Crucial role of asymmetric cell division in plant adaptation to hard and dry soils

The global climate change is progressively hardening soils due to the expansion of drylands. Soil compaction and mechanical resistance pose significant challenges to root growth, restricting plant development and crop productivity. Recent studies suggest that root cortex tissue plays a crucial role in enabling roots to penetrate hard and dry soils. Plants exhibit substantial interspecific variability in cortical layer number, ranging from a single layer, as seen in the model species A.thaliana, to several layers, as observed in horseradish. However, the relationship between this variability and the ability to penetrate soils of differing hardness and compactness remains unclear. The goal of my PhD project was to determine whether increasing cortical layer number enhances the ability of plants to penetrate harder soils. As first I developed an experimental set up that permits us to evaluate root penetration capacity in media mimicking progressively compacted soils. Exploiting this set up and utilizing closely related species with varying cortical layer number, I observed that an increased number of cortical layers enhances root penetration in compacted soils. To identify the molecular mechanisms that altering cortical layer number without disrupting global root development, I exploited the model species C.hirsuta, a close relative of A.thaliana with two cortical layers. Combining natural variation studies, biochemical and molecular biology experiments, I uncovered hormonal and genetic cross-talk involving the plant hormone Abscisic Acid (ABA) as a key regulator of cortical layer number. Using A,thaliana I confirmed that this ABA-dependent mechanism is sufficient to regulate cortical layer number and enhance root penetration by tailoring root diameter to specific soil conditions. Additionally, I identified cytokinin (CK) as another promising candidate controlling cortical layer number and improving plant capacity to penetrate compact soil. This work highlights the potential for optimizing root radial architecture through hormonal modulation and genetic strategies to develop crop varieties better suited to diverse soil environments, offering solutions for sustainable agriculture in a fast-changing environment.