Differentiation of Leaf Primordium in Maize Regulated by Exogenous Cytokinin

Abstract

The 12-day immature embryos of 2 maize(Zea mays L.) inbred lines with alternative phyllotaxy,8701d and H4d,were cultured on media with different contents of 6-BA for 14 d to produce calluses,which were further subcultured twice on the same media.Type II calli were established on the differentiation mediums with 0–3 mg L-1 of 6-BA content ladder to induce sprout formation.Sprouts in shoot tips were peeled off under a dissecting microscope and the images of the phyllotactic patterns of leaf primordium were taken by scanning electron microscope.The contents of 6-benzylaminopurine(6-BA),zeatin riboside group(ZRs),dehydro zeatin riboside group(DHZRs),isopentenyladenosine group(iPAs),napthalene acetic acid(NAA),and in-dole-3-acetic acid(IAA) in shoot apical meristem were measured by ELISA method.Three phyllotactic patterns,namely opposite,misshapen,and alternate were observed in the leaf primordium.In H4d plants,exogenous application of the cytokinin 6-BA in-duced a change in the phyllotaxy of shoot apical meristem(SAM) from alternate to opposite and misshapen.Higher 6-BA content and earlier treatment led to higher proportions of SAM with misshapen leaf primordia and opposite leaf primordia.The cyto-kinin/auxin(CTK/AUX) ratio in SAM with misshapen leaf primordia was higher than that with decussate leaf primordia,which in turn,was higher than that with alternate leaf primordia.The accumulation of cytokinin 6-BA increased the levels of cytokinin in SAM,but did not change the levels of endogenous cytokinins(ZRs,DHZRs,and iPAs).The comeback of CTK/AUX ratio in tissue cultured immature seedlings could induce the leaf primordium of them to come back to the original phyllotactic patterns.The CTK/AUX ratio is the determinative factor that regulates the differentiation of maize leaf primordium.