TOMATO CONTAINS 2 DIFFERENTIALLY EXPRESSED GENES ENCODING B-TYPE PHYTOCHROMES, NEITHER OF WHICH CAN BE CONSIDERED AN ORTHOLOG OF ARABIDOPSIS PHYTOCHROME-B

Tomato (Solanum lycopersicon L.) contains two B-type phytochrome genes (PHYB1 and PHYB2). Fragments of these two PHYB were cloned following amplification by the polymerase chain reaction of a portion of their r elatively well conserved 5' coding regions. Polypeptides encoded by th ese gene fragments exhibit 90% sequence identity. These two PHYB are i ndependently expressed in organ-specific fashion. In mature plants, PH YB2 mRNA is most abundant in fruit and PHYB1 mRNA in expanded leaves. A phylogenetic analysis fails to establish which tomato PHYB is orthol ogous to either Arabidopsis PHYB or PHYD, the latter being a second B- type phytochrome. Instead, this analysis indicates that following the divergence of the Solanaceae and Brassicaceae from one another, a PHYB gene duplicated independently in each lineage. Consequently, Arabidop sis PHYB mutants cannot be considered strictly equivalent to the tomat o tri mutants, which appear to be mutated at the PHYB1 locus. Similarl y, other putative PHYB mutants might not be equivalent to those descri bed for Arabidopsis and tomato. This situation complicates efforts to determine 'PHYB function' because there might be no one answer to this question.