THE ACCUMULATION OF ZEIN POLYPEPTIDES AND ZEIN MESSENGER-RNA IN CULTURED ENDOSPERMS OF MAIZE IS MODULATED BY NITROGEN SUPPLY

The effect of nitrogen (N) nutrition upon the accumulation of seed storage protein has been studied in wild-type and opaque-2 mutant (o2) maize endosperms grown in vitro, for 5 days, on a solid medium containing different amounts of N in the form of inorganic or organic compounds. As reference, the accumulation of zein proteins in endosperms grown to maturity in vivo under field conditions was also examined. The developmental stage up to 14-19 days after pollination (DAP) defines, both in vivo and in vitro, a phase of growth during which the major zein classes of 20 and 22 kDa are synthesized both in wild-type and o2 endosperms. In vitro, the corresponding transcripts of the genes coding the 20 and 22 kDa zeins, and of the zein synthesis associated gene b-32, are also present. Under in vitro conditions, the level of expression of the zein synthesizing system, as monitored by zein accumulation, is dependent on the level and form of N supplied in the medium. In vivo, at stages of development from 19 DAP to maturity, the synthesis of the 22 kDa zein class and of the b-32 protein is under control of the O2 transcriptional activator. It is hypothesized that at early stages of endosperm development (between 9 and 14-19 DAP) a 'metabolic control', based on N availability, is operative, possibly based on a regulatory factor different from 02.