Tomato fruit cuticular waxes and their effects on transpiration barrier properties: functional characterization of a mutant deficient in a very-long-chain fatty acid  -ketoacyl-CoA synthase

Abstract

Cuticular waxes play a pivotal role in limiting transpirational water loss across the plant surface. The correlation between the chemical composition of the cuticular waxes and their function as a transpiration barrier is still unclear. In the present study, intact tomato fruits (Lycopersicon esculentum) are used, due to their astomatous surface, as a novel integrative approach to investigate this composition- function relationship: wax amounts and compositions of tomato were manipulated before measuring unbiased cuticular transpiration. First, successive mechanical and extractive wax-removal steps allowed the selective modification of epi- and intracuticular wax layers. The epicuticular film consisted exclusively of very-long-chain aliphatics, while the intracuticular compartment contained large quantities of pentacyclic triterpenoids as well. Second, applying reverse genetic techniques, a loss-of-function mutation with a transposon insertion in a very-long-chain fatty acid elongase beta-ketoacyl-CoA synthase was isolated and characterized. Mutant leaf and fruit waxes were deficient in n-alkanes and aldehydes with chain lengths beyond C30, while shorter chains and branched hydrocarbons were not affected. The mutant fruit wax also showed a significant increase in intracuticular triterpenoids. Removal of the epicuticular wax layer, accounting for one-third of the total wax coverage on wild-type fruits, had only moderate effects on transpiration. By contrast, reduction of the intracuticular aliphatics in the mutant to approximately 50% caused a 4-fold increase in permeability. Hence, the main portion of the transpiration barrier is located in the intracuticular wax layer, largely determined by the aliphatic constituents, but modified by the presence of triterpenoids, whereas epicuticular aliphatics play a minor role.

Analyses of Tomato Fruit Brightness Mutants Uncover Both Cutin-Deficient and Cutin-Abundant Mutants and a New Hypomorphic Allele of GDSL Lipase

“…4B). The cutin layer is further covered by epicuticular waxes mostly made up of aliphatic compounds, which form a film at the surface of the fruit (Vogg et al, 2004;Buschhaus and Jetter, 2011). Because of these characteristics, wild-type fruit is moderately glossy.…”

“…6). Likewise, the high proportion of cyclic triterpenoids found in the wax-rich glossy (P4E2) or dull (P6D6) mutants may modify the water loss properties of the fruit (Vogg et al, 2004;Buschhaus and Jetter, 2012).…”

The genome of the stress-tolerant wild tomato species Solanum pennellii

“…15). The alkane abundance in S. pennellii and the higher ratio of alkanes to triterpenoids have previously been suggested as mechanisms for increasing resistance to water flux across the cuticle 9,10 . Additionally, the phenylpropanoid component of S. pennellii cutin was reduced to ~20% of that found in S. lycopersicum.…”

Cracking in sweet cherries: A comprehensive review from a physiological, molecular, and genomic perspective

“…Studies in cherry tomato (Solanum lycopersicum L.) demonstrated that the silencing of lecer6 elongase, involved in the extension of long-chain fatty acids, generated three to eight times more dehydration per unit surface area of fruit than the untransformed plant. These fatty acids are key components for the synthesis of aliphatic compounds like alkanes that are part of the cuticular wax (Vogg et al, 2004). The alkane's quantification for these tomatoes, showed a considerable decrease of the concentration of n-alkanes from 28 to 30 C (Leide et al, 2007).…”

“…Among the multiples genes found, there are wax biosynthesis transcriptional activators and genes involved in cell wall modification. Further evidence of the relationship between fruit cracking and properties and composition of the cuticle is provided by Vogg et al (2004) in tomato, where the mutation of the CER6 gene (β-ketoacyl-CoA synthase), leads to an alteration of the cuticular wax composition and increases water permeability. An interesting work about putative genes involved in the cuticle formation in sweet cherry was recently published by Alkio et al (2012).…”