THE BLUE LIGHT-RESPONSIVE ATHH2 GENE OF ARABIDOPSIS-THALIANA IS PRIMARILY EXPRESSED IN EXPANDING AS WELL AS IN DIFFERENTIATING CELLS AND ENCODES A PUTATIVE CHANNEL PROTEIN OF THE PLASMALEMMA

According to our previous studies the Arabidopsis gene AthH2 which is inducible by blue light and phytohormones codes for an intrinsic membr ane protein. It bears a resemblance to several distinct channel protei ns of plant and animal species classified as the MIP/NOD-26/GlpF famil y. In the present study biochemical analyses and electron microscopic immunochemistry were used to elucidate the subcellular location of the AthH2 protein. The results clearly demonstrate that it is an exclusiv e constituent of the plasmalemma. Furthermore, the expression of the A thH2 gene in transgenic Arabidopsis plants containing the promoter reg ion of AthH2 fused to the beta-glucuronidase (gus) reporter gene was s tudied. The in situ localization of gus activity revealed that the spe cific promoter is temporally activated by light in expanding and/or di fferentiating cells comprising newly formed tissues and organs: root e longation zone, guard cells of stomata, vascular bundle sheaths, filam ents of stamen and young siliques. Several sites of gus expression coi ncide spatially with those of in situ hybridization and the immunocyto chemical reaction, respectively, suggesting that the AthH2 promoter ha d correctly responded to light as an important exogenous factor with r elevance to the complex pattern of differentiation. Studies with proto plasts from plants transformed with an antisense construct revealed a water transport capacity of the AthH2 protein.