Molecular characterization of two cDNAs from Sinapis alba L. expressed specifically at an early stage of tapetum development

Flower formation in the long-day plant Sinapis alba is strictly dependent on an inductive light treatment. Differential screening of an apex cDNA library prepared 10 days after flower induction against cDNAs from vegetative apices has identified two cDNA clones, pSFD10.35 and pSFD10.44, which represent transcripts expressed transiently between day 10 and day 20 after induction and which disappear before flowers reach maturity. The corresponding full-length cDNAs which were isolated, Satap35 and Satap44, display 86% overall sequence identity. These cDNAs encode polypeptides with predicted molecular weights of 12.7 and 12.4 kDa, and isoelectric points of 10.4 and 7.5, respectively. The N-terminal portions of the open reading frames have characteristics of signal sequences. In situ hybridization reveals that both transcripts are localized exclusively in the tapetal cell layer of the anthers. Maximal expression is observed in flower buds of approximately 1.5 mm length (tetrad stage). Southern blot analysis demonstrates the presence of additional, closely related genes in the Sinapis genome, as well as the occurrence of homologous genes in Arabidopsis thaliana, Brassica napus and Nicotiana tabacum.