Rice phytoalexins have been investigated mainly for their antimicrobial activity, which is linked to their role in plant–pathogen interactions, and for their inhibitory activity on seed germination and plant development, which is instead correlated with their role in plant–plant allelopathic interactions. In recent years, a growing number of studies have revealed a wide variety of biological activities (Table 1) and possible applications of these biomolecules, especially in the pharmaceutical field.

Momilactones are secondary metabolites belonging to the (9β-H)-pimarane diterpene family, found not only in cultivated and wild rice ^{[1][6][55][56]}[27,28,44,127] but also in other Poaceae such as Echinochloa crus-galli (barnyard grass) ^[57][9], as well as in the mosses Calohypnum plumiforme ^[14][58][48,62], and Plagiomnium acutum ^[59][47]. Several biological activities have been attributed to momilactones (Table 14), some of which are directly related to their biological role in plant-pathogen and plant–plant interactions. Momilactones also exhibit pharmacological activities, which make them potential candidates for the development of novel drugs, cosmetics, and additives for health-promoting foods.

Magnaporthe grisea, the causal agent of rice blast disease, is a major devastating pathogen resulting in a loss of 40% of global yield ^[60][191]. This ascomycete can infect more than 130 Poaceae species, including barley, wheat, and millet ^[61][62][192,193]. The anti-blast activity of momilactones A and B was first reported in 1977 by Cartwright et al. ^[4][56]. Following this discovery, several other metabolites isolated from resistant rice strains were tested against this fungal pathogen. Among them, momilactone B exhibited the highest power against both spore germination and germ tube growth of M. grisea ^[6][28]. The superior antifungal activity of this compound was then confirmed by tests carried out on different fungal pathogens, including Botrytis cinerea, Fusarium solani, and Colletrotrichum gloeosporioides ^[15][19]. In addition, momilactone B exhibited significantly higher antibacterial activity than momilactone A against different bacteria such as Pseudomonas ovalis, Bacillus cereus, and B. pumilus ^[15][19].

The allelopathic properties of momilactones A and B were soon recognized ^[1][27]. It has been observed that, when co-cultivated with rice, the growth of other plant species like barnyard grass ^{[22][23][24][27]}[164,165,166,169] and Alisma plantago-aquatica (the common water plantain) ^[25][167], two of the most disruptive rice weeds, was inhibited. Similar results were obtained using model plants such as Medicago sativa (alfalfa) and Lactuca sativa (lettuce) ^[2][5][7][55,157,158].

Momilactone B inhibited the growth of cress (Lepidium sativum L.) and lettuce (Lactuca sativa L.) seedlings at concentrations above 3 and 30 μM, respectively ^[8][58]. Momilactones A and B exhibited strong herbicidal activity against duckweed (Lemna paucicostata Hegelm 381) ^[10][160] and quantitatively inhibited the germination and growth of three weed species (Amaranthus retroflexus L., Cyperus difformis L., and Leptochloa chinensis L.) at concentrations ranging from 4 to 20 ppm ^[10][160]. Further studies confirmed that momilactones A and B accumulate in the roots of rice seedlings and can be released into the environment as root exudates ^{[9][17][63][64]}[60,159,194,195]. Expression analyses of diterpene cyclase genes involved in the biosynthesis of momilactones and phytocassanes suggest that rice roots are not only responsible for the accumulation and exudation of these metabolites but also for their production ^[16][50]. The role of momilactones as allelochemicals was confirmed via reverse genetics, using knock-outs of relevant diterpene synthase genes ^[51][190].

An interesting study by Kato-Noguchi and Ino ^[27][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 allelochemical 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 ^[27][169].

The group of Kato-Noguchi and collaborators investigated the mode of action of momilactones A and B using the model plant Arabidopsis thaliana ^[65][66][67][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 ^[66][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 allelochemicals can be found in Table 1 in an excellent review recently published by Serra Serra et al. ^[68][12].

In 2005, Chung et al. ^[10][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 (IC₅₀ 0.07, 0.85, and 4.2, respectively) ^[29][171]. In 2007 Kim and colleagues ^[30][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. Through MTT-dye reduction, lactate dehydrogenase (LDH), and colony-forming ability assays, they highlighted the potential of momilactone B as a novel therapeutic agent to induce cell death in human colon cancer cells ^[30][172].

In a study by Lee and colleagues ^[32][174], the anticancer activity of momilactone B was demonstrated in blood cancer cells, including human HL-60 leukemia cells, Jurkat human leukemic T cells, rat basophilic leukemia RBL-2H3 cells, and p815 mouse mastocytoma P-815 cells, at concentrations below 6 mM. The cytotoxic effect of momilactone B on Jurkat cells was associated with its apoptosis-inducing activity via caspases and mitochondria.

Recently, Quan and collaborators ^[40][41][182,183] investigated the anti-diabetic and anti-obesity activity of momilactones A and B. By in vitro assays, they showed potent inhibitory activity of momilactones on key enzymes related to diabetes. The inhibition of pancreatic α-amylase and α-glucosidase was significantly higher than the known diabetes inhibitor γ-oryzanol. In addition, a strong anti-trypsin activity was recorded ^[41][183].

In 2016, Xuan and co-workers ^[35][177] investigated the contents of momilactones in 30 rice cultivars of different origins, including hybrid, foreign, local, sticky, upland sticky, and upland rice of the two subspecies Japonica and Indica. They found that momilactones in rice are more related to salinity and drought tolerance than weed resistance. The correlation between momilactone A and B content and weed resistance was very low, with r² coefficients of 0.001 and 0.09, while the correlation with drought tolerance was much higher, with r² of 0.65 and 0.27, respectively ^[35][177].

In 2019 Quan and colleagues ^[42][184] investigated the antioxidant and anti-skin-aging activities of momilactones A and B in comparison with tricin, a well-known antioxidant and antiaging rice flavonoid. ABTS assay and in vitro enzymatic assays on pancreatic tyrosinase and elastase highlighted the synergistic activity of momilactones A and B, whose mixture showed significantly greater activity than single momilactones and tricin ^[42][184].

Oryzalexins A–F are distinguished from oryzalexin S both by both the biosynthetic pathway and their molecular structure.

Contrary to what has been reported for momilactones, the literature concerning oryzalexins’ biological activity is surprisingly limited. The most widely known bioactivity associated with these diterpenoid compounds is the antimicrobic activity against M. grisea, the rice blast fungus ^[47][48][20,187]. Although many reviews from the last decade report this activity ^{[69][70][71][72][73]}[1,199,200,201,202], they typically refer to literature from the late 1900s that merely scratched the surface on this topic, focusing on M. grisea spore inhibition ^[44][45][46][33,185,186].

Interestingly, recent reports suggest that oryzalexins may have other potentially valuable capabilities. Cho and colleagues [43] recently reported a potential anti-inflammatory activity of oryzalexin A, which has been shown to possess an inhibitory activity on NO production by mouse macrophage RAW264.7 cells. Furthermore, Jain and Das ^[49][188] observed that oryzalexin B, in combination with other natural compounds, seems to be able to bind six potential receptors in estrogen receptor-positive breast cancer, suggesting another potential use in medicine.

Oryzalexin S also shows a mild allelopathic effect in lettuce and barnyard grass ^[51][190] and, along with other oryzalexins, seems to be able to affect stomatal closure, playing a role in drought resistance ^[50][189].

Regardless of their role, oryzalexins have been shown to accumulate after exposure to fungal proteins ^[74][203] and oligosaccharides ^{[75][76][77][78]}[204,205,206,207], as well as fungal ^[79][208] and nematode ^[80][209] infection. Abiotic factors also seem to induce the accumulation of oryzalexins, such as heavy metal ions ^[81][54], salicylic acid ^[82][210], and UV radiation ^[83][84][53,211].

Despite their known role as phytoalexins, literature regarding phytocassanes and their biological activity is surprisingly scarce. In the last decade of the 20th century, known members of this class of diterpenes (i.e., phytocassanes A–E) were found to effectively inhibit spore germination when rice plants were infected with the rice blast fungus M. grisea, the rice sheath blight fungus Rhizoctonia solani, and the pathogenic potato fungus Phytophthora infestans ^[52][53][29,30]. Over the last decade, Horie and co-workers ^[54][31] confirmed these observations, including the recently discovered phytocassane F in their tests. During this last work, an increase in the production of phytocassanes after exposure of rice leaves to UV light was also observed ^[54][31], providing insights into the role of these compounds in response to abiotic stress. Considering biotic factors, increases in phytocassane production have been observed after exposure to fungal inoculation ^[85][212], Tricoderma viride-derived xylanase ^[86][87][213,214], cerebrosides A, B, and C ^[88][89][215,216], cholic acid ^[90][91][217,218], and mannan oligosaccharides ^[92][219], demonstrating their implication in response to a wide array of biological challenges. More recently, phytocassanes have been confirmed to play an active role in plant response to stress in general. Knock-out lines with deletion of biosynthetic gene clusters from chromosome 2, associated with phytocassane biosynthesis, were shown to be more susceptible to fungal blast and bacterial leaf blight than lines with deleted biosynthetic gene clusters from chromosome 4, associated with momilactone biosynthesis. These mutants also exhibited a drought and temperature-sensitive phenotype ^[93][126].

Sakuranetin is the main phenolic phytoalexin in rice. It showed remarkable antifungal activity against phytopathogenic fungi, including M. grisea and R. solani ^[94][95][96][70,92,93]. In addition to the antifungal activity, this flavanone exhibits a wide range of other biological activities that makes it attractive to the pharmaceutical industry. Some of the major sakuranetin bioactivities are listed in Table 2.

In a recent paper, Moulishankar and Lakshmanan ^[108][228] investigated the 3D and 2D interactions between 26 naturally occurring flavonoids and 11 target enzymes through molecular docking (a key tool used in structural molecular biology and computer-assisted drug design). They found that sakuranetin binds to several targets related to specific bioactivities, namely 4KIK (anticancer activity by IkB kinase inhibition), 4HZ5 (antibacterial activity by DNA gyrase B and topoisomerase IV inhibition), and 3LN0 (anti-inflammatory activity by cyclo-oxygenase inhibition). Further studies on the interaction between sakuranetin and specific targets involved in human diseases will contribute to the elucidation of molecular mechanisms underlying the bioactivities of this compound, which are still unknown or not fully understood.

According to Miyazawa and colleagues ^[98][220], sakuranetin suppresses umu gene expression during the SOS response against AF-2 in Salmonella typhimurium. The SOS response is thought to be triggered by an alteration in DNA synthesis, either directly by DNA damage that blocks the replication fork or indirectly by antibiotics (e.g., novobiocin) that inhibit DNA synthesis. The umu assay was developed to evaluate the genotoxic effects of environmental mutagens and carcinogens by examining the expression of a gene from the SOS family to detect DNA-damaging agents.

Sakuranetin has been shown to inhibit cancer growth both in vitro and in vivo. The induction of cell death by apoptosis appears to be the main mechanism involved in this bioactivity. As shown by Park et al. ^[100][18], sakuranetin inhibits the proliferation of human colon cancer HCT-116 cells with an IC₅₀ value of approximately 68.8 μg/mL. According to Drira and Sakamoto ^[101][222], sakuranetin strongly promotes melanogenesis in murine B16BL6 melanoma cells by inhibiting ERK1/2 and PI3K/AKT signaling pathways, leading to increased expression of the Tyr family genes TRP1 and TRP2. Additionally, they found that sakuranetin reduced the proliferation rate of melanoma cells at concentrations ≥15 µmol/L without directly affecting cell viability. Based on these findings, sakuranetin appears to be a promising candidate for anticancer drug development.

In a study aimed at identifying antiallergic compounds in resin extracts of Xanthorrhoea hastilis R. BR. (Xanthorrhoeaceae), Ogawa and co-workers ^[102][85] isolated three chalcones and six flavanones, including sakuranetin, through bioassay-directed fractionation. In vivo assays and measurements of platelet aggregation demonstrated that sakuranetin is one of the active ingredients responsible for the antiallergic activity of X. hastilis extracts.

Between 1999 and 2005, several studies highlighted the presence of sesquiterpenes and flavonoids with anti-inflammatory activity in Inula viscosa (L.) Aiton (Asteraceae), an herbaceous plant known for its effectiveness against skin inflammations ^{[103][104][105][106][107][108][109][110]}[223,224,225,226,227,228,229,230]. In 2007 Hernández and colleagues ^[103][223] tested the anti-inflammatory properties of three flavanones isolated from I. viscosa, namely 7-O-methylaromadendrin, 3-acetyl-7-O-methylaromadendrin, and sakuranetin. Sakuranetin was the most active in vitro, inhibiting the production of LTB4, acting directly on the 5-LOX enzyme and regulating secretory processes such as elastase release. Although the anti-inflammatory activity of flavonoids is usually related to their antioxidant activity, the results of Hernández et al. ^[103][223] suggest a possible non-redox inhibition of lipoxygenases, as well as a blockage of some proteins implicated in exocytotic mechanisms.

Saito and colleagues ^[104][224] observed that, even in the absence of adipogenic hormonal stimuli, sakuranetin strongly promoted both the differentiation of 3T3-L1 preadipocytes into adipocytes and the expression of genes associated with the development of adipocyte phenotypes. They also observed that glucose uptake in differentiated 3T3-L1 fat cells was stimulated by sakuranetin, suggesting that it may contribute to the maintenance of glucose homeostasis in animals.

Sakoda and co-workers ^[105][225] evaluated the impact of sakuranetin on vascular and lung parenchyma alterations in a murine model of chronic allergic pulmonary inflammation. In most of the parameters evaluated by histopathological analysis (Table 2), the effects of sakuranetin were similar to those of the steroidal anti-inflammatory drug dexamethasone. The authors speculated that the reduction in the number of eosinophils and elastic fibers in pulmonary vessels and lung parenchyma, promoted by sakuranetin, results from the reduction of oxidative stress and the levels of transcription factors NF-kB and VEGF in the lung.

Zhang and collaborators ^[106][226] identified sakuranetin as a new inhibitor of the carrier protein β-hydroxyacyl-acyl dehydratase from Helicobacter pylori (HpFabZ). Sakuranetin was compared with two other flavonoids that had already been reported as HpFabZ inhibitors, namely quercetin and apigenin ^[106][226]. Sakuranetin exhibited significantly greater inhibitory activity than the other compounds (IC₅₀ in μM: 2.0, 39.3, and 11.0, respectively). Complex crystal structure analysis in combination with kinetic enzyme assays indicated that the compounds of interest act as competitive inhibitors of HpFabZ by binding to the B tunnel entrance of the substrate or by plugging into the C tunnel near the catalytic residues mainly through hydrophobic interactions and hydrogen-bond pattern.

Grecco and colleagues ^[107][227] studied the activity of sakuranetin against promastigotes and amastigotes of Leishmania spp. and trypomastigotes and amastigotes of Trypanosoma cruzi. Sakuranetin was found to be active against L. amazonensis, L. braziliensis, L. major, and L. chagasi (with IC₅₀ between 43 and 52 μg/mL) and against T. cruzi trypomastigotes (IC₅₀ = 20.17 μg/mL). Interestingly, sakuranetin was methylated to produce sakuranetin-4′-methyl ether, which was found to be inactive against both Leishmania spp. and T. cruzi, suggesting that the presence of a hydroxyl group at C-4′ together with a methoxyl group at C-7 is required for the antiparasitic activity. Further drug design studies targeting sakuranetin derivatives could contribute to the development of promising therapeutic agents for leishmaniasis and Chagas’ disease.

In a study conducted by Park and co-workers ^[111][74], phenolic rice phytoalexins were evaluated for their antimicrobial activity against phytopathogenic fungi and bacteria. Inhibition of Bipolaris oryzae (rice brown spot fungus) growth was observed with N-trans-cinnamoyltryptamine. In addition to B. oryzae, sakuranetin was active against Magnaporthe grisea (rice blast fungus) and Rhizoctonia solani (rice sheath blight fungus). Phenylamides (N-trans-cinnamoyltryptamine and N-p-coumaroylserotonin) and sakuranetin induced by UV exposure showed antibacterial activity against rice pathogens for blight (Xanthomonas oryzae pv. oryzae), grain rot (Burkholderia glumae), and leaf streak (X. oryzae pv. oryzicola) diseases.