A Proposed Role of Zein and Glutelin as N Sinks in Maize

Abstract

Zea mays grown with high levels of N fertilizer transports more sucrose into kernels than with low N. Sucrose translocation was greatest in genotypes with the highest capacity to deposit nitrogenous compounds as zein and glutelin in the kernel. These two proteins combined contain about 80% of the total N in the kernel and about 60% of the total N in the plant at maturity. They appear to serve as a functional N sink for the deposition of nitrogenous compounds. As the N sink capacity increases with additional available N fertilizer, more sucrose is transported into the kernel, resulting in increased kernel weight and grain yield. Zein functions as a more dynamic N sink than glutelin because the synthesis of zein is readily manipulated by N fertilization and genetic means. Increases in N deposition in the normal endosperm induced by N fertilizer are confined primarily to zein. Early termination of zein accumulation in the opaque-2 mutant results in a reduction of sucrose movement into kernels. By using plants heterozygous for normal and opaque-2 in these studies, interplant variability was eliminated and the hypothesis relating the kernel N sink capacity to productivity was strengthened.Our previous studies (21) have suggested that zein and glutelin serve as a N sink in maize (Zea mays L.) kernels to regulate the movement of photosynthates into kernels. Although both ammonium and nitrate ions can be taken up by maize roots, the assimilation of ammonia and its subsequent organic N interconversions require readily available organic acids, e.g. a-ketoglutaric acid (12), which are derived from sucrose. In response to high concentrations of ammonia, a greater amount of sucrose is translocated from leaves to provide energy and essential carbon skeletons for ammonia assimilation and organic N interconversions (25). Concurrently, the movement of sucrose to N-rich tissues may enhance CO2 fixation in leaves (13). The increase in photosynthetic efficiency, and the translocation of nitrogenous compounds and sucrose into the kernel sink for photosynthates, should further promote the synthesis of starch and thereby increase yield. Since nitrate, unlike ammonia, may accumulate without assimilation (18) be made available for assimilation (8). It seems that the effectiveness of the ammonium ion in enhancing sucrose movement is facilitated by the rapid assimilation of ammonia by roots and the resultant deposition of amides and amino acids in some tissue. These nitrogenous compounds are stored temporarily in the stalk and leaves during the period of vegetative growth. About 60% of the final N in maize kernels is present in the vegetative tissues at pollination; the remaining 40% is obtained from the soil subsequently (7). As the endosperm develops, the proteins in vegetative tissues are turned over, and the amino acids and newly assimilated N are transported into the kernel (2, 6). We propose that zein and glutelin function as N sinks for the deposition of these nitrogenous compounds to facilitate the movement of photosynthat...

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

“…5) were also similar to compositional modifications produced in kernels of plants from N fertility experiments (19,20,26). The greatest response to N supply occurred with zein, while relative increases in albumin/globulin and glutelin fractions were significant, but smaller (Fig.…”

“…This independence does not preclude a coupled regulation between starch and protein formation when N limits starch synthesis, but it does suggest that such regulatory mechanisms may not exist. For example, in this study starch and zein synthesis responded differently to alleviation of N stress, as indicated by the fact that zein is the primary N component of the endosperm (26) and the N use efficiency of starch production declined with increased N content of the endosperm (Fig. 3).…”

“…While the dilemma of identifying these factors remains, these in vitro studies help to qualify some of the proposals previously put forth. Tsai et al (26) hypothesized that accumulation of dry weight in kernels of maize is closely dependent upon protein, in particular zein, deposition in the endosperm. Under in vitro conditions, a distinct disconnection in maize starch and zein accumulation occurred when N in the medium exceeded that needed for optimum starch content (Figs.…”

“…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) (26) have proposed that the synthesis of zein, the primary storage protein of maize endosperm, may act as a determining factor in regulating kernel dry matter accumulation. Their contention is supported by the fact that reduced levels of zein and starch occur, respectively, in starch-and zein-deficient mutants of maize (27).…”

C and N Mobilization from Stalk and Leaves during Kernel Filling by 13C and 15N Tracing in Zea mays L.

“…However, translocation ofC and N have been often considered independently (11,13), even though metabolism of C and N are closely linked (3). Sucrose transfer from vegetative parts to the ear is linked to zein accumulation in maize kernels (21). All these data have been obtained from net variation in C and N content of the various organs.…”