Energy dissipation
NPQ exhibited a decreasing
tendency following a slow increase
(Fig. 3B). The slow NPQ development followed a single exponential
function and as a consequence
fast-activated qE component was absent, while the NPQmaxshowed a low value (Fig. 3C). Moreover, proteome data showed that the
luminal pH sensor PsbS protein, which was required for
qE, remained unchanged in response
to light exposure (Table S1).
Slightly upregulated zeaxanthin
epoxidase (ZEP) and violaxanthin de-epoxidase (VDE) levels indicated a
low level of xanthophyll cycle-dependent energy dissipation (Table S1).
The two key enzymes of chlororespiration, post-illumination chlorophyll
fluorescence increase and ubiquinol oxidase, remained unchanged.
Similarly, D-glycerate 3-kinase, the core enzyme in photorespiration
pathway, both ascorbate peroxidase (APX) and superoxide dismutase, the
key enzymes of the Mehler reaction, and malate dehydrogenase, the key
enzyme of malic acid synthesis,
remained
unchanged (Table S1).
These results suggested that
alternative electron flows
associated with energy dissipation
were
not significantly activated.
Antioxidant
system defense
Both the chlorophyll synthesis pathway and the early light-induced
protein were not significantly induced by light exposure (Fig. 2A and
Table S1), suggesting that there was no accumulation of free chlorophyll
which would have acted as a generator of reactive oxygen species.
Although Delta-aminolevulinic acid dehydratase, the enzyme committed to
tetrapyrrole biosynthesis, and geranylgeranyl diphosphate reductase,
which provides phytol, were
upregulated, the terminal key
enzyme NADPH-protochlorophyllide oxidoreductase of chlorophyll synthesis
was downregulated (Fig. 2A). The
expression of the stromal antioxidant enzymes glutathione peroxidase
(GPX) and APX remained unchanged (Table S1), further displaying that the
antioxidant ability was not significantly enhanced following light
exposure.