We have studied two lines of sunflower (Helianthus annuus L.) selected in the field as drought-tolerant (R1 genotype) or drought-sensitive (S1 genotype). When subjected to drought conditions, the R1 line was able to maintain high leaf water potential longer and wilted later than the S1 line. Therefore, this indicates that R1 tolerance includes a leaf-adaptive response. By subtractive hybridization, we have isolated six different cDNAs (designated sdi for sunflower drought-induced) corresponding to transcripts accumulated in R1 and S1 leaves during adaptive response. Analysis of transcript accumulation in response to drought in both genotypes suggests a preferential expression of three sdi genes in the tolerant line. Abscisic acid-mediated induction, analysed in R1 leaves, was observed for only four sdi genes. Sequence analysis of six sdi clones revealed that five clones were related to known proteins including non specific lipid transfer proteins (nsLTP), early light-induced proteins (ELIP), l-aminocyclopropane-l-carboxylate oxidase (ACC oxidase) or dehydrins, predicted to be involved in a wide range of physiological processes.

SummarySunTIP7 and SunTIP20 are closely related sunflower cDNAs showing a deduced amino acid sequence homologous to proteins of the tonoplast intrinsic protein (TIP) family. Their expression in Xenopus oocytes caused a marked increase in osmotic water permeability (demonstrating that they are water channels) which was sensitive to mercury. In leaves, in situ hybridization revealed that both SunTIP7 and SunTIP20 mRNA accumulated in the guard cells. The possible involvement of SunTIPs in stomatal movement was examined by comparing the time course of transcript accumulation and leaf conductance during the daily cycle and following a water limitation. SunTIP7 mRNA fluctuations fitted changes occurring in leaf conductance. The transcript levels were markedly and systematically increased during stomatal closure. It is suggested that aquaporin SunTIP7 facilitates water exit associated with a decrease in guard cell volume. In the same conditions, the transcript level of SunTIP20 remained constant indicating that SunTIP genes are differentially regulated within the same cell.