**5. Symbiosis flavonoids-microorganisms**

Flavonoids are considered signaling compounds in plant-microorganisms symbiosis; their function is signaling in response to pathogens, bacteria, fungi. Their participation is important in the nodulation of roots, where they are secreted to the rhizosphere [21], in the case of legumes under low nitrogen conditions, and where their interaction is specific with gramnegative bacteria called rhizobia that are nitrogen fixers [49]. They are also involved in the transcription of genes for the biosynthesis of rhizobial signaling molecules called Nod factors, which are perceived by the plant to allow symbiotic root infection [49]. Therefore, they can stimulate or inhibit rhizobial Nod gene expression. Root exudates can be flavonoids that participate as signaling compounds in the arbuscular mycorrhizal symbiosis [50]. Root exudation of flavonoids increases or decreases depending on the response to the symbiotic and pathogenic interaction of the plant and microbes.

It has been shown that certain flavonoids are stimulants for the germination of ectomycorrhizal fungi spores [50]. Depending on the arbuscular mycorrhizal fungi colonizers of the roots and the stage of symbiosis development [51].

The flavonoid role in the rhizosphere (**Table 2**) is important in agriculture because flavonoid interaction in the symbiosis with microorganisms can be a tool for nitrogen fixation in soils


**Table 2.** Role of flavonoids in the rhizosphere.

Another study related with metabolomic profiling is the analysis of flavonoid distribution in

**Crop Plant organs Analytical technique Compound Reference**

Fruit UPLC/QqQ-MS/MS Flavan-3-ol, quercetin-3,4-diglucoside,

H NMR Naringenin, rutin, quercetin, kaempferol and its glucosides, and total flavonoids

Catechin and epicatechin

7-O-β-d-glucopyranoside

luteolin, isorhamnetin

LC-QTOF-MS Tricin, tricin 7-O-rutinoside, and tricin

isorhamnetin

Naringenin chalcone and rutin [33]

quercetin, kaempferol-3-glucuronide, rutin, and the conjugates of isorhamnetin

Rutin, Naringin, kaempferol, apigenin,

malvidin, pentose, rhamnose, selgin, tricin, chalcone synthase, chalcone isomerase

quercetin 3-rutinoside-7-glucoside and methylisoorientin-2″-O-rhamnoside

[32]

[34]

[35]

[36]

[37]

[38]

[39]

Soybean Leaves (RP)-HPLC and <sup>1</sup>

194 Flavonoids - From Biosynthesis to Human Health

Avocado Fruit MS/MS

bran

Rice Leaf and

Red and yellow raspberry cultivars

Red tomato Fruit HPLC/DAD and LC/NMR,

LC/MS, and LC/MS/MS

LC-DAD-ESI-TOF MS

Maize Kernels LC-MS/MS Apigenin, luteolin, methyl chrysoeriol,

Wheat Flag leaf UPLC-QTOF MS/MS Isovitexin, isoorientin, Isoschaftoside,

Grape Berries LC-MS Quercetin and derivatives, kaempferol and

Broad bean Pods UHPLC-ESI-qTOF-MS<sup>2</sup> Glycosylated flavonoids [40] Tea Leaves UPLC-Q-TOF MS Flavan-3-ols, flavonols and their glucosides [41]

The novel approach of genetic metabolomics referred to as metabolite profiling combined with quantitative trait locus (QTL) analysis was applied to detect flux control points in flavonoids biosynthesis of *Populus*. It was found that flavonoid profile can be used for QTL analysis

Plant metabolism is disrupted by several types of stress. Flavonoids are involved in the response toward abiotic stress [46]; a good model to understand how flavonoids contribute to the mitigation of oxidative and drought stress is *Arabidopsis thaliana*. Another example is the metabolomic analysis of tea, where the effects of light intensity and temperature on the metabolites in tea grown in the shade were evaluated; they found that most flavonoids (flavan-3-ols, flavonols, and their glucosides) decreased significantly in the shading treatments. Their study also showed a greater effect of temperature on galloylation of catechins than light intensity [41]. The growing stage of plants has been studied in connection with flavonoid synthesis, an interesting metabolomic study is the analysis of soybean leaves [47], it showed significant changes in the content of flavonoids and isoflavonoids, and kaempferol derivatives were used

three *Momordica* species, where 13 flavonoids were found in a special pattern [44].

to reveal loci that control the flux of their biosynthesis [45].

**Table 1.** Metabolomic studies of flavonoids in agricultural crops.

poor in macronutrients. It can be a strategy to stimulate beneficial bacteria or inhibit harmful bacteria and fungi. It can also be considered in the genetic improvement of plant species.
