NTR1 encodes a floral nectary-specific gene in Brassica campestris L. ssp. pekinensis

We have characterized a gene specifically expressed in the floral nectaries of Brassica campestris L. ssp. pekinensis. Differential screening led to the isolation of a floral nectary-specific cDNA clone. Northern hybridization indicated that its mRNA transcript is 1450 nucleotides long and specific to the flower base. In situ hybridization and immunolocalization showed that its mRNA and protein are localized specifically to both the lateral and median nectaries of flowers. The cDNA codes for a 43.8 kDa polypeptide 392 amino acids long. The protein was named nectarin1 (NTR1) after floral nectary protein. NTR1 was located in the cytoplasm of nectariferous cells in the nectaries and was also observed in nuclei at a much lower level. The level of the transcript increases with flower development, especially during nectary development, but decreases abruptly with the opening of the flower. Genomic Southern blot analysis indicated that at least three copies of homologous genes were present in the genome of B. campestris, but that only a single copy was present in both Arabidopsis thaliana and Lycopersicon esculentum. The deduced amino acid sequence of NTR1 shows similarity to S-adenosyl-L-methionine:salicylic acid carboxyl methyltransferase of Clarkia breweri which is expressed mostly in petals. The function of the gene is speculated to be involved in the methylation of a plant secondary metabolite in the floral nectaries.