EFFECTS OF FLAVONOID METABOLISM ON PLANT RESPONSE TO STRESS
TOLERANCE
Flavonoids are a class of
secondary metabolites of polyphenols with C6-C3-C6 as the basic
structure or phenyl-benzopyran structure. Flavonoids also are a large
class of secondary metabolites formed by plants in the long-term
ecological adaptation process to withstand the stress of harsh
ecological conditions, animals, and microorganisms. They are widely
distributed in the plant kingdom and are abundant in flowers, fruits and
leaves of many plants (Du et al., 2010). Based on the different oxygen
rings and conformations of the basic molecular structure of flavonoids,
flavonoids are generally divided into six categories: flavone, flavonol,
isoflavone, flavanone, flavanol and anthocyanidin (Rice-Evans et al.,
2010). The starting substrate for plant flavonoid biosynthesis is
derived from coumaroyl-CoA of the phenylpropane metabolic pathway and
malonyl-CoA from acetyl-coenzymes. Under the action of chalcone synthase
(CHS), they first form chalcone (Aoki et al., 2000), and then the
naringenin is formed by the catalytic action of chalcone isomerase (CHI)
(McKhann and Hirsch, 1994). Under the catalysis of cytochrome P450
monooxygenase (CPM) and other enzymes, naringen can be used as a major
intermediate metabolite to synthesize other flavonoids (Akashi et al.,
1999; Falcone Ferreyra et al., 2012; Lam et al., 2014; Liu et al., 2003;
Uchida et al., 2015).
More than 10,000 plant flavonoids
have been discovered (Aoki et al., 2000; Jiang et al., 2010). They play
very important roles in plant resistance to various stress tolerances
(Agati et al., 2012; Yamasaki et al., 1997; Yan, et al., 2014). They
could remove free radicals under ultraviolet radiation (Li et al., 1993;
Treutter et al., 2005), improve seed storage capacity and prolong life
(Debeaujon et al., 2000), change petal color (Mola et al., 1998),
interfere with the polar distribution of auxin (Buer et al., 2004),
affect the accumulation and composition of fatty acids (Lian et al.,
2017), etc.
Early studies on the mechanism of flavonoids involved in stress
resistance mainly focused on their regulations on response to
ultraviolet radiation (Mellway et al., 2009; Tattini et al., 2006).
Later, flavonoids were found with strong antioxidant activity (Agati et
al., 2007; Hernández et al., 2009; Pourcel et al., 2007; Treutter et
al., 2006). Since various stresses can cause excessive peroxide to
accumulate in plants, the significant role of flavonoids in plants’
stress resistance attracts increasing interests (Fasano et al., 2014;
Qiu et al., 2008; Rai et al, 2016; Watkins et al., 2014). Tattini et al.
(2004) reported that European privet flavonoids as antioxidants respond
to strong light and drought stress.
Li et al. (2011) found a conserved
trans-acting element (G-box, CACGTG) in the promoter region of the
chalcone synthase family gene (AtCHS ) in Arabidopsis
thaliana , which regulates the accumulation of
H2O2 by responding to cGMP signals (Abu
Zahra et al., 2014). Yan et al. (2014) found that the cytochrome P450
monooxygenase GmFNSII/GmCPM in soybean was beneficial to the
accumulation of flavonoid aglycones in plants and the reduction of
H2O2 content. In previous studies, we
found that the content of flavonoids such as quercimeritrin in
salt-tolerant soybeans is relatively higher than that of salt-sensitive
soybeans, which is beneficial for soybeans to adapt to salt stress (Lu
et al., 2013). With the deepening of research, we further discovered
that enzymes related to the flavonoid metabolism pathway are important
salt stress response factors, and they can significantly regulate the
salt tolerance of plants such as Arabidopsis thaliana and soybean
(Pi et al., 2016). We recently found that the salt-triggered
phosphorylation of GmMYB173, subsequent elevates the transcription ofGmCHS5 for enhancing the accumulation of dihydroxy B-ring
flavonoids (such as cyaniding-3-arabinoside chloride) (Pi et al., 2018);
while salt-inhibited phosphorylation of GmMYB183 subsequently decreases
the transcription of GmCYP81E11 for reducing monohydroxy B-ring
flavonoids (such as ononin) (Pi et al., 2019). Actually, both GmMYB173
phosphorylation and GmMYB183 dephosphorylation contribute to soybean
salt tolerance.
The abovementioned studies showed that flavonoids played a very
important role in the response of plants to stress. However, there is no
direct evidence been found to show us whether bZIP could regulate
flavonoid accumulation for plant adaption to these stresses.
CONCLUDING REMARKS Due to their significant roles in plant
tolerances to various stress, the bZIP transcription factors have been
comprehensively studied, including their categorization and regulatory
mechanisms of target genes. However, there is at least one interesting
issue worthy of further investigation:
Whether bZIP transcription factor
regulates plant stress tolerance by modulating the synthesis of
flavonoids.
To date, plenty of literatures show that bZIPs regulate plant tolerances
to various abiotic stresses, such as low temperature, drought, and high
salt. Besides, there are many reports reveal that flavonoids participate
in various stress responses. Moreover, a lot of researches have now
confirmed that bZIP transcription factors play an important role in the
synthesis of flavonoids. Specially, bZIPs in subfamily H could bind to
G-box in promoter of cold responsive genes (Table I and II); members of
this subfamily also could modulate the synthesis of some flavonoids
(Table III). Since members in this group shares similar conversed
protein motifs (supplemental Figure S1 and S2), it is reasonable to
hypothesize that plant bZIPs in subfamily H could bind to G-box of
cold-responsive genes to further regulate the synthesis of flavonoids.
Similarly, it also makes sense that bZIPs in subfamily A could regulate
the synthesis of flavonoids by binding to G-box or ABRE of genes
involved in cold, salinity, drought and osmotic stresses; subfamily S
could regulate the synthesis of flavonoids by bind to G-box or C-box or
A-box or ABRE of genes involved in cold, salinity and drought stresses
(Table I, II and III). However, these hypotheses are still needed to be
further verified.
ACKNOWLEDGEMENTS
This work was supported by the grants 31970286 and 31301053 from the
National Science Foundation of China, LY17C020004 from the Natural
Science Foundation of Zhejiang Province.