McCormac, Alex C. and Terry, Matthew J.
The nuclear genes Lhcb and HEMA1 are differentially
sensitive to plastid signals and suggest distinct roles for
the GUN1 and GUN5 plastid-signalling pathways during
Plant Journal, 40, (5), . (doi:10.1111/j.1365-313X.2004.02243.x).
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Feedback mechanisms are critical to the regulation of chloroplast development and signals from functional
plastids are required to maintain nuclear gene expression of chloroplast proteins. To understand the role of
these signals in de-etiolating Arabidopsis thaliana L. seedlings, we followed the expression of three nuclear
genes, Lhcb, HEMA1 and GSA, under a variety of treatments (Norflurazon, lincomycin and a far-red light pretreatment)
leading to plastid damage in white light and in a range of genetic backgrounds known to modulate
plastid signalling: the genomes uncoupled mutants, gun1, gun4, gun5 and the gun1,5 double mutant, and in a
transgenic line over-expressing NADPH:protochlorophyllide oxidoreductase. The three nuclear genes were
differentially sensitive to changes in plastid signalling, with Lhcb the most strongly repressed and GSA
insensitive to all but the most severe treatments. Analysis of plastid morphology in seedlings grown under
identical conditions demonstrated that these responses corresponded closely to the degree of plastid damage.
Furthermore, although Lhcb and HEMA1 were responsive to both GUN1 and GUN5 signals, the relative inputs
from these pathways differed for each transcript with GUN1 being dominant for HEMA1 regulation. Further
analysis of HEMA1 expression in gun1 seedlings under non-photobleaching conditions indicates that GUN1 is
an important suppressor of HEMA1 expression in the dark and under saturating white light. These results are
consistent with plastid signals functioning in a feedback regulatory mechanism during chloroplast biogenesis,
and suggest a key role for GUN1 during the early stages of chloroplast development.
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