Changes in the Activities and Polypeptide Levels of Exo- and Endoglucanases in Cell Walls during Developmental Growth of Zea mays Coleoptiles

Abstract

Exo- and endoglucanases present in cereal coleoptile cell walls are capable of mediating hydrolysis of non-cellulosic /7-(l,3)(l,4)-glucan in situ. To assess the relationship with cell elongation, glucanase activities and the respective polypeptide abundance were determined as a function of Zea mays coleoptile development. Both exo- and endoglucanase activities were quite low initially, but increased to achieve maximum levels by days 5 or 6. Western blots revealed that the density of the protein bands increased with coleoptile development generally in correspondence to activity levels. However, in bioassays with 3 d old coleoptile segments we found that auxin stimulation of glucanase activities did not result from increased glucanase polypeptide levels. Hence, there was no evidence for de novo protein synthesis in excised coleoptiles in response to added auxin. While glucanase antibodies strongly inhibited IAAinduced elongation of coleoptile segments on days 2-4, these same antibodies had little effect on day 1. We conclude that glucanases contribute to auxin mediated coleoptile growth only during a limited developmental interval. We propose that when elongation is dominate, the physical properties of the cell wall adjust in response to metabolism of cell wall /?-(l,3)(l,4)-glucans but the enhancement of such activity is governed by factors other than glucanase protein levels.