Physiological and gene expression responses to water stress in drought tolerant and drought sensitive maize cultivars.
Physiological characteristics of the response to water stress of two maize cultivars, the one drought resistant (PAN473) and the other drought sensitive (SR52), were compared. Mature plants were grown in one of two treatments, control and water stress. The drought resistant cultivar had a higher growth rate and a greater amount of roots in the lower soil levels than the drought sensitive cultivar in the water stress treatment. There was no difference between the cultivars in physiological characteristics in the control treatment, but in the water-stress treatment the drought resistant cultivar had a higher transpiration rate during the onset of water stress, and higher relative water content and levels of abscisic acid and proline throughout the period of water stress. A comparison between the cultivars in their gene expression response was done to determine if a correlation could be made with the difference in physiological response. A differential screening of water stress cDNA libraries identified nine different cDNA species which gave a signal with the water stress probe but not the control probe. Three of these cDNAs were represented by more than one cDNA clone. The cDNAs occurred in both libraries, therefore there was no difference between the cultivars in the presence or absence of the water stress responsive genes. The three genes represented by these cDNAs were named rws7, rws16 and rws5. All three genes showed increased transcription in response to water stress in whole plants, and to desiccation and osmotic stress of detached leaves. No increase in transcription was found in response to exogenously applied abscisic acid or proline. No difference between the cultivars was observed in the pattern of transcription response. Two of the three cDNA species that are represented more than once were sequenced. Rws7 had an open reading frame. A BLAST search found no homologous amino acid sequences, but the characteristics of the polypeptide suggest that it is a dehydrin. Rws 16 had a partial coding sequence. A BLAST search found two homologous amino acid sequences, and both were chloride channel proteins. The remaining seven cDNA species were sequenced at their 5' ends, and no complete homologous nucleotide sequences were found.