COORDINATION OF PHYTOCHROME LEVELS IN PHYB MUTANTS OF ARABIDOPSIS AS REVEALED BY APOPROTEIN-SPECIFIC MONOCLONAL-ANTIBODIES

Accumulating evidence indicates that individual members of the phytoch rome family of photoreceptors have differential but interactive roles in controlling plant responses to light To investigate possible cross- regulation of these receptors, we have identified monoclonal antibodie s that specifically detect each of the five Arabidopsis phytochromes, phyA to phyE (phytochrome A holoprotein; PHYA, phytochrome A apoprotei n; PHYA, phytochrome A gene; phyA, mutant allele of phytochrome A gene ), on immunoblots and have used them to analyze the effects of phyA an d phyB null mutations on the levels of all five family members. In phy B mutants, but not in phyA mutants, a four-to six-fold reduction in th e level of phyC is observed in tissues grown either in the dark or in the light Coordinate expression of phyB and phyC is induced in the phy B mutant background by the presence of a complementing PHYB transgene. However, in transgenic lines that overexpress phyB 15- to 20-fold, ph yC is not similarly overexpressed. In these overexpressor lines, the l evels of phyA, phyC, and phyD are increased two-to four-fold over norm al in light-grown but not dark-grown seedlings. These observations ind icate that molecular mechanisms for coordination or cross-regulation o f phytochrome levels are active in Arabidopsis and have implications f or the interpretation of phytochrome mutants and overexpressor lines.