AUXIN INDUCES EXOCYTOSIS OF ACID-PHOSPHATASE IN COLEOPTILES FROM ZEA-MAYS

Auxin-stimulated elongation growth of maize coleoptiles has been sugge sted to be associated with enhanced exocytotic activity. However, the problem in plants is one of finding a soluble parameter, which can be used as a direct measure of exocytosis (H. D. Blackbourn and N. H. Bat tey [1993]. Physiol. Plant. 89: 27-32). In yeast, acid phosphatase (EC 3.1.3.2.) is used as a marker for secretory activity (E. Harsay and A . Bretscher [1995]. J. Cell Biol. 131: 297-310). Therefore, extracellu lar acid phosphatase activities in maize tissues were investigated. Co leoptile (7.36 nkat mg(-1)) and mesocotyl (8.9) showed higher specific extracellular acid phosphatase activities than primary leaf (6.0), ro ot (4.9) and root tip (2.7). In coleoptiles extracellular acid phospha tase activity was 6.7% of total homogenate activity (mesocotyls 10.6%) . Auxin (30 mu M IAA) increased the extracellular acid phosphatase act ivity of coleoptiles (146% of control). This effect was tissue-specifi c; extracellular acid phosphatase activity of mesocotyls was not enhan ced by IAA. The stimulating effect of auxin on extracellular acid phos phatase activity in coleoptiles was reversed by the protonophore niger icin (0.3 mu M) Furthermore, localization of an acid phosphatase activ ity in Golgi vesicles was shown by co-migration of the Golgi marker la tent IDPase (EC 3.6.1.6) and acid phosphatase activity (65% of total m icrosomal activity) on isopycnic continuous sucrose density gradients. Tonoplast-enriched membrane fractions (24% of microsomal acid phospha tase) and plasma membrane-enriched fractions (11%) contained lower amo unts of acid phosphatase. The data presented suggest that acid phospha tase activity is a useful marker for hormone-induced secretory activit y in plant cells.