Making the Surface of Fleshy Fruit: Biosynthesis, Assembly and Role of the Cuticular Layer - Fruit Ripening: Physiology, Signalling and Genomics - page 86

Epicuticular
waxes

Cuticle

Polymerization
(enzymatically via GDSL)

Cutin

LTP and ABC transporters

Cell wall

Plasma
membrane

Golgi?
Secretion vesicle?
Carrier protein?
ER?

Transport

HO

O

HO OH

O

OH

2-mono(10,16-dihydroxyhexadecanoyl)glycerol (2-MHG)

Glycerol esterification (GPAT)

Hydroxylation (CYP450)

Monomer biosynthesis
(in the ER)

Acyl activation (LACS)

O

HO

Free fatty acids

Fig. 6.2. Schematic of proposed routes of cutin monomer and polymer biosynthesis in an epidermal plant
cell. Enzymatic modifi cations of free fatty acids occur in the endoplasmic reticulum (ER) resulting in a
variety of acylglycerols (here represented by 2-mono(10,16-dihydroxyhexadecanoyl)glycerol (2-MHG)).
Important genes in this process include long-chain acyl-CoA synthetases (LACS), cytochrome P450s
(CYP450) and glycerol-3-phosphate-acyltransferases (GPAT). Acylglycerol oligomers are then transported to
the exterior of the cell via ATP-binding cassette (ABC) and lipid-transfer protein (LTP) transporters. It is
possible that secretion vesicles and carrier proteins play additional roles in the extracellular transport of
cutin. Cutin oligomers are further polymerized outside the cell by enzymes including GDSL-motif lipases.

to be responsible for Z -hydroxylase
and mid-chain hydroxylase activity,
respectively. It is not known if the
substrate for the P450 Z -hydroxylases are
acyl-CoAs or acylglycerol moieties or even
the non-acyl activated FFAs. Whilst P450s
have been shown to act on FFAs in vitro ,
this does not exclude their action on acyl-

CoAs and/or acylglycerols. P450s are not
the only enzymes to be able to act on
FFAs, as in the case of LACSs, which have
demonstrated activity on both normal and
Z -hydroxy FFAs. It is too early to assume
that we fully understand the metabolic
pathway(s) leading to Z -oxidized acyl-
glycerols.