Initiation of lateral root formation in Arabidopsis, one step at a time

Lateral root formation is a major determinant of root systems architecture. In Arabidopsis, lateral roots arise from a subset of stem cells situated in the pericycle at the xylem poles. These cells undergo - prior to a nuclear migration event - tightly coordinated asymmetric cell divisions to generate cell diversity and tissue patterns, resulting in the development of a lateral root primordium. The molecular mechanisms directing these first cell divisions during lateral root initiation (LRI) are largely unknown because very few molecular components have been identified in the control of this process.

Based on expression and genetic data, we discovered that the GOLVEN6 (GLV6) peptide is involved in a signalling process controlling the asymmetric cell divisions essential for correct LRI. A forward genetics screen led to the identification of the GLV6 secondary messenger MPK6 and loss of function reveals a role for MPK6 in LRI. Through tissue-specific modulation of MPK6 activity in the root we intend not only to understand the role of MPK6 during lateral root organogenesis but also the contribution of cell-to-cell signalling in the pericycle stem cell niche.

Moreover, in an attempt to identify new genes involved in LRI, we searched for homologs of the highly conserved nuclear distribution genes identified in different kingdoms, and selected the homolog that is specifically expressed during lateral root development for further investigation. Interestingly, loss of function of this homolog results in a severely obstructed embryogenesis, a process also relying strongly on correct asymmetric cell division patterns. Through complementary genetic approaches based on CRISPR/CAS9-technology and in-depth phenotypic analysis we aim to unravel its function in asymmetric cell division.

With the identification of these molecular players, we contribute to decipher the molecular mechanisms that operate in the pericycle controlling the initial formative divisions essential for the development of a lateral root primordium. De novo formation of lateral roots is the let-off for plants to compensate their lack of mobility but to still be able to search for water and nutrients in the soil.