2 TIP-LIKE GENES ENCODING AQUAPORINS ARE EXPRESSED IN SUNFLOWER GUARD-CELLS

SunTIP7 and SunTIP20 are closely related sunflower cDNAs showing a ded uced amino acid sequence homologous to proteins of the tonoplast intri nsic protein (TIP) family. Their expression in Xenopus oocytes caused a marked increase in osmotic water permeability (demonstrating that th ey are water channels) which was sensitive to mercury. In leaves, in s itu hybridization revealed that both SunTIP7 and SunTIP20 mRNA accumul ated in the guard cells. The possible involvement of SunTIPs in stomat al movement was examined by comparing the time course of transcript ac cumulation 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 systemati cally increased during stomatal closure. It is suggested that aquapori n SunTIP7 facilitates water exit associated with a decrease in guard c ell volume. In the same conditions, the transcript level of SunTIP20 r emained constant indicating that SunTIP genes are differentially regul ated within the same cell.