Origins of phytochrome-modulated Lhcb mRNA expression in seed plants

The levels of Lhcb mRNA in higher plants are regulated by phytochrome, cryp tochrome, and an endogenous circadian oscillator. To determine whether simi lar regulatory mechanisms operate in the ancient gymnosperm Ginkgo biloba, we measured Lhcb mRNA levels in seedlings in response to different light co nditions. Removal of a diurnally oscillating light stimulus caused dampenin g of maximal Lhcb mRNA accumulation levels, with little change in periodici ty. Although low fluence pulses of both red and blue light given to etiolat ed seedlings caused maximal accumulation of Lhcb mRNAs characteristic of th e phasic/circadian response seen in flowering plants, the additional initia l acute response seen in flowering plants was absent. The induction of Lhcb gene expression in both cases was at least partially reversible by far-red light, and appeared biphasic over a range of red fluences. Together, these data indicate that Lhcb genes in G. biloba appear to be regulated in a man ner similar to that of flowering plants, whereas signaling and attenuation of mRNA levels through the photoreceptor systems and circadian clock show f eatures distinct from those characterized to date. The implications for the se findings are discussed in light of the evolution of circadian clock inpu t signaling.