The cuticle as a barrier to foliar uptake

The outer leaf surface is covered with a waxy cuticle that waterproofs the leaf and provides the first line of defence between the plant and the environment. Its structure and chemical content are both varied and complex, but the successful passage across it is a vital aspect of herbicide efficacy. Generally, the cuticle is 0.1-13 ^m thick and contains three components: an insoluble cutin matrix, cuticular waxes, and epicuticular waxes (Figures 3.1 and 3.2) . It is not a homogeneous layer and varies greatly from species to species.

Waxes found on the surface of the cutin matrix are termed the epicuticular waxes and have a very diverse structure and composition. They can easily be removed by brief immersion of the leaf in organic solvents and analysis reveals a complex mixture of very long-chain fatty acids (VLCFAs), hydrocarbons, alcohols, aldehydes, ketones, esters, triterpenes, sterols and flavonoids (Table 3.1 and see Holloway, 1993; Post-Beittenmiller, 1996). Alkanes and ketones predominate in leek and brassica leaf epicuticular waxes, but are seldom observed in barley or maize. Similarly, peanuts are enriched in alkanes compared to maize where primary alcohols are abundant (Table 3.2).

Herbicides and Plant Physiology, Second Edition Andrew H. Cobb and John P.H. Reade © 2010 A.H. Cobb and J.P.H. Reade. ISBN: 978-1-405-12935-0

Figure 3.1 The upper leaf surface of fat hen (Chenopodium album L.) as shown by scanning electron microscopy at different magnifications: (a) x540, (b) x5,450 and (c) x11,000 (after Taylor et al., 1981).

When one class of homologue predominates, characteristic crystals of epicuticular wax form, which are very distinctive as rods, granules, crusts or aggregates. These structures may not be uniformly distributed over the whole leaf surface and differences may exist between upper (adaxial) and lower (abaxial) surfaces (e.g. Figure 3.1) , and are often less evident on stomatal guard cells. Their presence often gives the leaf a dull or transparent appearance, while leaves with no epicuticular wax projections appear shiny or glossy.

Cutin is a polyester based on a series of hydroxylated fatty acids, commonly with 16 or 18 carbon atoms, the relative proportion of which varies according to species. Although found in all plants, it is one of the least understood of the major plant

Table 3.1 Most common epicuticular wax components (after Holloway, 1993).

Class

Formula

Range of n

n-alkanes n-alkyl-monoesters n-aldehydes n-1-alkanols n-alkanoic acids

Less common components include:

CH3-(CH2)n-CH3

(often C29 or C31)

C32 C72 C16 C34

(often C26 or C28)

C18 C36

(often C26 or C28)

C14 C36

(often C26 or C28)

n-ketones

(often C29 or C31)

n-sec-alcohols

(often C29 or C31)

CH3-(CH2)n-C-CH2-C-(CH2)n-CH3

C29 C33

polymers. In most cutins the dominant monomer is an œ-hydroxy fatty acid, the self-polymerisation of which will produce a linear polyester chain (Table 3.3). The mid-chain oxygen-containing functional groups (such as hydroxyl and epoxy) may be esterified to other œ-hydroxy fatty acids by polyester synthases, creating a branched structure.

Table 3.2 Variations in epicuticular lipid classes (from Post- Beittenmiller, 1996 and Taylor et al., 1981). Values are percentage of total. Reproduced with permission of Annual Reviews, Inc, via Copyright Clearance Center.

Class

Leek

Barley

Maize

Brassica

Peanut

C. album

Fatty acids

6.4

1O.3

O

1.9

3B.1

O

Aldehydes

1B.O

1.7

2O.O

3.9

2.4

3O.3

Alkanes

31.0

O

1.O

4O.3

35.7

6.6

Sec-alcohols

O

O

O

11.9

O

O

Ketones

51.B

O

O

36.1

O

O

Primary-alcohols

O

B3.O

63.O

1.9

23.B

44.7

Esters

O

4.7

16.0

3.9

O

17.7

Table 3.3 Common cutin monomers, normally C16 and C18, if fatty acids (from Pollard et al., 2008).

Monomer type

Abundance (%)

Unsubstituted fatty acids

- -25

ra-hydroxy fatty acids

- -32

a,ra-dicarboxylic acids

<5

Epoxy fatty acids

0-34

Polyhydroxy fatty acids

- 6-92

Polyhydroxy dicarboxylic acids

Arace

Fatty alcohols

O-B

Glycerol

1-14

Phenolics (ferulic acid)

--1

We still do not know, however, how these components are precisely arranged or how they contribute to cutin function. The outer surface of the cuticle is most lipophilic and becomes more hydrophilic towards the underlying epidermal cells. Various workers have suggested that polar pathways may exist through the cuticle where herbicide movement can take place. Miller (1985) has reported the occurrence of such channels in many plant families but their significance in herbicide uptake remains to be demonstrated. Further transcuticular pathways may be provided by carbohydrate polymers extending into the cuticle from the cell wall (Figure 3.2) , but their role in herbicide movement is again obscure.

The biosynthesis of the cuticular waxes occurs almost exclusively in the epidermal cell cytoplasm. As detailed in Table 3.1, the majority of epicuticular wax components are derived from VLCFAs, primarily 20-32 carbons in length. They are synthesised from C16-C18 plastidic fatty acid precursors by cytoplasmic membrane-bound elongases, using malonyl-CoA as the two-carbon donor. The wide diversity of wax components arises from the operation of three parallel pathways (Figure 3.3).

The recent use of forward and reverse genetic approaches in Arabidopsis has led to the identification of oxidoreductase and acyltransferase genes involved in cutin biosynthesis (as reviewed by Pollard et al., 2008).

Decarbonylation i aldehydes \

odd chain alkanes secondary alcohols

ketones

Fatty acid precursors

Elongated fatty acids (B)

Acyl reductio n

aldehydes primary alcohols plus elongated fatty acids wax esters

P-Ketoacyl elongation i

P-diketones

J^CO2

methyl ketones ,R

alkan-2-ol acyl esters oxoalkan-2-ol acyl esters

Figure 3.3 An overview of the three primary pathways of epicuticular wax biosynthesis (from Post-Beittenmiller, 1996). Reproduced with permission of American Institute of Biological Sciences via Copyright Clearance Center.

There appear to be three enzyme families involved:

• Fatty acid oxidases of the CYP86A sub-family

• An acyl-activating enzyme of the long-chain acyl-CoA synthase (LACS) family

• Acyltransferases of the glycerol-3-P acyl-CoA sn-1 acyltransferase family (GPAT)

These reactions of acyl activation, m-oxidation of acyl chains and acyl transfer to glycerol could take place in several sequences and pathways, although the m-oxidised-acylglycerols are considered to be the polyester building block.

These enzymes are thought to be located in the endoplasmic reticulum, although the cellular site of the polyester synthases is not known. How the waxes find their way to the cuticle remains uncertain. Passage along the polar pores has been suggested, or simple diffusion may occur through spaces in the cell wall. Lipid transfer proteins (LTPs) have now been demonstrated in the epidermal cells of several species and located in the cell wall. It is speculated that these LTPs transport lipids through the endoplasmic reticulum and deposit them outside the cell, although this pathway remains to be proven. LTPs are small (9-10 kDa), basic proteins that are widespread and abundant in plants, constituting as much as 40% of the soluble protein pool in maize seedlings. They consist of 91-95 amino acids differing widely in sequence but always containing 4 disulphide bridges, with a three-dimensional structure that contains an internal hydrophobic cavity. They are able to bind acyl chains and transport them from the endoplasmic reticulum to the cell wall for cutin biosynthesis (Kader, 1997).

Cuticular lipid biosynthesis is very sensitive to environmental conditions and signals such as light intensity, photoperiod, humidity, chilling, soil moisture content and season, which all have an effect on cuticular development and hence herbicide efficacy. In particular, the change from high to low humidity can trigger wax production by more than an order of magnitude, an important factor to consider when extrapolating data on herbicide trials from the glasshouse to the field environment.

Generally, the cuticle will thicken during conditions that are unfavourable to plant growth, including low temperatures, photon flux density and water availability, and so herbicide absorption is maximised when opposite conditions prevail.

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