DEPOSITION OF GLYCINE-RICH STRUCTURAL PROTEIN IN XYLEM CELL-WALLS OF FRENCH BEAN SEEDLINGS IS INDEPENDENT OF LIGNIFICATION

Lignin biosynthesis was inhibited in young bean seedlings by 2-aminoindan-2-phosphonic acid (AIP). AIP is a specific and potent inhibitor of phenylalanine ammonialyase, an enzyme involved in lignin biosynthesis. At a concentration of 100 μM AIP in the growth medium, no lignin could be detected in roots and hypocotyls of 7- or 9-day-old seedlings when stained with phloroglucinol/HCl. At an AIP concentration of 70 μM only a very weak lignification was observed, whereas at 30 μM, no inhibition of lignification was detectable. Glycine-rich protein GRP 1.8, a cell wall protein present in protoxylem of beans, was studied by immunocytochemistry in hypocotyls grown in the presence of 100 μM AIP. No difference of the GRP deposition pattern at sites of normally lignified secondary cell wall thickenings, as well as along the protoxylem vessels, was found in unlignified tissue when compared to controls. The cell-type specific synthesis of GRP 1.8 was not affected by AIP. Thus, deposition of the GRP 1.8 structural cell wall protein is independent of lignification, and lignin does not act as an essential scaffold for correct GRP 1.8 deposition in the complex wall structure of xylem. © 1990.