STRUCTURE AND DISTRIBUTION OF LIGNIN IN PRIMARY AND SECONDARY CELL-WALLS OF MAIZE COLEOPTILES ANALYZED BY CHEMICAL AND IMMUNOLOGICAL PROBES

Lignin is an integral constituent of the primary cell walls of the dar k-grown maize (Zen mays L.) coleoptile, a juvenile organ that is still in the developmental state of rapid cell extension. Coleoptile lignin was characterized by (i) conversion to lignothiolglycolate derivative , (ii) isolation of polymeric fragments after alkaline hydrolysis, (ii i) reactivity to antibodies against dehydrogenative polymers prepared from monolignols, and (iv) identification of thioacidolysis products t ypical of lignins. Substantial amounts of lignin could be solubilized from the coleoptile cell walls by mild alkali treatments. Thioacidolys is analyses of cell walls from coleoptiles and various mesocotyl tissu es demonstrated the presence of guaiacyl-, syringyl- and (traces of) p -hydroxyphenyl units besides p-coumaric and ferulic acids, There are t issue-specific differences in amount and composition of lignins from d ifferent parts of the maize seedling. Electron-microscopic immunogold labeling of epitopes recognized by a specific anti-guaiacyl/syringyl a ntibody demonstrated the presence of lignin in all cell walls of the 4 -d-old coleoptile. The primary walls of parenchyma and epidermis were more weakly labeled than the secondary wall thickenings of tracheary e lements. No label was found in middle lamellae and cell corners, Ligni n epitopes appeared first in the tracheary elements on day 2 and in th e parenchyma on day 3 after sowing. Incubation of coleoptile segments in H2O2 increased the amount of extractable lignin and the abundance o f lignin epitopes in the parenchyma cell walls, Lignin deposition was temporally and spatially correlated with the appearance of epitopes fo r proline-rich proteins, but not for hydroxyproline-rich proteins, in the cell walls. The lignin content of coleoptiles was increased by irr adiating the seedlings with white or far-red light, correlated with th e inhibition of elongation growth, while growth promotion by auxin had no effect. It is concluded that wall stiffness, and thus extension gr owth, of the coleoptile can be controlled by lignification of the prim ary cell walls. Primary-wall lignin may represent part of an extended polysaccharide-polyphenol network that limits the extensibility of the cell walls.