# Identification and Isolation Techniques for Plant Growth Inhibitors in Rice
## 4 Major PGIs in Rice
### Terpenoids  
Together with phenolic acids and polyphenols, terpenoids are also a major component in rice and are grouped into monoterpenoids, sesquiterpenoids, triterpenoids. Their distribution and content depend on the part of rice or rice varieties. This section discusses some typical terpenoid-allelochemicals, phytohormones, and phytoalexins. The structures of these terpenoids are shown in Figure 4. Rice bacterial blight (_Xanthomonas oryzae_ pv. _Oryza_, Xoo) is a severe rice disease, which seriously decreases the crop yield by 50% and even leads to the death of seedlings after a few weeks if the infection occurs at the seedling stage [18]. In 2016, [PERSON] et al. [20] indicated that monoterpene (S)-limonene severely inhibited Xoo growth at 1 mM concentration, as confirmed by disc diffusion and liquid culture assays. Similarly, [PERSON] et al. [21] detected diterpene phytoalexin, which can resist blasts in rice plants. In reality, phytocassanes A-E, oryzaeas A-F, oryzaeasin S, and momilactones A (MA) and B (MB) in response to biotic stresses were found, and these discoveries provide a strong premise for the control of bacterial blights in rice based on plant-derived natural pesticides [22; 97].  
More specifically, MA and MB were detected in rice seedlings, rice hulls, rice straws, rice leaves, and exudate roots [5; 8; 25; 26; 98]. MA and MB act as growth inhibitors. Barnyard grass (_Echinochloa crus-galli_) and dececan grass (_Echinochloa colona_), the most popular and detrimental breeds for rice growth, are inhibited by MA and MB at 10 and 1 uM, respectively [98]. In more detail, [PERSON] et al. [99] conducted inhibitory experiments of MA, MB, momilactone E (ME), and the mixture MAB on _L. sativa_, _Echinochloa crus-galli_, and _Solidago altissima_. The results showed that MB and MAB were prominent in the inhibition of the germination of _L. sativa_ (IC\({}_{50}\) = 178.46 and 327.20 ug/mL, respectively), while the value of the germination rate of _Echinochloa crus-galli_ and _Solidago altissima_ was least affected by MA, with an IC\({}_{50}\) reaching 229.67 and 119.80 ug/mL, respectively. The suppression activity of MA on the growth of _L. sativa_, _Echinochloa crus-galli_, and _Solidago altissima_ is weaker compared to MB and MAB. The above evidence proves that the inhibitory effects of momilactones on weeds are strong. The role of MA and MB in countering pests and diseases has been suggested. Indeed, the concentration of MA reached 100 to 500 \(\mathrm{\SIUnitSymbolMicro g}\)/g, which is enough to restrict the growth of the rice blast fungus _Pyriculara oryzae_[100].  
Figure 4: The structure of allelopathy-terpenoids in rice.  
Momilactones are significant allelochemicals that can be released from rice seedlings into the environment [98] or extracted from parts of rice [26; 101]. The content of MB in the rice is lower than other momilactones, but its biological activities are stronger compared to others [102; 103; 104]. [PERSON] et al. [103] reported that the expression of an enzyme that causes ketogenesis, 3-hydroxy-3-methylglutaryl-CoA synthase-2 (HMGCS2), can be suppressed by MB. The anti-cancer role of MB has also been examined. MB accelerates the hypoxia-induced apoptosis of human breast cancer cells through the STAT5b pathway [105] and induces the apoptosis and G1 arrest of the cell cycle in human monocytic leukemia U937 cells [106]. Additionally, MA and MB, extracted from rice hulls, were found to have cytotoxic effects against P388 murine leukemia cells [107]. Regarding antioxidant activities, MA is slightly stronger than MB (EC\({}_{50}\) was 783.9 and 790.7 ug, respectively) [38]. In 2019, [PERSON] et al. [24] suggested that both MA and MB may be prominent candidates for anti-diabetic therapy by inhibiting \(\alpha\)-amylase and \(\alpha\)-glucosidase. Based on the above evidence, terpenids, especially momilactones, are a great resource for developing natural compound-based medicines.