ROLE OF AUXIN IN MAIZE ENDOSPERM DEVELOPMENT - TIMING OF NUCLEAR-DNA ENDOREDUPLICATION, ZEIN EXPRESSION, AND CYTOKININ

The timing of developmental events and regulatory roles of auxin were examined in maize (Zea mays L.) endosperms. Zeatin, zeatin riboside, a nd indole-3-acetic acid (IAA) levels were determined by enzyme-linked immunosorbent assay (ELISA). Zeatin and zeatin riboside increased to m aximal concentrations at an early stage (9 d after pollination [DAP]), corresponding to the stage when cell division rate was maximal. In co ntrast, IAA concentration was low at 9 DAP and abruptly increased from 9 to 1 1 DAP, thus creating a sharp decline in the cytokinin to auxin ratio. Coincident with the increase in IAA was an increase in DNA con tent per nucleus, attributed to postmitotic DNA replication via endore duplication. Exogenous application of 2,4-dichlorophenoxyacetic acid ( 2,4-D) at 5 or 7 DAP hastened the time course of DNA accumulation per nucleus and increased the average nuclear diameter, whereas 2-(para-ch lorophenoxy)isobutyric acid delayed such development. Exogenously appl ied 2,4-D hastened the accumulation of the zein polypeptides of appare nt molecular masses of 12, 14, and 16 kD and the expression of mRNA hy bridizing with a zein DNA probe. We conclude that an abrupt increase i n auxin induces cellular differentiation events in endosperm, includin g endoredupliction and expression of particular zein storage proteins.