Phytochrome apoproteins in angiosperms are encoded by a small gene family. Tomato {Solanum lycopersicum L.) serves well as a dicotyledonous model system for elucidating the extent of this gene family, its expression patterns, and the roles of individual members of the family. Five phytochrome genes (PHYA, PHYBl, PHYB2, PHYE and PHYF) have been characterized in tomato. Quantitative measurements of transcript abundances from each tomato PHY throughout the life cycle indicate that transcript levels generally range from 10 to 100 ^umol moP^ total mRNA, in the following order of decreasing abundance: PHYA, PHYBl, PHYE, PHYB2 and PHYF. PHYA transcripts were found to be most abundant in seedling roots, while PHYB2 and PHYF transcripts were expressed preferentially in fruit. PHYA mutants (fri) have been found to be the consequence of a single nucleotide substitution adjacent to the 3' terminus of an intron. What are almost certainly PHYBl mutants have also been described, although the molecular nature of these mutants remains to be revealed. Efforts to obtain PHYB2, PHYE and PHYF mutants are currently underway.

SummaryTomato (Lycopersicon esculentum Mill., recently redesignated Solanum lycopersicum L.), an agronomically important crop plant, has been adopted as a model species complementary to Arabidopsis in which to characterize the phytochrome family. Here we describe the cloning and molecular characterization of the gene encoding the apoprotein of phytochrome A in wild-type tomato and in the far-red-light-insensitive (fri 1 and fri 2 ) tomato mutants. The physical organization of this gene is similar to that of other angiosperm phytochromes with the four exons of the coding region interrupted by three introns. The pool of transcripts is heterogeneous due to multiple transcription start sites and to three modes of alternative splicing of the 5Ј leader. The leader in each alternative transcript carries multiple upstream open reading frames of considerable length. At the genomic level, both fri mutants share an identical base substitution which changes a consensus AG/ to TG/ at the 3Ј end of the intron between exons 1 and 2. This mutation leads to aberrant processing of the resultant pre-mRNA. While most mature transcripts retain the mutated intron, both cryptic splicing and exon skipping were also detected. Cryptic splicing occurred both upstream and downstream from the wild-type splice site. These observations are consistent with the hypothesis that exon definition in splicing of plant pre-mRNAs plays a secondary role to that of intron definition. Analysis of

A homologue of the ABI3 gene was isolated from the conifer species, Chamaecyparis nootkatensis. The deduced protein of 794 amino acids exhibited sequence similarity to other VP1/ABI3 proteins within four regions. Expression occurs exclusively in seeds, with no detectable mRNA in leaves and roots. Unlike the homologues of angiosperms, CnABI3 may be encoded by more than one gene.

No abstract