Geoscience Reference
In-Depth Information
adjustments also enable plants to keep their stomata open and
continue taking up CO
2
for photosynthesis under conditions of
moderate water stress. One amino acid that appears to be par-
ticularly sensitive to stress is proline. A large number of plants
synthesise proline from glutamine in the leaves. The role of
proline is demonstrated by experiments with tomato cells in
culture. Cells subjected to water stress by exposure to hyper-
osmotic concentration of GEG responded with an initial loss of
turgor and rapid accumulation of proline.
14.5
State transition mechanism for energy
distribution
State transition is one of the best mechanisms for short-term
regulation of energy distribution and is based on reversible
phosphorylation of light-harvesting complex II (LHCII) pro-
tein. The phosphorylation of protein is a ubiquitous mechanism
for regulating many aspects of gene regulation and response
to environmental stimuli in all eukaryote organisms. The
phosphorylation of proteins is catalysed by a class of enzymes
known as protein kinases. Chloroplasts contain a thylakoid
membrane-bound protein kinase capable of phosphorylating
LHCH. The activity of this kinase is sensitive to the redox state
of the thylakoid membrane and is activated when excess energy
drives PSII, resulting in a build-up of reduced plastoquinone.
Plants that are exposed to conditions that result in the preferen-
tial excitation of PSII are considered to be in state second.
The resulting phosphorylation of LHCII increases the nega-
tive charge of the protein, causing LHCII to dissociate from
PSII. The same negative charge also loosens the appression of
the thylakoid membranes in the grana stacks, freeing a certain
portion of LHCII to migrate into the PSI-rich stroma thyla-
koids. This shifts the balance of energy away from the PSII
complexes, which remain behind in the appressed region, in
favour of PSI. The preferential excitation of PSI is referred to
as state I. Recently, it has been shown that the PSI-H subunit
of
Arabidopsis thaliana
is required for reversible transitions
between state I and state II.
14.6
Long-term acclimation
Long-term processes
are less transient and thus usually exhibit
a longer lifetime. However, the lifetimes of these processes
