Patterns of incorporation of D-galactose into cell wall polysaccharide of growing maize roots.

(14)C-1-D-galactose was rapidly taken up by excised corn root-tips and efficiently converted to hexose units in cell wall polysaccharides. The label recovered in both hydrolysed pectin and hemicellulose was predominantly in galactose and only the α-cellulose contained appreciable amounts of labelled glucose. There was no evidence for breakdown of labelled units after incorporation into the cell wall. It is suggested that the utilisation of this free galactose has not appreciably affected the normal metabolic pathway by which galactose is incorporated into plant cell walls.Advantage was taken of the specificity of this labelling to follow patterns of galactosyl incorporation in roots. Autoradiographs were prepared from adjacent longitudinal sections that had been extracted with ammonium oxalate solution and 24% (w/v) KOH respectively. The distribution of silver grains over these sections was compared with that over an unextracted section. Galactosyl units of pectin were incorporated in young cell walls in all tissues investigated. The pattern closely resembled that noted in earlier work for uronosyl and pentosyl incorporation. In pith and cortical cells, galactosyl units of hemicellulose were deposited at a maximum rate in walls approaching the end of their growth when pentose incorporation was low. Because branched alkali-soluble polysaccharides containing galactose and pentose have been isolated from several tissues of corn, similar compounds are likely to exist in the root. It is proposed that the continued elaboration of such a polysaccharide might continue after deposition, and the addition of galactosyl units may be a factor which limits further plastic extension of the wall.