Structure and development of Medicago truncatula pod wall and seed coat

Medicago truncatula has gained much attention as a genomic model species for legume biology, but little is known about the morphology of its pods and seeds. Structural and developmental characteristics of M. truncatula pod walls and seed coats are presented.

Plants of M. 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 (3, 6, 13 and 20 d 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 an inner epidermal cell layer and three to five layers of sclerenchyma cells adjacent to it. 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. Seed coat features include an epidermal layer of 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.

This characterization provides a needed understanding of pod structure and development in this model legume, and should facilitate various molecular investigations into legume fruit and seed biology.