The Development of Root/Shoot Polarity During Early Embryogeny of Pea (Pisum Sativum cv. “Alaska”)

Abstract

Histological studies showed that day 3 Pisum embryos were undifferentiated in terms of cell lineage and cytology. By day 4, the embryo had undergone rapid development and had a definite root/shoot axis. Little further development had occurred by day 5. A morphometric ultrastructural analysis revealed that changes in cell structure accompanied differentiation between day 3 and day 5. The most striking changes involved cell volume, rough endoplasmic reticulum surface area and the degree of vacuolation. The data showed that day 3 embryonic cells are quite different from day 5 meristematic cells. Autoradiographic studies showed that day 5 and day 7 embryos had a requirement for exogenously-supplied carbon but not for thymidine. The possible significance of the quiescent centre in early embryogeny is discussed. A study of plasma membrane polypeptide composition from four-day dark-grown seedlings was initiated and led to a review of the reliability of the periodic acid, chromic acid, phosphotungstic acid (PACP) stain, reputedly specific for the plasma membrane. PACP was found to stain other cellular components in addition to the plasma membrane including plasmalemmasomes and morphologically-similar cytoplasmic vesicles. These cytoplasmic vesicles were occasionally found in association with vacuoles and a possible role for plasmalemmasomes in plasma membrane turnover is hypothesised. Plasmalemmasomes were serial sectioned to reveal their partial three-dimensional structure. It was not possible to isolate an organ region-specific antibody against total soluble protein from four-day dark-grown seedlings using immunological techniques. Results are discussed in terms of the nature of factors influencing the development of root/shoot during early embryogeny of pea and suggestions for further research are proposed.