Axial changes in apoplast properties in the elongation zone of maize mesocotyl

Abstract

Changes in extensibility of cell walls and composition of apoplastic solution along the elongation zone were studied in mesocotyls of etiolated seedlings of maize (Zea mays L.). It was found that plastic and elastic extensibility of cell walls was much greater in the cells with a high rate of elongation. Basipetal decrease in hydrogen peroxide concentration in the apoplast (from 5.1 to 2.0 μM) was detected. We determined the activity of cell wall enzymes participating in H2O2 metabolism and found that in basal direction, potential ability of these enzymes to decompose H2O2 rises stronger than the ability to produce it. We found a basipetal decrease in polyamine oxidase activity, an increase in oxalate oxidase activity, and a rise in the ratio between peroxidase and NADH-oxidase activities of peroxidases. IAA (10−6 M) promoted elongation of mesocotyl segments, induced a steady elevation of H2O2 content in the apoplast, an increase in NADH-oxidase activity of peroxidases, and a transient decrease in oxalate oxidase activity. Treatment with ABA (10−4 M) suppressed elongation of mesocotyl segments, induced a transient elevation of H2O2 content in the apoplast, and a decrease in oxalate oxidase activity. It was shown that the main metabolites of apoplastic solution are glucose (20–30 mM), fructose (6–7 mM), malic acid (3 mM), and amino acids, namely, Asp, Glu, Asn, Gln, Ala, Val, Ser, Thr, and Phe. In basal direction, we observed a decrease in the content of glucose (from 30 to 20 mM), inositol (from 0.24 to 0.08 mM), and total amino acids (from 5.5 to 3.3 mM), whereas concentration of orthophosphate (3 mM) and malate (3 mM) did not change significantly. A relationship between the detected changes in the apoplast composition and basipetal decrease in the elongation rate of mesocotyl cells is discussed.