Submitted to: Annals Of Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/20/2004
Publication Date: 4/20/2005
Citation: Wang, H.L., Grusak, M.A. 2005. Structure and development of medicago truncatula pod wall and seed coat. Annals of Botany 95:737-747. Interpretive Summary: Agronomic legumes, such as pea, bean, chickpea, and soybean, are important sources of dietary nutrients. Efforts to improve the nutritional value of these grain crops requires a sound understanding of how sugars, amino acids, and minerals are moved from the leaves, to the legume pod, and on to the developing seeds. We need to understand what cellular pathways in the plant are used to move these nutrients, what other types of cells are present along this pathway, and what genes and proteins might be involved that influence the overall movement of nutrient molecules. One way to advance our knowledge base is to study legumes that are genetically simpler plants, and to extend the understanding learned in these simple plants to the more complex agronomic legumes. One such simple legume that is being developed as a model to study legume biology is the annual plant called Medicago truncatula, or burr medic. Although much genetic information has been learned for this plant, we unfortunately know little about the structure of its pod walls or seed coats; these are important components of the leaf-to-seed nutrient pathway. So, we characterized the structural attributes of Medicago pod walls and seed coats at four stages of reproductive development: three, six, 13 and 20 days after pollination. We showed when various cell types reached maturity, and discussed differences and similarities between Medicago truncatula pods, and the pods of other legumes. We concluded that this plant could serve as a valid model organism to study pod biology in important agronomic legumes.
Technical Abstract: Medicago truncatula has gained much attention as a model species for legume biology, but little is known about the morphology of its reproductive tissues. In order to assess this plant's potential as a model to study reproductive biology in agronomic legumes, the structural characteristics of Medicago truncatula pod walls and seed coats were characterized in developing pods. Plants of Medicago truncatula, ecotype A17, were grown under controlled conditions in a greenhouse. Flowers were date-tagged at anthesis, so that pods of known age could be collected. Harvested pods were fixed and sectioned for light microscopy. Structural attributes of pod walls and seed coats were characterized at four time points throughout early to mid stages of pod development (three, six, 13, and 20 days post-pollination). Basic features of the pod wall are an exocarp comprised of a single epidermal layer, a mesocarp with seven to 14 layers of parenchyma cells, and an endocarp composed of three to five layers of sclerenchyma cells adjacent to an inner epidermal cell layer. Vascular bundles are abundant in the pod wall and include one lateral carpellary bundle, one median carpellary bundle, and nine to 12 vascular bundles, all embedded within the mesocarp parenchyma. General features of the seed coat are an epidermal layer of lignified macrosclereids, a sub-epidermal layer of osteosclereids, and two to five rows of internal parenchyma cells. The hilar region contains the tracheid bar and the chalazal vascular bundle, the latter of which expands to form only two short branches. Medicago truncatula pod walls and seed coats possess several structural similarities to those of agronomic legumes, thus it has strong potential as a valid model for the study of reproductive biology in this important plant family.