Effect of sucrose and asparagine on the synthesis of storage products in in vitro grown maize endosperms
--D. Bosio, C. Balconi, E. Rizzi, L. Nembrini, A. Morselli and M. Motto

Regulation of nitrogen supply to the developing maize kernel is a subject of great interest since it is responsible for the accumulation of starch protein and corresponding increases in dry weight of the kernel. In addition, the synthesis of storage protein and the potential for cell and/or starch granule formation have been associated, through correlative studies, to the control of starch synthesis. Therefore, understanding the relative importance for these factors and how they interact in controlling endosperm growth could be useful in developing strategies for improving maize productivity.

The aim of this research was to examine the effect of C and N supply on growth, starch, and protein composition of maize endosperms. In vitro culture of maize endosperms, on well defined media, offers a convenient opportunity to study various factors affecting kernel growth and endosperm starch and protein synthesis; this will avoid the complex relationship between the growing seed and the mother plant. Immature endosperms of 26 maize inbred lines (Table 1), differing in starch and protein content in the grain, were collected at 9 days after pollination (DAP) and grown for five days on solid media containing different sucrose to asparagine ratios (Table 2).

Table 1. Inbred lines with high (HP), medium (+) and low (LP) protein content as percent (%) of dry matter, in the mature seed.
 
No. Inbred line % of proteins LP<10.5% HP>13
IHP 25.50 HP
1 Lo5 14.85 HP
2 A69Y 14.50 HP
3 W25 14.42 HP
4 38-11 13.52 HP
5 Lo881 13.48 HP
6 W64A 13.41 HP
7 B14A 13.11 HP
8 Pa83 13.05 HP
9 B14 13.02 HP
10 R193 12.61 + +
11 A637 12.55 + +
12 Lo863 12.18 + +
13 B37 11.52 + +
14 A632 10.61 + +
15 Lo1066 10.40 LP
16 N7B 9.39 LP
17 Lo1069 9.37 LP
18 Lo964 8.93 LP
19 Lo904 8.92 LP
20 Ms213 8.83 LP
21 ND385 8.78 LP
22 N28 8.75 LP
23 Lo1016 8.41 LP
24 K44W 8.24 LP
25 R227 8.10 LP
26 B84 7.61 LP
ILP 3.90 LP

Table 2. Culture media. All media contained 0.4 mg/l thiamine, 100 mg/l inositol, salts as described in Nitsch and Nitsch and 8 g/l agar.
 
Medium Sucrose (g/l) Asparagine (g/l)
1 10 0.02
2 10 4
3 30 0.02
4 30 4

For all the inbred lines tested, it was evident that increased dry weight accumulation by cultivated endosperms and increased sucrose and asparagine concentrations in the media were positively correlated. In addition, from this study it was possible to identify groups of lines differing in the trend of total nitrogen and starch accumulation in the endosperm, during in vitro culture. The data suggest that for some inbred lines the control of synthesis of endosperm proteins and of starch was, at least in part, at the source level rather than at the sink level.

The antagonism between starch and N accumulation, observed in some inbred lines, could be explained by the fact that the highest amount of asparagine supplied was greater than that required for maximum N content in the endosperm; in these conditions the efficiency of N use declined and less starch was deposited.

Our data suggest that, in the maize endosperm, starch and protein accumulation were interdependent and were controlled by carbon and nitrogen nutrient supply. However, a large variability among the inbred lines in the trend of response to the nutrients was evident. 


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