FIGURE 9.

Model of the regulatory circuitry regulating the gymnosperm (pine) or angiosperm (tobacco or Arabidopsis) LHCB promoters. The pine promoter, in either pine itself (left) or heterologously expressed in tobacco, is regulated by positive plastid biogenic signals, largely independently of light. The angiosperm LHCB promoters, in tobacco or Arabidopsis (right), are dependent on positive plastid signals and GLK transcription factors in the dark or the light, but the ability to respond to the plastid signals is suppressed (as shown in Arabidopsis) by the DET1 and COP1 proteins in the dark, and so expression only takes place when phytochrome photoreceptors repress DET1 and COP1 and allow the response to plastid signals. Several other photosynthesis-associated gene promoters are regulated in the same fashion but are not dependent on GLKs. DET1 and COP1 regulate the expression of the light-dependent but photosynthesis-unrelated CHS gene without the requirement for positive plastid biogenic signals. Components (including general photomorphogenesis genes) known in angiosperms and whose presence is hypothesized in gymnosperms are shown in gray on the left.