## 2 Diversity and Distribution of Rice Phytoalexins
### Phenolic Phytoalexins in Rice  
Sakuranetin (4\({}^{\prime}\),5-dihydroxy-7-methoxyflavanone) (Figure 4A) is the major phenolic phytoalexin in rice. It belongs to the class of flavonoids known as flavanones. Sakuranetin is the O-methylated derivative of the better-known flavanone naringenin, which is predominantly found in some edible fruits such as _Citrus_ species, graprefruit, and tomatoes [67]. It was first isolated from the bark of Chinese cherry (_Prunus pseudocerasus_) [68] and wild Himalayan cherry (_Prunus censoides_ or Prunus puddum) [69] and was later established as a phytoplankton in rice [70]. Over a century of investigations have shown that this flavanone is widely distributed in angiosperm families, generally in its glycosylated form (primarily Sakuranin). Some of the sakuranetin-producing plants are listed in Table 2, which is largely based on an excellent review paper recently published by [71] focusing on the pharmacological aspects and distribution of this compound. The induction of sakuranetin accumulation in response to the rice blast fungus infection [70; 72; 73], as well as its antimicrobial activity against a wide range of phytopathogenic fungi (e.g., _M. grisea_, _Bipolaris oryzae_, and _R. solani_) and bacteria (e.g., _Burkholderia glumae_, _Xanthomonas oryzae_ pv. _Oryza_, and _X. oryzae_ pv. _Oryza_) strongly suggests its biological role as phytoplankton [73; 74]. To understand the role of sakuranetin in rice blast resistance, [PERSON] et al. [73] compared the fungus-responsive characteristics in resistant- and susceptible-type rice lines (IL7 and Nipponbare, respectively). They found that sakuranetin has stronger antifungal activity to blast fungus than momilactone A and that the resistant rice line accumulated sakuranetin in infected regions at an adequate concentration to restrict the fungus, while the amount in the susceptible line was too low to be effective. Moreover, different time-dependent sakuranetin accumulation profiles and hypersensitive responses (HR) were observed in resistant and susceptible lines. Leaves of the resistant line showed HR within three days post-inoculation (dpi) with _M. grisea_ spores, with a four-fold increase in sakuranetin accumulation at 4 dpi. Conversely, a susceptible line had an increase in sakuranetin accumulation at 4 dpi but not at 3 dpi, resulting in a large fungus mass without HR. \begin{table}
\begin{tabular}{l l l l} \hline \hline
**Species** & **Family** & **Part of the Plant** & **Reference** \\ \hline _Prunus pseudocerasus_ Lindl. & Rosaceae & Bark & [68] \\ _Prunus cerasoides_ D.Don (Prunus puddum) & Rosaceae & Bark & [69] \\ \hline _Eupatorium havanense_ Kunth & Asteraceae & Whole plant & [75] \\ \hline _Ribes nigrum_ L. & Grossulariaceae & Leaves & [76] \\ \hline _Iris milesii_ [PERSON] ex Foster & Iridaceae & Rhizes & [77] \\ \hline _Artemisia campestris_ subsp. _glutinosa_ (Gay ex [PERSON].) Batt. & Asteraceae & Aerial parts & [78] \\ \hline _Hyptis salzmanii_ Benth. & Lamiaceae & Leaves & [79] \\ \hline _Bonnetia dinizii_ Huber & [PERSON] & Wood & [80] \\ \hline _Primula siebollai_ E. [PERSON] & Primulaceae & Bud exudate & [81] \\ \hline _Eridictyon californicum_ (Hook. \& Arn.) Torr. & Boraginaceae & Leaves & [82] \\ \hline _Teurium stocksianum_ Boiss. & Lamiaceae & Aerial parts & [83] \\ \hline _Dodonaea viscosa_ Jacq. & Sapindaceae & Aerial parts & [84] \\ \hline _Xanthorrhoea hasttilis_ R. Br. & Xanthorhoeaceae & Dried resin & [85] \\ \hline _Daphne aurantiuca_ Diels. & Thymelaeaceae & Stem bark & [86] \\ \hline _Dodonaea viscosa_ Jacq. & Sapindaceae & Aerial parts & [87] \\ \hline _Baccharis retusa_ DC. & Asteraceae & Twigs & [88] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Plant species in which sakuranetin has been identified in recent decades. As will be discussed later, the direct precursor of sakuranetin is supposed to be naringenin. Sakuranetin showed significantly higher antifungal activity than naringenin against _M. grisea_[70; 92].