Structure of the Developing Pea Seed Coat and the Post‐phloem Transport Pathway of Nutrients

The seed coat - protector and provider

Many of us regard the main function of the seed coat as the protection of the embryo prior to germination. Of course that view is partially correct but we must not forget the role of the seed coat in seed development. There is considerable evidence from many dicots but especially from the �grain� legumes, such as peas and beans, that the seed coat has a major function in transferring nutrients to the developing seed. This is the subject of the paper by Van Dongen and colleagues from Utrecht and Wageningen (pp. 729-737), working on pea, Pisum sativum. A particularly noteworthy feature of their work is the combination of anatomical and cytological observations with studies of solute movement. For readers unfamiliar with the anatomy of the legume seed coat, its structure is surprisingly complex. The coat is vascularized by the branched chalazal vein and it is from the phloem of this vein that nutrients are unloaded into the parenchyma of the seed coat. The parenchyma actually consists of three layers, the chlorenchyma, the ground parenchyma and the �branched� parenchyma with extensive intercellular spaces. Outside of the parenchyma are the hypodermis and the epidermis. As the embryo grows, the seed�s liquid endosperm is absorbed and the developing cotyledons begin to contact the seed coat parenchyma. In the latter, the branched parenchyma has now collapsed, leaving liquid-filled spaces, and it is the ground parenchyma cells that are closest to the cotyledons. Experiments involving an inert fluorescent tracer molecule, HPTS, clearly revealed its movement from the chalazal phloem into the ground parenchyma and transfer out of those cells to the apoplast from which it was taken up by the cotyledons. Whether the ground parenchyma has specific �efflux cells� is not yet clear; doubtless the authors are already working to answer this question.