Uptake and utilization of nutrients by developing kernels of Zea mays L.

Leszek Aleksander Lyznik, Purdue University

Abstract

The mechanisms involved in amino acid and sugar uptake by developing maize kernels were investigated. In the pedicel region of maize kernel, the site of nutrient unloading from phloem terminals, amino acids are accumulated in considerable amounts and undergo significant interconversion. A wide spectrum of enzymatic activities involved in the metabolism of amino acids is observed in these tissues. Subsequently, amino acids are taken up by the endosperm tissue in processes which require energy and the presence of carrier proteins. Conversely, no evidence was found that energy and carriers are involved in sugar uptake. This process of sugar uptake is not inhibited by metabolic inhibitors and shows nonsaturable kinetics, but the uptake is pH-dependent. L-glucose is taken up at a significantly reduced rate in comparison to D-glucose uptake. Based on analysis of radioactivity distribution among sugar fractions after incubations of kernels with radiolabeled D-glucose, it seems that sucrose is not efficiently resynthesized from D-glucose in the endosperm tissue. Thus, the proposed mechanism of sucrose transport involving sucrose hydrolysis in the pedicel region and subsequent resynthesis in endosperm cells may not be the main pathway. The evidence that transfer cells play an active role in D-glucose transport is presented. To test the effects of amino acids on storage protein synthesis, endosperm cell suspension cultures were used. Storage protein synthesis is temporally coordinated during cell growth cycle and is affected by the kinds of amino acids present in the medium. Like asparagine (standard medium component), glutamine and alanine were good sources of nitrogen in sustaining cell growth and protein synthesis indicating a high potential of de novo amino acid synthesis by these cells. However, methionine was not a good nitrogen source. This amino acid inhibited protein accumulation and induced hydrolysis of zeins. The pattern of zeins accumulated by endosperm cells was different from that of native zeins isolated from maize kernels. Nevertheless, the pattern of zeins translated in vitro from polysomal RNA preparations from these two sources were similar. This suggests that mechanisms involved in synthesis and deposition of zein polypeptides are controlled independently and can be regulated by culture conditions.

Degree

Ph.D.

Advisors

Tsai, Purdue University.

Subject Area

Botany

COinS