Oxylipins

Derivatives of fatty acids.

• mono-/dioxygenases add 1 or 2 O

• hydratases add H2O

• other spontaneous chemical reaction (unsaturated fatty acids)

Present in any living organism

Eukaryotic oxylipins are incredibly diverse while prokaryotic oxylipins only originate from one or two metabolic steps.

Horizontal gene transfer via microorganisms (endosymbiontic or pathogenic).

Note that JA is not present in nonvascular landplants (moss, liverworts, etc.). Even though OPDA is, it might perform different functions in different lineages because it seems to have evolved rather independently.

It is not present in angiosperms but in oomycete pathogens. It is used to interfere with the SA and JA signaling pathways.

In nonvascular plants it is present, primarily in plants that do not have JA.

There are incredibly many functions, most of them related to biotic/abiotic stress response or growth/development.

Check Wasternack & Feußner for more detail.

Lipoxygenase (LOX)

(oxidation of cis-diunsaturated fatty acids - hydroxyl/hydroperoxide groups)

Both start from galactolipids.

The 9-LOX pathway takes place in the cytoplasm. It only works with free fatty acids. Initiation is most likely achieved by an acyl-lipid hydrolase / -thiolase (e.g., DAD1 in Arabidopsis).

The 13-LOX pathway takes place in the plastid and peroxysome, the products can also end up in the cytosol and the nucleus. It works with both free and esterified fatty acids. Initiation is most likely achieved by a specific LOX.

The two pathways have different products.

The initiation topic is not fully understood.

LOX13 can oxygenate complex lipids (or esterified lipids), LOX9 cannot. The reason is most likely the shape of the active site. Substrate fatty acids enter LOX13 tail-first, LOX9 head-first.

allene oxide synthase (AOS)

• forms unstable epoxides

hydroperoxyide lyase/isomerase (HPL)

divinyl ether synthase (DES)

The AOS branch and the HPL branch compete for substrate - it is thought that AOS activity results in a direct defense response via JA synthesis while the HPL product hexenyl acetate mediates indirect response (attracting natural enemies of the attacking pathogens)

They all work via disproportionation so that they do not require molecular oxygen or cytochrome P450 reductases.

They are all very similar to one another and can sometimes be converted into one another by a few amino acid exchanges. The AOS from Arabidopsis also shows HPL activity.

The enzyme family also exists in nonvascular plants.

AOC: allene oxide cyclase

The AOS leads to the formation of instable epoxides which spontaneously hydrolize to alpha- and gamma-ketols.

AOC leads to the exclusive formation of cis-OPDA, which can then be further processed to jasmonate (in angiosperms).

The length of the substrate binding cavity determines the substrate specificity (in terms of chain length). This has been shown in P. patens (AOC1 and 2).

OPR3 (12-Oxo-Phosphodienoic Acid Reductase 3) converts cis-OPDA to OPC8.

It is probably deactivated by dimerization.

It has a second function in root growth under phosphate deficiency (transcriptional level).

• Substrate availability (release of linoleic acid from the plastidial membrane)

• short half-life of the COI proteins

• enhancement of JA-Ile degradation

• positive feedback loop

• tissue specificity

Regulatory means (selection):

• posttranslational modifications (the response to wounding is superfast, JA-levels in unwounded leaves are low even if genes are expressed)

• JAZ repressors

• heterodimerization of AOC in Arabidopsis

JAR1. It is a GH3 enzyme! AtGH3.11

It is part of the ANL superfamily and works like this:

Formation of an adenylated intermediate (-AMP) under release of pyrophosphate, transfer on the amino acid

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