Do polyamine contribute to plant cell wall assembly by forming amide bonds with pectins?

Two new reducing glycoconjugates [N-D-galacturonoyl-putrescinamide (GalA–Put) and N,N0-di-D-galacturonoyl-putrescinamide (GalA–Put–GalA)] and homogalacturonan–putrescine (GalAn–Put) conjugates were synthesised as model compounds representing possible amide (isopeptide) linkage points between a polyamine and either one or two pectic galacturonate residues. The amide bond(s) were stable to cold acid and alkali (2 M TFA and 0.1 M NaOH at 25 C) but rapidly hydrolysed by these agents at 100 C. The amide bond(s) were resistant to Driselase and to all proteinases tested, although Driselase digested GalAn–Put, releasing fragments such as GalA3–Put– GalA3. To trace the possible formation of GalA–polyamine amide bonds in vivo, we fed Arabidopsis and rose cell-cultures and chickpea internodes with [14C]Put. About 20% of the 14C taken up was released as 14CO2, indicating some catabolism. An additional 73% of the 14C taken up (in Arabidopsis), or 21% (in rose), became ethanol-insoluble, superficially suggestive of polysaccharide–Put covalent bonding. However, much of the ethanol-inextractable 14C was subsequently extractable by acidified phenol or by cold 1 M TFA. The small proportion of radioactive material that stayed insoluble in both phenol and TFA was hydrolysable by Driselase or hot 6M HCl, yielding 14C-oligopeptides and/or amino acids (including Asp, Glu, Gly, Ala and Val); no free 14C-polyamines were released by hot HCl. We conclude that if pectin–polyamine amide bonds are present, they are a very minor component of the cell walls of cultured rose and Arabidopsis cells and chickpea internodes.