A 146 BP FRAGMENT OF THE TOBACCO LHCB1-ASTERISK-2 PROMOTER CONFERS VERY-LOW-FLUENCE, LOW-FLUENCE AND HIGH-IRRADIANCE RESPONSES OF PHYTOCHROME TO A MINIMAL CAMV 35S PROMOTER

The occurrence of very-low-fluence responses (VLFR), low-fluence respo nses (LFR) and high-irradiance responses (HIR) of phytochrome was inve stigated for the expression of the gene of beta-glucuronidase (gusA) u nder the control of the tobacco Lhcbl2 promoter, in etiolated transge nic tobacco seedlings. The activity of beta-glucuronidase (GUS) showed biphasic responses to the calculated proportion of Pfr provided by li ght pulses. The first phase (i.e. the VLFR) showed a maximum for Pfr l evels characteristic of far-red light. The second phase (i.e. the LFR) was observed at higher Pfr levels and was reversible by far-red light pulses. The strong effect of continuous far-red light (i.e. HIR) was fluence-rate-dependent and could not be replaced either by hourly puls es of the same spectral composition and total fluence or by very low f luences of red light. Deletion of the Lhcbl2 promoter to -453 caused little loss of GUS activity. The -453 to -31, -270 to -31 and -176 to -31 fragments of the Lhcbl2 promoter conferred proportionally normal VLFR, LFR and HIR to a truncated (-46 to +8) CaMV 35S minimal promoter . This is the first demonstration of the presence of three phytochrome action modes in the control of the transcriptional activity of a sing le gene. The cis-acting regulatory elements necessary for VLFR, LFR an d HIR are present in a 146 bp fragment of the tobacco Lhcbl2 promoter .