In all plant studied species, the 13-AOS branch of the LOX pathway is known to produce the well-characterized plant hormones, 12-oxo-phytodienoic acid (12-OPDA) and jasmonic acid (JA), and their derivatives collectively called jasmonates. The 12-OPDA is produced by the sequential action of 13-LOX, 13-AOS, and 13-AOC, which are all localized to plastids. Subsequently, 12-OPDA is transported to peroxisome for further conversion to JA. The 13-AOS pathway also produces 13-ketols, including 9-hydroxy-12-oxo-10(E)-octadecenoic acid (9,12-KOMA), 9-hydroxy-12-oxo-10(E),15(Z)-octadecadienoic acid (9,12-KODA), 13-hydroxy-12-oxo-9(Z)-octadecenoic acid (13,12-KOMA), and 13-hydroxy-12-oxo-9(Z),15(Z)-octadecadienoic acid (13,12-KODA). The 9-AOS pathway produces 10-oxo-11(Z)-phytoenoic acid (10-OPEA), 10-oxo-11(Z),15(Z)-phytodienoic acid (10-OPDA), and their derivatives collectively called “death acids” due to their strong programmed cell death-inducing activity
[17]. Recently, an additional AOS branch of oxylipins, collectively named ‘graminoxins’, was identified in wheat, barley, sorghum, and rice roots
[18], the function of which is currently unknown. Products from the 9-AOS pathway can undergo spontaneous cyclization into 10-OPEA and 10-OPDA, with speculation that a putative 9-AOC exists in this pathway
[17,19][17][19]. The allene oxides produced by the 9-AOS pathway are also converted non-enzymatically to 9-ketols, including 9-hydroxy-10-oxo-12(Z)-octadecenoic acid (9,10-KOMA), 9-hydroxy-10-oxo-12(Z),15(Z)-octadecadienoic acid (9,10-KODA), 13-hydroxy-10-oxo-11(E)-octadecenoic acid (13,10-KOMA), and 13-hydroxy-10-oxo-11(E),15(Z)-octadecadienoic acid (13,10-KODA).
2. Biosynthesis and Occurrence of Ketols
Ketols are C18 compounds that contain a hydroxide group (-OH) present at the 9- or 13-carbon of the fatty acid backbone, a ketone group (C=O) present at the 10- or 12-carbon and ending in a carboxyl functional group. Ketols are designated as ‘α-’ or ‘γ-’ ketols based on the location of the hydroxide functional group in relation to the ketone functional group. If two double-bonds are present, then these ketols are referred to under the abbreviation of ‘KODA’ and are produced from C18:3 substrate, while if a single double-bond is present, then the ketols are referred to as ‘KOMA’ and are produced from the C18:2 substrate
[22]. Ketols are grouped into ‘9-AOS-derived ketols’ (9-ketols for short) or ’13-AOS-derived ketols’ (13-ketols) based on the regiospecific 9- or 13-AOS enzymes that produce them. It should be noted that the following four ketols, 9,12-KODA, 9,12-KOMA, 13,10-KODA, and 13,10-KOMA, are classified as reactive electrophilic species (RES) because they contain an α,ß unsaturated carbonyl group. α,ß-unsaturated carbonyls are known to interact with nucleic acids and proteins to initiate adverse biological effects
[23].
The proposed mechanism for the synthesis of ketols begins with the conversion of 9- or 13-hydroperoxide fatty acid substrates into epoxide intermediates via AOS activity. In this specific example, hydrogen donated from a hydronium (H
3O
+) ion attaches to the oxygen in the epoxide ring, leading to the breakage of the epoxide ring and the formation of a hydroxyl functional group on the 10-carbon position. Subsequent hydrolysis reactions occur to add another hydroxyl functional group to the 13-carbon position, leading to the transfer of electrons through double bonds and the formation of an ‘enol’ product that undergoes tautomerization. In the final step of ketol synthesis, a compound acting as a base attracts the hydrogen attached to the carbonyl functional group on the 10-carbon, leading to the ketol product of 13,10-KODA in this specific example.
In addition to 13-AOS that all characterized plant species possess, many monocot species encode an additional clade of putatively extraplastidic AOS isoforms that possess dual 9/13-AOS activity
[6,24][6][24]. It is likely that these 9/13-AOS isoforms are responsible for the biosynthesis of 9-oxylipins, death acids, and 9-ketols in addition to 13-ketols. Currently, little information is known about the regulation of the synthesis of ketols. It is suggested that ketol production is dependent on JA, as the expression of LOX and AOS genes involved in ketol production is dependent on JA
[6], and disruption of JA biosynthesis in maize results in reduced levels of wound-induced ketols
[25].
The 13-AOS enzymatic branch occurs primarily in plastids where it acts upon hydroperoxides of either C18:3 or C18:2 acids produced by 13-LOXs
[5]. The products of 13-AOS are short-lived epoxide intermediates that are either non-enzymatically hydrolyzed to form α- and γ- ketols or undergo enzymatic cyclization by 13-AOC, the latter pathway leading to the eventual production of JA. Due to C18:3 being the predominant fatty acid found in the glycerolipids present in plastid membranes, a higher percentage of products derived from C18:3 than C18:2 is expected to be produced in these organelles.
Typically, AOSs exhibiting 13-AOS activity are categorized as ‘CYP74A’ enzymes
[26]. It is important to note that cyclization of the allene oxide intermediate synthesized under the 13-AOS pathway is possible with the presence of 13-AOC. It has been recently shown that 13-LOXs, 13-AOS, and 13-AOC form a protein complex to channel substrate specifically for the synthesis of JA
[27]. However, it is not known if the formation of this complex interferes with ketol synthesis through substrate competition.
The majority of tested 9-LOXs and 9-AOSs have been reported to be localized to the cytosol and/or organelles other than the chloroplast
[6,28,29][6][28][29]. To date, all characterized cytosolic AOSs possessing 9-AOS activity display a strong affinity for both 9- and 13-hydroperoxide substrates; thus, they are characterized as dual-specific enzymes
[6,19,23,28,29,30,31,32][6][19][23][28][29][30][31][32]. Typically, AOSs that have this dual-specific activity are grouped within the CYP74C subfamily
[26,28][26][28]. Examples of dual-specific AOS genes include a barley enzyme capable of producing α-ketols from both 9- and 13-hydroperoxides of fatty acids
[23]. The 9/13-AOSs were identified in other monocot species, including maize
[6[6][19],
19], rice
[33], and sugarcane
[34], based on their phylogenetic relationship and ability to produce both 9- and 13-ketols. Some dicot species also contain a similar mixed-function AOS, including tulips
[35], tomatoes
[26], and potatoes
[28].
The best-characterized JA-producing and plastid-localized 13-AOS enzyme is present in both dicot and monocot plant species. Plant species are known to contain different numbers of AOS genes ranging from a single 13-AOS gene in
Arabidopsis to three 13-AOS genes and two 9/13 mixed function AOS genes in maize
[6], to twelve mixed function 9/13-AOS genes in sugarcane
[34]. The mixed function 9/13-AOSs have been reported in fewer dicot species than monocot species. Flax, barley, maize, rice, tulips, petunia, potato, and tomato are among a few of the plant species identified so far that contain a dual-specific 9/13-AOS enzyme and therefore synthesize both 9- and 13-ketols
[6,19,24,28,29,30,31,32][6][19][24][28][29][30][31][32]. Little is known about the evolution of CYP74 enzymes, especially AOS genes. Although
Arabidopsis AOS shares structural similarities to other enzymes in mammals
[14], it is not known whether mammalian systems possess AOS-like enzymes. Interestingly, 9/13-AOS genes are present in lancelets, which are considered to be an evolutionary intermediate between vertebrates and invertebrates
[36]. This recombinant enzyme from the lancelet,
Branchiostoma belcheri Gray, was active towards both 9- and 13-hydroperoxides, producing ketols 13,12-KOMA, 13,12-KODA, 9,10-KOMA, and 9,10-KODA
[36]. Both soft and stony corals also possess AOS enzymes, though it is not reported if these enzymes produce ketols
[37,38][37][38]. Because liverworts, mosses, and green algae contain AOS enzymes
[29,30[29][30][32],
32], it is suggested that the introduction of AOS genes occurred before the evolution of terrestrial plants, and AOS enzymes were likely present in the last common ancestor of plants and animals
[14,34][14][34]. Recently, a metabolic analysis of wounded
Physcomitrium patens Mitt knockout mutants of the PpAOS1 gene uncovered that this enzyme is responsible for the synthesis of two α-ketols
[39]. Available data suggest that dual-specific AOS enzymes localized either to cytosol or chloroplast likely evolved before the evolution of flowering plant species
[14,29,34][14][29][34].
3. Ketols Serve Signaling Roles in Plant–Pathogen Interactions
Several studies implicated ketols in playing a role in defense against biotic stresses. Such a role was first suggested for ketols produced in below-ground organs, as these tissues are characterized by especially high activity of 9/13-AOS enzymes and high levels of ketol production
[26,28,35][26][28][35]. Recent research suggests that ketols are involved in plant–pathogen interactions in above-ground tissues. Exogenous application of 9,10-KODA results in strong induction of expression of several SA-inducible PR genes in tobacco (
Nicotiana tabacum) leaves to the extent similar to the effect of the application of SA, suggesting that 9,10-KODA may be associated with defenses against pathogen infections and systemic acquired resistance (SAR)
[40]. While this study provided strong evidence for the potential relevance of 9,10-KODA to SAR induction, it is unknown if 9,10-KODA acts independently of SA on the induction of SAR marker genes. A further report supported the defensive role of 9,10-KODA as an exogenous application of 9,10-KODA to grape berries suppressed the disease progression by
Glomerella cingulata Stonem
[41]. Recent genetic and pharmacological evidence obtained by the analysis of maize knockout mutants identified 9,10-KODA as a major xylem-mobile long-distance signal required for the activation of induced systemic resistance (ISR) against leaf pathogens triggered by root colonization with the beneficial fungal symbiont
Trichoderma virens [42], suggesting a signaling role for this molecule. Moreover, several γ-ketols were subsequently shown as additional ISR priming agents induced transiently in leaves of maize in response to
T. virens-triggered ISR
[22]. Importantly, these studies indicated that both 9- and 13-ketols serve as important signals for the induction of ISR in maize
[22,42][22][42]. Additionally, several ketols, including 9,10-KODA, 9,12-KODA, 13,12-KODA, and 13,12-KOMA, were upregulated in
Colletotrichum graminicola-infected maize plants that were treated with pentyl leaf volatiles (PLVs); suggesting a positive correlation with PLV-mediated pathogen resistance
[43]. The 9,10-KODA and other 9-LOX products were highly induced upon infection of maize stems by the hemibiotrophic pathogen
Fusarium graminearum Petch, the causal agent of Gibberella Stalk Rot (GSR)
[44]. Significantly higher accumulation of this ketol was observed in the maize line resistant to GSR, whereas
Zmlox5 mutants disrupted in 9,10-KODA production displayed increased susceptibility to GSR
[44].