The expression of an abundant transmitting tract-specific endoglucanase (Sp41) is promoter-dependent and not essential for the reproductive physiology of tobacco

In angiosperms the interactions between the secretory matrix of the stylar transmitting tract and the growing pollen tubes have central roles in determining a successful fertilization. Sp41 is a major glycosylated component of the soluble proteins of the transmitting tract matrix and exhibits (1-3)-beta-glucanase activity. It is a member of the pathogenesis-related protein superfamily, but shows developmental regulation as opposed to pathogen induction. In order to investigate the mechanisms regulating Sp41 expression, we isolated and characterized genomic clones corresponding to the sp41a gene. Sp41a contains an intervening sequence localized between the sequences encoding for a putative signal peptide and the mature protein. A fragment of 2.5 kb that lies 5' to the coding region of the gene was sufficient to confer transmitting tract specific expression to a beta-glucuronidase reporter gene in transgenic tobacco plants. The sp41 transcripts have unusually long 5'-untranslated sequences. The leader sequences contain small open reading frames, include secondary structures, and may be involved in post-transcriptional regulation. A possible function for Sp41 in reproductive physiology was tested by monitoring tobacco plants transformed with antisense stylar sp41a RNA. Transgenic antisense plants with immunologically and enzymatically undetectable levels of (1-3)-beta-glucanase were obtained and their offspring analyzed. The progeny plants did not show any detectable phenotypic modifications as they had a normal flower morphology and were fully fertile.