## 5 Biological Activities of Rice Phytolexins
### Bioactivities of Diterpeneoid Phytolexins
#### 5.1.1. Momilactones
\begin{table}
\begin{tabular}{p{56.9 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline
**Metabolite/s** & **Activity** & **Experimental Model** & **References** \\ \hline \multirow{6}{*}{Oryzalezxin S} & \multirow{6}{*}{Allebotathic} & Evaluation of allebotathic activity of oryzaexin S against _L. sativa_ cv. Black-seeded Simpson and _E. crus-galli cv. Kudiraivali in O. sativa_ & [190] \\  & & Oskls4 knockouts. & \\ \cline{2-4}  & Antimicrobial & Evaluation of inhibitory activity of oryzaekins S against _M. grisea_. & [20] \\ \cline{2-4}  & Regulation of stomatal closure & Evaluation of stomatal closure and susceptibility to drought in _cps2_ and _cps4_ knockout lines. & [189] \\ \hline \multirow{6}{*}{Phytocassanes} & \multirow{6}{*}{Antimicrobial} & Evaluation of inhibition of _M. grisea_ spore germination by phytocassanes A-D from rice leaves infected with _M. grisea_ and from rice stems infected with _Rhizoctonia solani_. & [29] \\  & & Evaluation of inhibition of _M. grisea_ spore germination by phytocassanes E from rice suspension-cultured cells treated with a mycelial extract of the pathogenic potato fungus & [30] \\ \cline{1-1}  & & _Phytophthora infestans_. & \\ \cline{1-1}  & & Evaluation of inhibition of _M. grisea_ spore production by phytocassanes A-F from rice leaves elicited by UV light. & [31] \\ \hline \end{tabular}
\end{table} 
Table 4: _Cont._and phytocassanes suggest that rice roots are not only responsible for the accumulation and exudation of these metabolites but also for their production [50]. The role of momilactones as allelechemicals was confirmed via reverse genetics, using knock-outs of relevant diterpene synthase genes (_OsCPS4_ and _OsKSL4_, see Section 3.1 and Figures 6 and 7) [190]. An interesting study by [PERSON] and [PERSON] [169] showed that rice can perceive some chemicals released into the environment by barnyard grass. Rice plants respond to the presence of this weed by producing and secreting momilactone B into the surrounding environment. On the other hand, this metabolite induces allelechemical activity in barnyard grass. This suggests that during their evolution, rice and barnyard grass may have developed chemical crosstalk to promote the defense mechanisms against biotic stress conditions by detecting certain key compounds [169]. The group of [PERSON] and collaborators investigated the mode of action of momilactones A and B using the model plant _Arabidopsis thaliana_[196; 197; 198]. They first observed that momilactones were absorbed by _A. thaliana_ in proportion to their exogenous levels and that their inhibitory effects on root and hypocotyl growth were related to their endogenous levels [197]. They then investigated protein expression in the same model plant in response to momilactone treatment. In Arabidopsis plants treated with momilactones A and B, it was observed a higher amount of cruciferina, cruciferin 2, and cruciferin 3 compared to the control. The breakdown of cruciferins and cruciferina is indeed essential for seedling growth as it provides the initial source of nitrogen for seed germination. These results suggest that momilactones may inhibit the germination of Arabidopsis seeds by inhibiting the degradation of these proteins. Further insights into the role of momilactones as allelechemicals can be found in Table 4 in an excellent review recently published by [PERSON] et al. [12]. * Anti-cancer activity: In 2005, [PERSON] et al. [160] evaluated the cytotoxic activity of seven compounds isolated from rice hulls. Three of these, namely orizaterpenol and momilactones A and B, showed cytotoxic effects against murine P388 leukemia cells. Momilactone B was found to be significantly more active than momilactone A and orizaterpenol (IC\({}_{50}\) 0.07, 0.85, and 4.2, respectively) [171]. In 2007 [PERSON] and colleagues [172] evaluated the cytotoxic activity of momilactone B on human colon cancer HT-29 and SW620 cells, which exhibited strong tolerance to anticancer agents in vitro and in vivo in previous studies.