A radical role in lateral root development

While roots may appear tangled, their architecture is strictly controlled by plants to maximise their uptake of water and nutrients from the soil. Root development is regulated by many hormones and signals, including reactive oxygen species (ROS), which were once considered to be toxic by-products of cellular processes, but are now known to function in a wide range of plant functions and developmental stages.

Image: A developing lateral root primordium before emergence from the parental root tissue. Credit: Fernández-Marcos et al. (2016).
A developing lateral root primordium before emergence from the parental root tissue. Credit: Fernández-Marcos et al. (2016).

Growth in plants depends on the balance between cell division and differentiation. In the growing tips of plants (meristems), the cells rapidly divide, leaving daughter cells behind to elongate and differentiate into specific cell types required in the newly developing organ. Too much cell division results in tumour-like unregulated growth, while too little means that there are not enough cells to properly form the new tissue. In roots, ROS have been shown to control the transition from cell division to differentiation, defining the boundary between the sites of these two processes.

MYB36 and root development

In a new study published in New Phytologist, a gene called MYB36 was shown to affect the development of branched (lateral) roots, with far fewer lateral roots produced by plants lacking a functional copy of this gene (myb36 mutants). The authors – Fernández-Marcos and colleagues – showed that early lateral root development was normal in these myb36 mutants, but that the cells failed to differentiate and the developing root could not grow out of the parental tissue. Researchers recently showed that MYB36 regulates the transition from cell division to differentiation in the primary root, so Fernández-Marcos and colleagues hypothesised that it may be important in lateral root differentiation too.

Image: MYB36 is expressed in the cells surrounding the developing lateral root, defining the boundary between the parent and progeny root tissues (blue: MYB36-GUS). Credit: Fernández-Marcos et al. (2016).
MYB36 is expressed in the cells surrounding the developing lateral root, defining the boundary between the parent and progeny root tissues (blue: MYB36-GUS). Credit: Fernández-Marcos et al. (2016).

MYB36 is expressed in the cells surrounding the developing lateral root, as shown in the image above. It encodes a transcription factor that regulates the expression of a suite of different genes, which define the boundary between the new lateral root and its parent. One group of genes regulated by MYB36 encode peroxidases, enzymes that alter the levels of ROS in the cells. Since ROS control the transition from cell division to differentiation in the primary root, Fernández-Marcos and colleagues investigated their role in the development of lateral roots.

ROS regulates root development

When the authors treated myb36 mutant plants with ROS-reducing potassium iodide, their defect in lateral root development was almost fully corrected.

Why should less ROS have opposing effects in these plants? Fernández-Marcos and colleagues believe that it’s all to do with the balance of ROS in the lateral root boundary cells ensuring proper growth, and that MYB36 acts via the peroxidases to regulate this ROS balance.

This was further corroborated by the similar phenotype of the myb36 mutants (this study) and plants that overexpress MYB36; in both cases, the lateral roots do not develop properly, likely because of the mis-regulation of ROS at their boundaries. ROS appear to act in lateral root boundary cells in a similar manner to the primary root, controlling the transition from cell division to differentiation.

Image: Figure 2(c) from Fernándex-Marcos et al. (2016).
Role of Arabidopsis thaliana MYB36 in mid stage of lateral root primordia (LRP) development. Detail of DAPI-stained wild-type (Col-0) and myb36-5 LRPs indicating the position of cells at their base (left panel) and quantification of cell number at different stages, as indicated (right panel). Differences for stages IV–V were statistically significant (***, P ≤ 0.01).

New insights into root development

This new study from Fernández-Marcos and colleagues provides new insights into the role of MYB36 and ROS in lateral root development. The authors propose that MYB36 is expressed in the boundary cells of the developing lateral root, inducing the expression of a subset of the peroxidase genes, which regulate the levels of ROS to define the boundary of the growing root. This novel function resembles MYB36’s previously described regulation of the transition from cell division to differentiation in the primary root, suggesting the existence of a similar developmental mechanism.

Read the paper: Fernández-Marcos, M., Desvoyes, B., Manzano, C., Liberman, L. M., Benfey, P. N., del Pozo, J. C. and Gutierrez, C. (2016), Control of Arabidopsis lateral root primordium boundaries by MYB36. New Phytologist. doi: 10.1111/nph.14304


 

Sarah Jose
@JoseSci