Phenoxyacetic acid residue incorporation in cell walls of soybean (Glycine max.).

The metabolism of [(14)C] phenoxyacetic acid (POA) and the formation of bound residues were studied in soybean leaves and stems. POA was metabolized to 4-HO-POA and to 4-HO-POA glucoside, and a significant fraction of the radioactivity was incorporated in the cell walls (CW). An extraction procedure of CW polymers was developed to specifically isolate the radioactivity associated with each of them. In leaves, the radioactivity showed a preferential distribution into the hemicelluloses and lignins, while pectins and lignins were the most radioactive CW polymers in stems. The identified bound metabolites were 4-HO-POA, POA, and phenolic residues. The latter and POA were essentially incorporated into the lignin fractions and were linked to the benzylic carbons of lignin monomers. 4-HO-POA, the major bound residue, was more evenly distributed in CW polymers. It was esterified to noncellulosic polysaccharides and lignins, but in the latter, contrary to other POA residues, it was mainly linked at the gamma-carbon of propanoid side-chains of lignin monomers. That type of linkage suggested an enzymatic incorporation of 4-HO-POA in CW, contrary to others residues which have an opportunistic lignin incorporation. That incorporation of 4-HO-POA in CW polymers looks like that of endogenous hydroxycinnamic acids.