Structural Aspects of Protein Accumulation in Developing Pea Cotyledons. II. Three-Dimensional Reconstructions of Vacuoles and Protein Bodies From Serial Sections

Abstract

The three-dimensional structure of vacuoles and protein bodies seen in developing cotyledons from pea (Pisum sativum L.) have been reconstructed from serial sections. At days 12 and 15 after flowering, serial sections 1 �m thick of epoxy-embedded seed tissue were used to determine vacuole morphology while, at day 20, serial sections 0.25 �m thick were examined by electron microscopy to ascertain protein body morphology. At day 12 there are one or two large vacuoles having extremely complex protrusions emanating from a larger central vacuolar volume. This gives rise to up to 20 apparently discrete vacuole profiles in a given section through a cell. By day 15, there are many smaller, approximately spherical, vacuoles and also some that are more complex. At day 20 most protein bodies are discrete, spherical structures, although a few irregularly shaped bodies are seen. The results support the concept of a large highly convoluted central vacuole fragmenting to give rise to the protein bodies seen towards seed maturity.

Assembly of storage protein oligomers in the endoplasmic reticulum and processing of the polypeptides in the protein bodies of developing pea cotyledons

“…It is worth noting in this context that protein bodies are part of the vacuolar/lysosomal compartment of plant cells. Recent evidence shows that protein bodies are derived from vacuoles during seed maturation (5,6,27) and that they contain numerous acid hydrolases (14,17,20,26). Protein bodies therefore have an enzymic complement and function similar to that of the central vacuole of plant cells or the lysosomes of animal cells.…”

Role of posttranslational cleavage in glycinin assembly.

“…Our findings may also have relevance for the development of seed protein body organelles. Craig et al (1980) have shown that large vacuoles of developing pea (P. sativum) cotyledon parenchyma cells fragment to give rise to the small protein bodies found in mature seeds. Prior to this fragmentation, seed storage proteins accumulate in the vacuoles in an aggregated form at the periphery of the vacuole and are associated with tonoplast membranes (Craig et al, 1979 …”

Sorting of proteins to vacuoles in plant cells

“…They may store proteins; vacuolar storage proteins are most prominent in seeds but may also occur in many different vegetative tissues [27,31,32,106]. Previous concepts of vacuole biogenesis and function hypothesized that protein storage vacuoles represented portions of the central vacuole that subdivided as deposits of storage protein accumulated [17,115]. It is now clear, however, that vacuoles with proteases active at low pH (so-called lytic or vegetative vacuoles) and protein storage vacuoles are structurally and functionally distinct organelles [44,86].…”