| Plant survival relies on resisting episodes of adverse environmental stresses. Water stress is one of the most severe constraints in crop production. Plants reduce water loss by decreasing stomatal aperture when water supply is limited. The stress hormone ABA activates a signal transduction pathway culminating in stomatal closure. Numerous studies have been conducted to understand in-depth mechanisms involved in ABA signaling and stomatal functions, and numerous components including bzip transcription factors such as ABFs/AREBs and phosphoinositides such as IP3 have been identified. However, most of progress has been from model plant Arabidopsis, and still the biochemical, cellular, and molecular mechanisms underling plant drought response are not completely understood. The objective of this study was to increase the resistance of crops (tobacco and tomato) to water deficit stress. To achieve this goal transgenic plants were generated using two genes, a putative tomato type I inositol 5 polyphosphatase (5PTase) that terminates inositol 1,4,5 trisphosphate (IP3) signaling and ABRE binding factor ABF4 derived from Arabidopsis.; Inositol 1,4,5 triphosphate (IP3) as one of phosphoinositides, is known to transduce a stress signal by changing its level in response to water deficit, osmotic stress, and low temperature in plants. Recently it was shown that type I inositol 5 phosphatases (5PTases), At5PTase1 and AtIP5PII/At5PTase2, regulate IP3 level in plant like in animals and that up-regulation of these genes decreases IP3 levels which leads to a reduction in the expression of ABA-/drought-responsive genes in Arabidopsis. On the basis of sequence similarity to Arabidopsis 5PTases, four tomato cDNAs ( Le5PT1-4) that encode putative tomato type I 5PTase proteins were identified. Predicted protein sequences of identified Le5PTs had conserved catalytic domains that are required for 5PTase enzyme activity. Two clones, Le5PT1 and Le5PT2 were similar to AtIP5PII/At5PTase2 and At5PTase1, respectively. The expression of Le5PT1 was down-regulated in early time point under dehydration, NaCl, and exogenous ABA treatment, indicating that Le5PT1 may play a negative role in stress signaling. Transgenic tobacco plants with 35S:Le5PT1 did exhibit weak expression of the drought inducible gene, NtERD10B, but did not show correlation with resistance to water deficit stress. Transgenic tobacco and tomato plants, however, exhibited retarded growth, suggesting that Le5PT1 may have a relation to plant development.; AtABF4, a bzip transcription factor, is known to induce the expression of ABA-responsive genes. The expression of Arabidopsis ABF4/ AREB2 gene under the control of guard cell specific KST1 promoter was shown to significantly increase drought tolerance in tobacco and tomato plants. The transgenic plants exhibit significantly lower water loss per unit leaf area compared to wild type plants. ABF4-mGFP protein strongly accumulated in guard cell nuclei. The possibility that ABF4 protein can activate 4XKST1-rd29B:mGFP-GUS fusion construct in a water stress dependent manner was tested. The results show that ABF4 itself may not be sufficient to induce GUS expression even under water stress. These results demonstrate that drought-inducible transcription factors may be an agronomically useful tool to alter crop drought tolerance. |
