How Do Real Roots Work? (Some New Views of Root Structure).

Abstract

Root systems of plants growing in the field are marvelously successful at foraging for nutrients and water in a hostile, competitive environment where supplies of them are very limited, local, and variable. This success is not surprising because this is the environment in which they have evolved. We are uncovering increasing complexities of this life of roots in the soil, such as their interactions with a broad range of microbes well beyond the better-known relationships with mycorrhizal fungi and root-nodule bacteria. More is being discovered about the two-way interchange of materials and messages across the root-soil interface. Growth movements, the groping for nutrients and water, have intrigued plant biologists since they were so well described by Darwin, and they continue to be puzzles that become more complex as they are better understood. Additionally, the variety and success of responses of roots to environmental stresses multiply and raise more puzzling questions. Useful background material concerning these topics is available in Waisel et al. (1991). Despite these advances, the model of root structure used to interpret root system function is inadequate. This model is too often the stylized, simplistic, textbook interpretation of old studies of seedling roots of a very few species and is derived from roots and shoots still living heterotrophically on reserves, transpiring little or not at all. Also, these roots have never met the rigors of the field soil. Furthermore, traditional anatomical study has focused on the apical meristem and the patterns of the young tissues within a few millimeters of the tip, rarely beyond a few centimeters. One laboratory manual, a standard reference for almost 30 years, includes a table of the distances from the root tip at which major developmental events occur in seven different species. The farthest distance indicated is 8.3 cm in a woody plant. For the herbaceous species, the last developmental event is even closer to the tip. Corn xylem lignification is listed at less than 1.7 cm. The limitations of such a view can be calculated from Dittmer's (1937) famous data for total lengths of roots (main and three orders of branches) of a single soil-grown rye plant. The first 10 cm of all 143 main roots total about one millionth of the whole root length. In 1980, together with a brave graduate student named Janet Vermeer, I began to study the roots of corn plants in the field. We started by washing the roots to get rid of as much of the dirt as possible, so worrying to an electron microscopist. But a freshly excavated plant, unwashed as shown in Figure 1A, impressed us with the heterogeneity in size, origin, and soil binding, which have intrigued me and my students ever since. This interest has led us to explore the structure and functioning of root systems growing in the field. The results have been replete with surprises. In this paper I will include some of these surprises and the work of other groups, which provide new insights into three aspects of root structure and function, with important implications for many root-related studies: (a) delayed development of xylem, (b) the development of the interface with the soil, and (c) branch roots and their role.