# Identification and Isolation Techniques for Plant Growth Inhibitors in Rice
## 4 Major PGIs in Rice
### Polyphenols  
Polyphenols are the main constituents of rice, and their distribution is varied among rice varieties and rice plant parts. Based on their structure, polyphenol compounds can be classified into flavones, flavonoids, flavanols (flavan-3-ols), flavanols, isoflav, flavanones, anthocyanins, and chaonnes with \(O\)- or \(C\)-glycos forms, of which some of the flavonoids are alledechenicals [79]. Published papers showed that the polyphenols were simultaneously determined in rice and evaluated their inhibitory effects on weeds and rice diseases [33; 45; 80; 81; 82]. The structures of phenolic compounds are shown in Figure 3. For example, Sakuraneit, a flavanone, was investigated from ultraviolet-irradiated rice leaves in 1992 by [PERSON] et al. [45]. This compound not only prevented the spore germination of _Pyriculara oryzae_ at 15 ppm but was also observed in blast-infected rice leaves. Sakuraneit provides typical evidence of the negative aspect of alledechenicals [45]. In 2013, five new compounds, including the derivatives of flavonols and flavanones, were elucidated in rice straw by using one- and two-dimensional NMR, combined with IR, electrospray ionization-mass spectrometry (ESI-MS), and high resolution-electrospray ionization-Fourier transformation-mass spectrometry (HR-ESI-FT-MS). The algicidal activities of the new compounds were evaluated with blue-green algae (_M. aeruginosa_) by bioassays. In the results, oleioly-\(\beta\)-d-arabinoside had the highest growth inhibition (92.6%) against _M. aeruginosa_ UTEX 2388 at a concentration of 100 mg/L [44]. From rice seedlings, two kinds of flavone \(O\)-glycosides (5,4\({}^{\prime}\)-dihydroxy-3\({}^{\prime}\),5\({}^{\prime}\)-dimethoxy-7-\(O\)-\(\beta\) glucopyranosyl flavone, and 7,4\({}^{\prime}\)-dihydroxy-3\({}^{\prime}\),5\({}^{\prime}\)-dimethoxy-5-\(O\)-\(\beta\)-glucopyranosyl flavone) have been identified, which act as agents to interfere with the growth of weeds and microbes in rice  
Figure 2: The structure of allelopathy-phenolic acids in rice. fields [46]. One of the significant phenolic allelochemicals is tricin, which was isolated from allelopathic rice cultivars and other plant species. Tricin exists in different parts of rice in either free or conjugated forms such as tricin-free, tricin-glycosides, tricin lignans, and tricin-lignan-glycosides [83]. Tricin and its isomer can suppress the fungi _Fusarium oxysporum_ [PERSON] and _Rhizoctonia solani_ [PERSON], which are the main causes of rice seedling rot disease [19]. Figure 3: The structure of allelopathy-polyphenols in rice. Polyphenols, as one of the most abundant secondary metabolites in rice, have been acknowledged as having benefits for human health. The published studies indicate that they help to reduce the risk of many chronic disorders, such as cardiovascular diseases, inflammation, type 2 diabetes, obesity, COVID-19, and some types of cancers [23; 78; 84; 85; 86]. This section focuses on summarizing some prominent polyphenols, which have preventive effects on the above-mentioned diseases. Tricin is one of the seven major flavonoids that are usually reported in rice, accounting for 77% of all seven flavonoids (131.5 mg/100 g DW) [79]. Tricin and its derivatives were also reported as being potentially applicable in pharmaceuticals due to their preventive efficacy, low toxicity, and reasonable bioavailability [87]. Tricin and flavonolignan ether have been isolated from medicinal rice Njavara (_Oryza sativa_ L.) and were tested for their anti-inflammatory properties in rats. The result showed that carrageenanan-induced paw edema effects of more than 65% after 5 h at a dose of 2 mg/kg [88]. In another experiment, 50 uM of extracted tricin in brown rice was the most potent anticholinergic effect of the phenols with IC\({}_{50}\) of 16 uM in the SW 480 colon cells and 0.6 uM in the MDA-MB-468 breast cells [78]. In addition, anthocyanins and proanthocyanidins are polyphenols, which were found in the pigment rice grain.