Apoplastic barriers: Their structure and function from a historical perspective

Abstract

The multicellular plant body is a complex structure that is internally organized into organs and tissues specialized for particular functions. The outer boundary of a plant body is delimited by the epidermis, and its specific modifications allow controlled material exchange with the surrounding environment. The inner space of a plant body is subdivided into functional domains. Such division is known to take place in both the symplastic and apoplastic spaces of organs and tissues. The apoplast is composed mostly of intercellular spaces and porous cell walls. It surrounds the symplast, which is bordered by the plasma membrane. The internal subdivision of apoplastic space is carried out by so-called apoplastic barriers, which are cell layers with modified cell walls where lowered porosity decreases the passive flow of solutes, water, gasses, and regulatory molecules. There is a well-established role of the endodermis in the function of the root in vascular plants. However, the root endodermis is not the only “barrier” essential for plant function and development. Similar barriers are known to be present in stems, leaves, and the root periphery. This review focuses on the historical course of our understanding of the development, structure, and function of these protective layers.