The overview and prospect of chemical composition of plant cuticular wax

Plant cuticle covering the aerial surfaces of land plants serves as a hydrophobic barrier.Plant cuticle possesses two layers of waxes: the intracuticular waxes embedded in the cuticle and the epicuticular waxes covering the cuticle.The functions of plant waxes in the cuticle are vital and include regulation of nonstomatal water loss and gas exchange,protection against UV radiation and pathogens,and construction of a microenvironment suitable for certain plants.The chemical composition of plant waxes is complicated and can vary not only from species to species,but also among different parts of the same plant.In the past decades,knowledge on the composition of cuticular waxes from diverse plant species has been accumulated.This review summarizes the current research progression on chemical composition of the plant waxes,discuss problems and foregrounds that exist in the present studies and explore potential research topics of the future.Plant cuticular waxes are complex mixtures of long chain(ranging from 20 to almost 40 carbons) aliphatic and cyclic compounds.There are two wax biosynthetic pathways to synthesize aliphatic wax components,including acyl-reduction pathway,which leads to the formation of primary alcohols and wax esters,and decarbonylation pathway,which gives rise to aldehydes,alkanes,secondary alcohols,and ketones.Aliphatic compounds with unbranched,fully saturated hydrocarbon backbones,including n-alkanes,n-aldehydes,n-alcohols and fatty acids,have been detected in the plant cuticle in most of the plant species studied to date and proportions of these compounds vary in different species.Except for these ubiquitous constituents,some specific wax compounds were also discovered from the cuticular waxes of special plants.The taxonspecific wax constituents contain fully saturated aliphatic chains with 29 or 31 carbons,which usually possess two alcohol or keto functional functional groups,leading to the possibility of positional isomerism,such as secondary alcohols,corresponding ketones,alkanediols and ketols.Most cyclic compounds discovered in plant waxes are triterpenoids,and are at trace levels in most plant species.However,triterpenoids can accumulate to very high concentration in some specific plants,such as Prunus laurocerasus,Vitis vinifera,Tilia tomentosa and so on.Although there are more than 200 basic triterpenoid carbon skeletons detected to date,the most abundant triterpenoid constituents detected in plant waxes are pentacyclic triterpenoids and its derivatives.Moreover,there are some compounds extracted from plant surface,such as phytosterols,alkaloids,palmitic and stearic acids,but whether they are components of cuticular wax mixtures was not confirmed to date.In some plant species,constituents other than waxes,including diterpenoids and flavonoids,are located at or near the plant surface and can be extracted with wax compounds simultaneously.In the end,to promote the understanding of the chemical composition of plant cuticular waxes,there are some problems needed to be solved.First of all,we should improve the wax extraction method to make sure we can extract the different layer waxes quickly and effectively.Secondly,the exact wax composition from different wax layers must be studied to elucidate the biological function of each wax layer.Finally,explore new strategies to identify some special wax compounds that are difficult to be identified due to the lacking of standard samples.