Growth & Development

Thicker roots: to grow or not to grow? Investigating the dynamics of secondary root growth

Secondary growth of the roots of annual dicots has functional significance with regards to soil resource acquisition and transport, interactions with soil organisms and carbon sequestration.

Whilst we are under lockdown, have you been eating potatoes, carrots and other root vegetables? Roots are not only a tasty source of nutrients but their growth is a fascinating process. Tracking how roots grow in the soil is a bit of a challenge and crop breeders might focus more on yields rather than investigating how roots would be able to adapt to local conditions.

There are two main root growth processes. Primary growth leads to longer roots as the cells divide at the tip of the root or stem. Secondary growth increases root thickness in mostly woody plants which have lateral meristems. 

Strock and Lynch at Penn State University’s Roots Lab have written a fascinating Viewpoint on the secondary root growth process and highlighting its importance to plant fitness. Adjusting secondary root growth can help some crops to tolerate environmental stresses (e.g. drought) or to take up nutrients from the soil better than other plants.

As there are many knowledge gaps about the secondary root growth, the authors propose hypotheses and future research opportunities by focusing on annual dicot plants such as cassava, potato and common bean.

 Figure from Strock and Lynch (2020) highlighting the dynamic progression of growth along the length of axial roots of  a common bean (Phaseolus vulgaris).

The first section of the Viewpoint discusses the regulation of secondary root growth, how to measure it and metabolic processes and what are the costs to the plant. The authors quote Savidge (1993) who described the secondary root growth where the root is an “inverted cone, which volume expands exponentially with time”. Secondary root growth might be inhibited or increased during for example a drought so the root can either avoid or escape the stress. As soil is a three dimensional space, secondary root growth could also help with nutrient uptake and root foraging. The authors propose that radial and axial transport can be a dynamic process, helping plants adapt to local conditions. 

The second part of the paper reviews the processes and strategies behind secondary root growth as the roots interact with the soil and soil organisms. Increased growth can lead to the lignification (i.e. thickening) of the periderm which can either provide protection against harmful pests and pathogens but might also stop symbiotic organisms from colonising the roots. Strock et al. (2019) have also recently published findings using laser ablation tomography in the Journal of Experimental Botany to investigate root growth of several plant species and interactions with soil organisms. In the last three sections, the authors discuss the effect of secondary root growth on the production of root exudates, its development under hypoxic conditions and role in carbon sequestration. 

In each section, the authors propose hypotheses and future fundamental research possibilities so plant breeders could use secondary root growth as an important trait when selecting crop varieties.