Form matters: morphological aspects of lateral root development

Abstract

Background: The crucial role of roots in plant nutrition, and consequently in plant productivity, is a strong motivation to study the growth and functioning of various aspects of the root system. Numerous studies on lateral roots, as a major determinant of the root system architecture, mostly focus on the physiological and molecular bases of developmental processes. Unfortunately, little attention is paid either to the morphological changes accompanying the formation of a lateral root or to morphological defects occurring in lateral root primordia. The latter are observed in some mutants and occasionally in wild-type plants, but may also result from application of external factors.

Scope and conclusions: In this review various morphological aspects of lateral branching in roots are analysed. Morphological events occurring during the formation of a typical lateral root are described. This process involves dramatic changes in the geometry of the developing organ that at early stages are associated with oblique cell divisions, leading to breaking of the symmetry of the cell pattern. Several types of defects in the morphology of primordia are indicated and described. Computer simulations show that some of these defects may result from an unstable field of growth rates. Significant changes in both primary and lateral root morphology may also be a consequence of various mutations, some of which are auxin-related. Examples reported in the literature are considered. Finally, lateral root formation is discussed in terms of mechanics. In this approach the primordium is considered as a physical object undergoing deformation and is characterized by specific mechanical properties.

Keywords: Lateral root formation; arabidopsis mutant; growth rates; mechanical stress distribution; plant organ morphology.

Fig. 1.

Examples of the most characteristic types of lateral root primordium deformation in wild-type arabidopsis seedlings. (A) Lack of symmetry in relation to the primordium axis. (B) Flattened surface. (C) Spindle-like form. (D) A pocket formed at the primordium base. Notice that the vascular bundles bend towards the main root axis in (A–C). Insets in the upper left corners are schematic representations of each type of deformation. (A, B) Phase contrast; (C, D) Nomarski contrast. Scale bar = 20 μm.

Fig. 2.

Deformation of the initially rectangular figure under the field of growth rates specified for the lateral root. Application of a stable field of growth rates results in formation of a regular dome-shaped primordium (upper); application of the field displaced constantly in one direction (middle) or randomly (lower) results in formation of an atypically shaped primordium (for details see Szymanowska-Pułka, 2007).