4.4 Molecular studies
The results of gene expression analysis showed that allelochemical
compounds altered the expression of PsbA and PsbS genes directly or
indirectly. Interestingly, these changes were dependent on plant
species; so it seems that the strategy and ability of each plant for
tolerating allelopathic stress determines the fate of changes in gene
expression. Increasing PsbA gene expression in wheat plant (which
encodes D1 subunit of photosystem II due to increasing chlorophyll
fluorescence) declines the use of reduced coenzymes, reduction of
stomatal opening, and subsequently reduction of CO2input and carbon fixation and ultimately reduction of photosystem II
efficiency and increasing risk of D1 subunit degradation in this
photosystem are quite obvious. In fact, by continuously replacing
degraded D1 protein, wheat plant retains the production of
NADPH/H+ and ATP to fixate respiratory carbon and
avoid water loss, whereas the decline in cucumber’s PsbA gene expression
caused cucumber not to be able to continuously replace degraded D1
protein. According to the increasing chlorophyll fluorescence in this
plant and extreme risk of D1 subunit degradation, cucumber plant
increased the stomatal opening and photosynthesis rate to reduce the
excess electron in the photosynthetic electron transport chain. The
results of PsbS gene expression confirm the interpretations provided for
both plants. The expression of this gene was decreased in both plants,
but it was statically significant (p ≥ 0.01) only in cucumber
plant (99.93%). Decline in PsbS gene expression led to the destruction
of the optical antenna components. Destruction of antenna structure,
reducing the energy supply for excitation of reaction centers,
especially in photosystem II, led to better control of excess electron
entry into the photosynthetic electron transfer chain. The results of
molecular docking showed that the proteins encoded by PsbA and PsbS can
interact directly with thymol, carvacrol, beta-elemene,
trans-caryophyllene, germacrene-D, spathulenol, and 5alpha-pregnane
compounds. These compounds are well-known allelochemicals in plants.
Therefore, it can be concluded that allelochemicals not only indirectly
(such as induction of oxidative stress) but also directly are able to
decrease protein’s function by binding to their structure. All of the
above mentioned compounds have the ability to bind to proteins encoded
by the PsbA and PsbS genes, and among them the 5alpha-pregnane found in
wheat shoot has the best structure for this binding.