G. Neuhaus et al., CALCIUM CALMODULIN-DEPENDENT AND CALCIUM/CALMODULIN-INDEPENDENT PHYTOCHROME SIGNAL-TRANSDUCTION PATHWAYS/, Cell, 73(5), 1993, pp. 937-952
Phytochrome is a well-characterized plant photoreceptor, able to modul
ate many morphological, physiological, and biochemical events through
as yet undefined mechanisms. By developing single-cell assays to visua
lize phytochrome responses, we have studied the effects of microinject
ing putative signaling intermediates into phytochrome-deficient tomato
cells. We demonstrate that phytochrome phototransduction initially in
volves the activation of one or more G proteins that are coupled to at
least two different pathways; one pathway requires calcium and activa
ted calmodulin and can stimulate expression of a photoregulated cab-GU
S reporter gene together with the synthesis and assembly of some, but
not all, of the photosynthetic complexes. The other pathway, controlli
ng anthocyanin biosynthesis, does not require calcium. Furthermore, ou
r results reveal that phytochrome signaling is cell autonomous and is
not likely to require any light-activated steps downstream of the G pr
otein.