The adaptation of crop plants to environmental stress conditions:
Environmental stress due to drought and salinity are the most serious factors limiting the productivity of agricultural crops, which are predominantly sensitive to low soil moisture and the presence of high concentrations of salts in the soil. The relevance of solutions to these problems is of obvious importance for world’s agriculture in general and the State of California in particular. Crop production in both the Imperial Valley and the San Joaquin Valley is severely affected by limiting water resources and the progressive salinization of the soil. In addition, water resources are becoming increasingly scarce and water quality is decreasing, thus increasing the severity of the problem.
We have identified a family of plant sodium transporters and showed in the laboratory that the overexpression of these proteins could confer salt tolerance to valuable crops like tomato, alfalfa, rice, etc. Our assumption was that if plants were genetically modified to have an enhanced ability to sequester sodium in their vacuoles, the transgenic plants would be able to utilize salty water for cell expansion and growth. Our work has generated a number of patents that have been licensed by the California biotechnology industry to develop cultivars that not only produce better on saline soils, but also accumulate the salt in the foliage, presents an opportunity for improving soil quality for subsequent more sensitive high value crops such as rice, tomato, etc.
We have also generated transgenic plants with a remarkable adaptive response to extreme drought conditions. On the basis of the assumption that senescence is a type of cell death program that could be inappropriately activated during drought, we hypothesized that it may be possible to enhance drought tolerance by delaying drought-induced leaf senescence through the stress-induced synthesis of cytokinins. We generated transgenic plants expressing an isopentenyltransferase gene driven by pSARK, a stress- and maturation-induced promoter. Remarkably, the suppression of drought-induced leaf senescence resulted in outstanding drought tolerance as shown by, among other responses, vigorous growth following a long drought period that killed the control plants. We have generated several drought-tolerant lines in several crop species (tobacco, rice, tomato, wheat, etc.) and are now initiating field trials at different locations.
The biochemical and molecular basis of sugar and acid accumulation in citrus fruits
In general, the TSS (total soluble solids) to total acidity (TA) ratio determines whether citrus fruit can be marketed. The TSS/TA ratio in citrus fruits is dominated by two main components: (1) overall vacuolar juice cell acidity and (2) juice cell sugar content. The understanding of the physiological and biochemical determinants for TSS and TA content in fruits will allow the enhancement of fruit quality during post-harvest practices, the improvement of citrus fruit sweetness, and the characterization of physiological disorders that depend on TSS and TA fruit content. We have identified novel organic acid transporters that control the fruit acid load and sugar transporters that control the fruit sugar composition and content. We are now studying the developmental and environmental conditions that control the expression of these genes and the activity of the gene products in order to adapt fruit pre- and post-harvest conditions aimed at the increase of fruit sweetness and acidity.
The development of genomic and proteomic resources for the improvement of fruit quality.
We are developing of a citrus fruit Proteome that will serve to identify all of the proteins in the juice cells and will also serve as an aid to the Genomics efforts of the Citrus research community (validating the annotation of the fruit genes and the different ESTs). We have already identified more than 2,500 specific fruit proteins and were able to define a function to more than 2,100 proteins. In addition, we are constructing a “functional database” that will serve as a tool for growers and field extension specialists. We have developed a novel Differential Quantitative LC-MS/MS Proteomics Methodology for the identification and quantification of key biochemical pathways in fruits and are applying this methodology to identify determinants of key traits for fruit quality. We are now processing the information in order to build “biosynthesis maps” that will aid in defining key pathways associated with the development of key fruit quality traits.