Lignin composition and structure differs between xylem, phloem and phellem in Quercus suber

Thecompositionandstructureofligninindifferenttissues—phellem(cork),phloemandxylem(wood)—ofQuercussuberwasstudied.Wholecellwallsandtheirrespectiveisolatedmilledligninswereanalyzedbypyrolysiscoupledwithgaschromatography/massspectrometry(Py-GC/MS),two-dimensionalnuclearmagneticresonancespectroscopy(2D-NMR)andderivatizationfollowedbyreductivecleavage(DFRC).Differenttissuespresentedvariedp-hydroxyphenyl:guaiacyl:syringyl(H:G:S)lignincompositions.WhereasligninfromcorkhasaG-richlignin(H:G:Smolarratio2:85:13),ligninfromphloempresentsmoreS-units(H:G:Smolarratioof1:58:41)andligninfromxylemisslightlyenrichedinS-lignin(H:G:Smolarratio1:45:55).Thesedifferenceswerereflectedintherelativeabundancesofthedifferentinterunitlinkages.Alkyl-arylethers(b–O–4′)werepredominant,increasingfrom68%incork,to71%inphloemand77%inxylem,asconsequenceoftheenrichmentinS-ligninunits.Corkligninwasenrichedincondensedstructuressuchasphenylcoumarans(b-5′,20%),dibenzodioxocins(5–5′,5%),ascorrespondstoaligninenrichedinG-units.Incomparison,ligninfromphloemandxylempresentedlowerlevelsofcondensedlinkages.Theligninfromcorkwashighlyacetylatedattheg-OHoftheside-chain(48%ligninacetylation),predominantlyoverG-units;whiletheligninsfromphloemandxylemwerebarelyacetylatedandthisoccurredmainlyoverS-units.Theseresultsareafirsttimeoverviewoftheligninstructureinxylem,phloem(generatedbycambium),andincork(generatedbyphellogen),inagreementwithliteraturethatreportsthatligninbiosynthesisisflexibleandcellspecific