Growth and Composition of Maize Kernels Cultured in Vitro with Varying Supplies of Carbon and Nitrogen

Abstract

This study employed in vitro seed culture to determine how C and N supply influence the growth (i.e. starch accumulation) and protein composition of maize (Zea mays L.) endosperm. Immature kemels were grown to maturity on liquid medium containing various concentrations of C (sucrose at 234 millimolar [low] and 468 millimolar [high]) and N (amino acid mixture ranging in N from 0 to 144 millimolar). Low C supply limited starch, but not N, accumulation in the endosperm. With high C, endosperm starch and protein content increased concomitantly as N supply increased from 0 to 13.4 millimolar. Endosperm growth was unaffected by additional N until concentrations exceeding approximately 72 millimolar reduced starch accumulation. A similar inhibition of starch deposition occurred with lower N concentrations when kernels were grown with low C. Endosperm total N content reached a point of saturation with approximately 36 millimolar N in the medium, regardless of C supply. Zein synthesis in the endosperm responded positively across all N levels, while glutelin content remained static and albumin/globulin proteins were reduced in amount when N supply was greater than 36 millimolar. A reciprocal, inverse relationship was observed in mature endosperm tissue between the concentrations of free amino acids and soluble sugars. Our data suggest that under N stress starch and protein accumulation in the endosperm are interdependent, at least in appearance, but are independent otherwise.Maize endosperm comprises approximately 83% of the mature kernel and is composed of about 88% starch and 8% protein (8). Because starch and protein deposition in the endosperm constitute the bulk of kernel growth and represent the majority of kernel dry weight and N accumulation, respectively, these terms are often used interchangeably (i.e. starch for dry weight and protein for N) in this paper. Learning how processes of starch and protein deposition are controlled in the endosperm is important to understanding factors which affect grain development. The supply of C and N substrates provided to the seed by vegetative portions of the plant is one obvious factor which can affect the synthesis of these storage products. Regulation may also occur through an interdependency of the starch and protein synthetic pathways. For example, maize kernel dry weight and protein content often increase concomitantly in response to N fertilization (25,28) (27). On the other hand, starch and protein formation are also commonly thought to be negatively related, as grain yield and protein percentage are inversely correlated in cereals (6, 21). This inverse relationship is based on the concentration, rather than the content, ofprotein in the grain. In contrast to theories of coupled regulation between starch and protein formation, source-sink and postanthesis fertilization experiments with maize have shown that the N concentration of the grain can be increased without significantly affecting kernel size (11,22,29). These results suggest that starch and protein dep...

The accumulation of zein polypeptides and zein mRNA in cultured endosperms of maize is modulated by nitrogen supply

“…Dry matter and storage proteins are accumulated, suggesting that in the endosperm cells the nOmxil developmental processes continue after isolation from the plant, if sucrose and a source of N are supplied. The finding is in good agreement with previous observations indicating that in vitro cultures of maize kernels and endosperm cells allow the study of grain growth and protein synthesis during development (Balconi eta/., 1991 ;Lyznick and Tsai, 1989;Manzocchi, 1991 ;Singletary and Below. 1989;Singletary etal., 1990).…”

“…Available evidence indicates that the coordinate expression of zein gene classes occurs rnostty at the levd of transcription (reviewed in Motto eta/., 1989). In addition, recent data (Lyznick and Tsai, 1989;Singletary and Below, 1989 The results reported here indicate that wild-type and 02 endosperms of 9 DAP continue to develop for at least 5 days when cultured on simple defined media. Dry matter and storage proteins are accumulated, suggesting that in the endosperm cells the nOmxil developmental processes continue after isolation from the plant, if sucrose and a source of N are supplied.…”

“…Circumstantial evidence has been in this sense obtained by nutrient manipulation in field studies ( l s a i eta/., 1978), long-term kernel cultures (Singletary and Below, 1989;Singletary et al, 1990), and endosperm cell cultures (Balconi et a/., 1991;Lyznick and Tsai, 1989).…”

Response of Enzymes and Storage Proteins of Maize Endosperm to Nitrogen Supply

“…Exemplifying this relationship, it has been demonstrated numerous times that kernel weights increase when crops of maize are supplemented with increasing levels of N fertilizer (3,7). Recently, with the aid of in vitro techniques, it was demonstrated that starch and N accumulation in the endosperm of maize kernels increased as the concentration of N available to the kernel increased (23). Similar studies have shown corresponding responses for wheat (4) and barley (14).…”

“…From a similar standpoint, it is well known that the accumulation of zeins, the primary storage proteins of maize kernels, responds dynamically to N nutrition (23,26). Because the amino acid composition of the zein fraction does not change significantly as a function of N accumulation in the kernel, it would seem that different zeins are always produced in equally proportional amounts (16).…”