Handbook of Plant and Crop Physiology

the transcription of small subunit Rubisco [128], and cabgene transcription (reviewed in Ref. 129) as well
as the expression of phytoene synthase [130]. Horwitz et al. [131] demonstrated that the abundance of
CabmRNA is not the factor limiting Chl accumulation.

C. In Fully Matured Leaves

It has been observed that in plants many cellular activities occur with a daily rhythmicity. These rhythms
are called circadian. The input factors such as light and temperature are connected to the central oscilla-
tor(s), which generates output rhythms via a range of signaling pathways. Interestingly, it has been shown
that the glutamyl tRNA reductase (see Figure 1) and cabgenes present circadian variations in barley. The
phase at which these genes are expressed is slightly earlier than that of the cabgenes (Figure 6) [132,133].
It is important to note that among all the genes expressed according to the circadian rhythms, only pro-
moters of Arabidosis cab2and wheat cab1have been shown to confer clock regulation to a reporter gene
(reviewed in Ref. 134).
In plants cultivated under a light/dark regime, the amount of Chl slightly decreases during the dark
period [28]. Therefore, Chl should be resynthesized at the beginning of each day [82]. This can be
achieved by accumulation during the dark period of aggregates of LPORA-Pchlide-NADPH complexes
according to the mechanism identified by Reinbothe et al. [80]. This regeneration also triggers the for-
mation of small prolamellar bodies [83].

ACKNOWLEDGMENTS

The author thanks the Ministery of Education, Youth and Sports of the Czech Republic (grant VS96085)
and the Grant Agency of the Czech Republic (grant 204/98/P120) for their financial support.

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274 SCHOEFS

Figure 6 Northern blot analysis of glutamyl-tRNA reductase and cabmRNAs during the first 16 hr of green-
ing of 6-day-old dark-grown barley seedlings. (From Ref. 132.)