Functional analysis of yeast‐derived phytochrome A and B phycocyanobilin adducts

Investigations of phytochrome mutants of Arabidopsis suggested that the expression of chalcone synthase (chs) and anthocyanin accumulation is predominantly controlled by phytochrome A. To test the functionality of phytochrome A and B at the molecular level recombinant, yeast‐derived phytochrome‐phycocyanobilin adducts (phyA*, phyB*) and oat phytochrome A (phyA) were microinjected into etiolated aurea tomato seedlings. Subsequent to microinjection anthocyanin and chlorophyll accumulation was monitored as well as β‐glucuronidase (GUS) expression mediated by light‐regulated promoters (chs, chlorophyll a/b binding protein (lhcb1) and ferredoxin NADP+ oxidoreductase (fnr)). Microinjection of phyA* under white light conditions caused anthocyanin and chlorophyll accumulation and mediated chs—GUS, lhcb1—GUS and fnr—GUS expression. Microinjection of phyB* under identical conditions induced chlorophyll accumulation and mediated lhcb1—GUS and fnr—GUS expression but neither anthocyanin accumulation nor chs—GUS expression were observed. The characterization of Arabidopsis phytochrome mutants and the microinjection experiments suggested that phyB cannot induce the accumulation of juvenile anthocyanin. Microinjections under far‐red light conditions demonstrated that phyA can act independently of other photoreceptors. By contrast, phyB* injections under red light conditions indicated that phyB* needs interactions with other photoreceptors to mediate a rapid and efficient de‐etiolation signal.