress tolerance.
A potential limitation of recombinant DNA technology is that stress tolerance is complex, requiring the response of many genes. A genetic engineering strategy we are pursuing, namely the over-expression of transcriptional activators such as the CBF/DRE family has a much higher potential for success than strategies relying on single stress adaptive transgenes because multiple adaptive genes would be overexpressed.
CBF/DRE transcriptional activators:
The CBF/DRE transcriptional activators, CBF1 (DREB1B), CBF2 (DREB1C) and CBF3 (DREB1A) are some of the master switches for drought, salinity and cold tolerance. Over-expression of CBF1 in Arabidopsis increased cold tolerance by 3.3° C. Over-expression of CBF3 also increased the freezing, drought and salinity tolerance of Arabidopsis, and rice. Constitutive over-expression of each of the CBFs in Brassica napus increased the freezing tolerance of both cold acclimated and non-acclimated plants without any detrimental effects.
My Objectives for Wine Grape Research at UNR are:
1) To enhance drought, salinity and cold tolerance in grapevine by the over-expression of CBF/DREs
2) To develop cDNA libraries from drought, salinity, and cold stressed leaves, roots and fruits of V. vinifera.
3) To discover other genetic tools for engineering improved stress tolerance.
4) To conduct comparative metabolite profiling in grapes from well-watered and water-deficit treated vines and work towards determining the genetic basis of the factors responsible for improving the quality of wine produced from drought-stressed plants.
The long-term goal of our research is to understand and enhance the cold, drought, and salinity stress tolerance mechanisms in V. vinifera. This research will not only have immediate practical benefit by producing V. vinifera with improved stress tolerance, but also provide important insights into the molecular basis of stress tolerance and underlying mechanisms of how abiotic stress improves wine quality. Investing now in the development of comprehensive molecular genetic resources for V. vinifera, will greatly facilitate future gene discovery efforts of novel stress tolerance mechanisms and lead to improvements in both production efficiency and wine quality under adverse growing conditions.