## 2 Diversity and Distribution of Rice Phytoalexins
### Diterenoid Phytotalexins in Rice
#### 2.1.1. Momitactones  
Momitactones belong to the small family of (9\(\beta\)-H)-pimarane compounds, which are characterized by a \(\beta\)-orientation of the hydrogen bonded to carbon-9 of the pimarane scaffold [20]. To our knowledge, momilactones A and B (Figure 1A,B) were the first phytoalexins to be characterized by any member of the Poaceae family. They were first identified as plant growth inhibitors from rice seed husks [27] and only later recognized as phytoalexins due to their blast-induced biosynthesis and antifungal activity [28]. The name \"momilactone\" is the combination of two words, namely _momi_, which is Japanese for rice husk, and lactone, which refers to the chemical structures of momilactones A and B [20]. Other rice momilactones have been identified, whose role as phytoalexins has so far not been demonstrated. Momilactone C (Figure 1C) was isolated in 1976 by [PERSON] and collaborators [42] as a minor constituent of growth inhibitors from rice seed husks, while momilactones D and E (Figure 1D,E) were isolated in 2015 by [PERSON] and co-workers [43] from rice roots. Momilactone E is a 19-_nor_-(9\(\beta\)-H)-pimarane and does not contain a lactone residue in its molecular structure; therefore, its name is not chemically correct [20].  
Although momilactones were initially discovered in cultivated rice (_Oryza sativa_) [27; 28], comparative genomic and biochemical studies have recently demonstrated the ability of several wild rice species (i.e., _O. barthii_, _O. brachyantha_, _O. glaberrima_, _O. glumapatula O. meridionalis_, _O. punctata_, _O. rufipogon_) to biosynthesize momilactones A and B, suggesting that gene clustering for momilactone biosynthesis (see Section 3) had already been accomplished before rice domestication [9; 44].  
Surprisingly, momilactone biosynthesis has also been found in barnyard grass [45; 46], which falls into a separate clade within the Poaceae family. Even more surprising is the identification of momilactones A and B in the moss species _Plagionnium acutum_ (Mniaceae) [47] and _Calobynum plumforme_ (_Hypum plumazoforme_ prior to 2019, Hypnaceae) [48], which represent a very early diverging lineage of land plants. Ethanol extracts of _C. plumiforme_ showed significant growth inhibitory activity against angiosperms, mosses, and liver-worts. This suggests that momilactones in bryophytes, as well as in Poaceae, play a role in allelopathy.  
Intra- and inter-specific variation in diterpen phytoalexin production in rice has not yet been extensively investigated. Recently, [PERSON] and colleagues [9] analyzed the abundance of diterpenoid phytoalexins in UV-light-irradiated leaves of rice cultivars from the World Rice Core Collection (WRC) (covering a wide range of genetic diversity of rice [49]), and in several wild rice species. Momilactone A was found in most WRC cultivars and wild species, while momilactone B was generally accumulated at lower or undetectable levels. In cultivated rice, the highest content of momilactone A was found in the japonica cultivars (up to 495 nmol g\({}^{-1}\) FW in 'Urasan 1'). In wild rice, the greatest accumulation was observed in species with AA and BB genomes (up to 667 nmol g\({}^{-1}\) FW). In contrast, momilactones were not detected in _O. brachyantha_, which has a FF genome. They also discovered a novel phytoalexin, oryzaelactone (Figure 1F), which was only detected in three cultivars in the WRC and in a few strains of wild rice species _O. rufipogon_ and _O. meridionalis_. The abietane skeleton of oryzaelactone distinguishes it from other momilactones, which are instead characterized by a pimarane skeleton.