Submitted to: Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/17/2002
Publication Date: N/A
Interpretive Summary: Developing corn seeds receive nutrients through their pedicels, the neck-like structure that attaches them to the cob. Within the pedicel, the sugars and nitrogen compounds are released from the plants vascular system so that they can be taken up by the seed. Modification of the transport compounds within the pedicel affects final kernel quality. Pedicels are also important ports of entry into the seed for various disease organisms. Previously, we have shown that a unique form of glutamine synthetase (GS), an important enzyme in nitrogen interconversions, is found in abundance within this region. Now, we have cloned the pedicel GS gene and used its promoter - the portion that tells the gene where and when to make its protein - to express a reporter protein, GUS, in corn plants. GUS activity was found in the pedicel and pericarp (skin) tissues, but not in the seed itself, nor in the roots or leaves. This pattern of GUS gene expression is exactly the same as the GS gene's natural expression pattern. Therefore, we now have a genetic tool for expressing disease resistance and other beneficial genes in the pedicel region of the kernels. This offers great potential for improvements in kernel quality in terms of increased disease resistance as well as other agronomic traits.
Technical Abstract: One of the five maize (Zea mays L.) genes for the cytoplasmic forms of glutamine synthetase, namely GS1-2, encodes a unique form of the enzyme that is specifically expressed in the basal maternal tissues of the developing kernel. The gene has now been isolated, partially sequenced and its 5' upstream regulatory region has been characterized by both transient and stable expression in maize tissues. Deletion and linker-scanner experiments revealed a minimal requirement of approximately 40 base pairs upstream of the putative transcription start site to drive transient gene expression. Stable expression of a heterologous gene, consisting of the GS1-2 genomic clone extending from 664 base pairs upstream of the putative transcriptional start site to the middle of exon III, fused in-frame with GUS, resulted in the predicted tissue-specific expression in the basal maternal seed tissues, including the surrounding pericarp. Gene expression within the pedicel parenchyma that subtends the basal endosperm transfer cells and embryo was particularly notable. In contrast, GUS expression was completely absent from the endosperm, embryo, leaves or roots. The GS1-2 expression pattern is consistent with the GS isozyme's role in nitrogen metabolism during grain fill. Potential applications of the promoter in modifying of grain characteristics or increasing disease resistance of the kernel are discussed.