|Samac, Deborah - Debby|
Submitted to: Molecular Plant Microbe Interactions
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/6/1999
Publication Date: N/A
Citation: Schoenbeck, M.A., Samac, D.A., Fedorova, M., Gregerson, R.G., Gantt, J.S., Vance, C.P. 1999. The alfalfa (Medicago sativa) TDY1 gene encodes a mitogen-activated protein kinase homolog. Molecular Plant Microbe Interactions. 12(10):882-893. Interpretive Summary: Plants in the bean family, such as alfalfa, are able to form a symbiotic relationship with a soil bacterium that allows them to use nitrogen gas from the atmosphere, in a process called nitrogen fixation, to form amino acids. This process occurs in an organ called a root nodule. Plants lacking this ability rely on nitrogen in the form of nitrate or ammonia that is usually found in only limited amounts in soil and must be added as fertilizer for crop production. The symbiotic bacteria produce a chemical signal that causes alfalfa root cells to begin to divide and form the root nodule. An alfalfa gene encoding a mitogen-activated protein kinase was isolated and its DNA sequenced. The gene appears to be part of a signaling pathway leading to nodule and root formation. The gene is active primarily in dividing cells in the root nodule and root tips. Young nodules and secondary roots were visible before they emerged from the root when roots were stained for gene activity. In mature nodules the gene is active in th dividing cells at the tip of the nodule and in the dividing cells in the water-conducting vessels in nodules, stems, and leaves. The gene was activated upon wounding leaves and infection by a fungal pathogen. The protein encoded by the gene has features unique from previously isolated proteins carrying out a similar function. This is the first time a gene encoding this kind of protein has been isolated from a plant and the first study of this kind of gene in root nodules. The gene appears necessary for nodule formation. Understanding the process of nodule formation will improve our ability to modify the process of biological nitrogen fixation, which would have significant benefits for agriculture by reducing the need for and use of nitrogen fertilizer.
Technical Abstract: Development of root nodules, specifically induction of cortical cell division for nodule initiation, requires expression of specific genes in the host and microsymbiont. A full-length cDNA clone and the corresponding genomic clone encoding a MAP (mitogen-activated protein) kinase homolog were isolated from alfalfa (Medicago sativa). The genomic clone, TDY1, encodes a 68.9 kDa protein with 47.7% identity to MMK4, a previously characterized MAP kinase homolog from alfalfa. TDY1 is unique among the known plant MAP kinases, primarily due to a 230 amino-acid C-terminal domain. The putative activation motif, Thr-Asp-Tyr (TDY), also differs from the previously reported Thr-Glu-Tyr (TEY) motif. TDY1 messages were found predominantly in root nodules, roots, and root tips. Transgenic alfalfa and Medicago truncatula containing a chimeric gene consisting of 1.8-kbp of 5' flanking sequence of the TDY1 gene fused to the b-GUS coding sequence exhibited GUS expression primarily in the nodule parenchyma, meristem, and vascular bundles, root tips, and root vascular bundles. Stem internodes stained intensely in cortical parenchyma, cambial cells, and primary xylem. GUS activity was observed in leaf mesophyll surrounding areas of mechanical wounding and pathogen invasion. The promoter was also active in root tips and apical meristems of transgenic tobacco. Expression patterns suggest a possible role for TDY1 in initiation and development of nodules and roots and in localized responses to wounding.