Molecular biology of water stress responsive genes in wheat



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Drought stress, the major limiting factor in worldwide wheat production, has long been an area of high interest to plant scientists. Drought stress in plants results in various physiological changes including reduced growth, photosynthetic activity and overall protein synthesis. Water deficit also results in accumulation of the plant hormone abscisic acid (ABA) which in tum induces expression of ABA responsive genes. The function of these gene products are not known although they are believed to play a role in osmoprotection and osmoregulation during water stress and desiccation. The objective of this study was to isolate cDNAs corresponding to the rab (responsive to ABA) family of water stress inducible genes from winter wheat (Triticum aestivum L. cv TAM W-101) and determine changes in steady state RNA rab levels during water stress. A eDNA library was constructed in the lambda Uni-Zap XR vector system using poly (At mRNA isolated from roots of water stressed wheat seedlings. The eDNA library was screened and cDNAs representing two different size members of the rab gene family were isolated. Sequence analysis revealed sizes of 781bp and 1068bp with long open reading frames encoding polypeptides of 15,766Da (pTawsp15 (Triticum aestivum water stress protein)) and 23,229Da (pTawsp23), respectively. The predicted amino acid sequences of wheat Ta WSP 15 and 23 have significant regions of identity with reported amino acid sequence of other reported RAB polypeptides from both monocots and dicots. Northern blot analysis of total RNA hybridized with pTawsp15 revealed increases in steady state RNA levels with increasing water stress. RNA transcripts corresponding to pTawsp began to accumulate in both leaves and roots after a mild stress (93% relative water content (RWC)) and continued to accumulate even as stress became severe (48% RWC). Upon rewatering of severly stressed seedlings, Tawsp transcripts return to control levels within 24 hours. These results support the idea that ABA responsive gene products may accumulate in order to protect the plant during severe desiccation and that once the stress is relieved and protection is no longer necessary the transcripts are rapidly degraded. This scenario would approximate the events which occur during seed development and maturation. Perhaps through a better understanding of drought responsive genes, we may be able to elucidate the molecular mechanisms involved in drought tolerance and use this understanding to develop more tolerant wheat cultivars using molecular techniques.



Winter wheat -- Drought tolerance.