What is apical and what is basal in plant root development?

Baluška, František and Barlow, Peter W. and Baskin, Tobias I. and Chen, Rujin J. and Feldman, Lewis and Forde, Brian G. and Geisler, Markus and Jernstedt, Judy and Menzel, Diedrik and Muday, Gloria K. and Murphy, Angus and Šamaj, Jozef and Volkmann, Dieter (2005) What is apical and what is basal in plant root development? Trends in Plant Science, 10 (9). pp. 409-411. ISSN 1360-1385

Abstract

Plant architecture is complex but well described by an established terminology that includes clear definitions of organismal polarity [1]. However, the definitions of polarity that apply to most stages of plant development cannot be applied to early zygotic development. Recent introduction of terminology reserved for early embryonic anatomy to post embryonic seedling anatomy have created some confusion. In this letter, we highlight the issue with the intention of clarifying terminology and bringing about a consensus regarding usage. The original Latin word ‘apex’ refers to the summit of a hill, mountain or building. According to both the Oxford and Webster dictionaries, ‘apex’ is defined as ‘the highest or topmost point’ of a structure. In plants, an apex constitutes the tip of a shoot or a root. The word ‘apical’, therefore, means relating to, located or situated at, or constituting, an apex. A ‘base’ is defined as the ‘lowest or bottom part of an object on which it stands’ or the ‘main part to which other parts are added’. In biology, ‘base’ means the part of a plant or animal organ that is near the point of attachment to the ground or to a more basal part of the body. Because we cannot say that plants stand on their roots, the base of both stems and roots is actually the same point, and is where the two organs meet and are attached to each other. Similarly, for lateral organs their base refers to their point of attachment to the main plant body: for example, lateral roots are attached at their base to the main root, just as lateral shoots are attached at their base to the stem. In all standard text books on plant anatomy, including Plant Anatomy, the tips of shoots and roots are referred to as apices (Figure 1) [1]. It is here that their ‘apical meristems’ are to be found [2]. The attachment point between stem and root is referred to as a base – stem-base or root-base – in each case. Therefore, in roots (possessing their own apex and base, both of which are well defined and instantly recognisable) the proper usage of the term ‘apical’ can also define the polarity of the constitutent cells and hence direct attention to the cellular pole that faces the apex (or tip). By the same token, ‘basal’ can refer to the pole that faces the base of the organ (i.e. the basal attachment point of the root to the stem).