Further Evidence for an Elongation-Decarboxylation Mechanism in the Biosynthesis of Paraffins in Leaves

Abstract

Abstract. In isolated tobacco leaves L-valine-U-14C gave rise to labeled even-numbered isobranched fatbty acids containing 16 to 26 carbon atoms and iso C29, iso C31, and iso C3, paraffins. L-Isoleucine-U-14C on the other hand produced labeled odd-numbered anteiso C17 to C97 fatty acids and anteiso C30 and C32 paraffins. Trichloroace'tic acid inhibited the incorporation of isobutyrate into C90 and higher fatty acids and paraffins without affecting the synthesis of the Cl. and C,, fatty acids. Thus the very long branched fatty acids are biosynthetically related to the paraffins. In Senecio odoris leaves acetate-1_14C was incorporated into the paraffins (mainly n-C,1 ) only in the epidermis although acetate was readily incorporated into fatty acids in the mesophyll tissue. Similarly only the epidermal tissue incorporated acetate into fatty acids longer than C18 suggesting that the epidermis is the site of synthesis of both paraffins and the very long fatty acids. In broccoli leaves n-C., acid labeled with 14C in the carboxyl carbon and 3H in the methylene carbons was incorporated into C, paraffin without the loss of 14C relative to 3H. Since n-C18 acid is known to be incorporated into the paraffin without loss of carboxyl carbon these results suggest that the condensation of C32 acid with CV, acid is not responsible for n-C29 paraffin synthesis in this tissue. Thus all the experimental evidence thus far obtained strongly suggests that elongation of fatty acids followed by decarboxylation is the most likely pathway for paraffin biosynthesis in leaves.The epidermis of plants is covered with a thin l.ayer of waxy !chemicals. The chemistry of this suirface wax 'ha.s been reviewed thoroughily in the past few years (2, 3, 4). The 2 most common componen-ts of the surface wax are waxy esters and hydrocarbons. The waxy esters are synithesized in the leaf by the 'esteri.ficaition of aliphatic prima.ry alcohols with fatty acyl moieties from fatty acyl coenzvme A, pho-spholliipid, o,r free fatty acid (12).The stra,ight chain 'hydrocarbons in planits originate from acetate (9). Longer fatty acicds suich as C, ,, and C,8 are inco,riporated into the nt-C..9 hydrocarbon of Brassica oleracea more efficientlv and wiithoutt prio,r degradation of their carboni chains (10). TII thi.s tissule C,, and C,8 fatty acidls are also incorporaited inito very long fatty ac,ids (C.,,)-C.,,). The effects o,f light, 3-(4-,chl'orophenl)l)-1,1-climethyl uirea (CMU), and trichilo,roacetate oni the incorporation of laibeled prectirsors 'into paraffins and 'very long fatty acid's ssuggested thait ni-paraffins are synthesized by elongation of a common fattv acid (C1,,,) followed by decarboxylaition as shown in Scheme I (11). Althouigh this hypothesis was derived from work on the biosyntbhesis of it-C.,9 paraffin in B. oleraceo, it seemed likely that stuch a hypoithesis would be of wider application. In order to tesft suich a poss bility Sencecio odoris, a plant whiich has priniarily it-C: l paraffin and tobacco wihich has considerabl'e prolport:onis of ...

Thecer-cqu region in barley: Gene cluster or multifunctional gene

“…Although considerably less is known about elongation beyond lengths of 16 carbons, as must occur in wax biosynthesis, presumably an analogous set of enzyme reactions is required at each elongation step. Epicuticular wax biosynthesis which is carried out by epidermal cells (2, 14) consists not only of the elongation steps but also of the associated pathways giving rise to the various lipid classes. Thereafter the wax is deposited on the cuticle surfaces.…”

Transcriptomic and Reverse Genetic Analysesof Branched-Chain Fatty Acid and Acyl Sugar Production inSolanum pennelliiandNicotiana benthamiana

“…Similar studies indicate a-KAE for short branchedchain production in tobacco and Petunia acyl sugars (Kroumova and Wagner 2003). FAS elongation appears to be employed for branched-chain wax components in tobacco species, however (Kaneda, 1967;Kolattukudy, 1968;Kroumova and Wagner, 1999).…”

“…In primary metabolism, these steps are associated with Leu synthesis, but here choice of substrate is evidently wider (Kroumova and Wagner, 2003;Field et al, 2006). Interestingly, iso-and anteiso-branched chains are also significant components of wax alkanes, epicuticular esterified alcohols, and fatty acids in tobacco species, but according to isotope-labeling studies, the latter are generated by FAS-mediated elongation of branched-chain precursors rather than a-KAE (Kaneda, 1967;Kolattukudy, 1968;Kroumova and Wagner, 1999).…”

Effect of inhibitors on synthesis of fatty acyl chains present in waxes on developing maize leaves

“…The esters were identified by co-GLC with barley esters and a C50 standard (synthesized by P. AVATO). The conclusions drawn were confirmed by subjecting the total esters to MS. Transmethylation of the maize esters was not complete in 24 …”

“…The effect of CI3AcONa on wax synthesis has been examined in a number of plant systems (8,21,23,24,26,30,32,38). Initial studies with intact leaves of B. oleracea revealed that Cl3AcONa concentrations up to 5 vmol/leaf inhibited incorporation of acetate into hydrocarbons, secondary alcohols and ketones much more severely than that into primary alcohols, free fatty acids and esters (2 I).…”