A NOVEL PHLOEM-SPECIFIC GENE IS EXPRESSED PREFERENTIALLY IN AERIAL PORTIONS OF VICIA-FABA

We have isolated a gene from bean (Vicia faba L.), called Vein 1, that encodes a novel protein. The Vein 1 cDNA was isolated as a result of a differential screen for genes that are expressed in leaves but not i n the most common cell type, the mesophyll cell. Northern blot analysi s revealed that Vein 1 transcripts are differentially expressed in the plant with expression in leaves, stems and sepals but not in petals, mesophyll cells or roots. In situ hybridization studies of stem and le af sections indicate that the expression of Vein 1 is localized to the phloem tissue. Interestingly, Vein 1 was differentially expressed in stem tissue with the highest expression in the oldest internodes. The deduced Vein 1 protein sequence does not share homology with any known protein sequences. The 17 kDa Vein 1 protein is highly hydrophilic an d contains a histidine-rich motif, where six out of seven amino acids are histidines. The function of Vein 1 is unknown, although the expres sion patterns suggests that it may play a role in mature phloem tissue in the aerial parts of the plant.