Willemsen Group - Plant architecture in relation to the environment

Plant shape, or architecture, is a primary determinant of productivity and yield. The shape of the above-ground part of the plant determines light interception and photosynthesis, whereas the below-ground root system determines the interaction with the soil, including uptake of water and nutrients and anchoring. Determining the molecular mechanisms that drive plant architecture in relation to the environment is the central aim of this group. This central question originates from the ambition to gain enough knowledge on plant architecture development to engineer crops that are adapted to their environment and can survive with scarce resources, and thus do not depend on nutrient application and copious watering. Sustainable crops that are adapted to poor soil conditions by generating a large root system in suboptimal conditions can lead to higher yields than non-adapted plants.

As plant cells are bound by a cell wall and cannot move, shape is an outcome of the reorientation of the cell division plane of new cell divisions and subsequent cell growth. In the model plant Arabidopsis, distribution patterns of the plant growth regulator auxin have been linked with altered cell division planes during embryo development, lateral root initiation  and in primary roots. During early stages of plant embryogenesis, the orientation of cell divisions is crucial; defects in these first divisions have severe developmental consequences. New cell layers are constantly established in the ground tissue and epidermis/lateral root cap stem cells of Arabidopsis roots. 

The importance of orienting cell planes also becomes apparent in asymmetric cell divisions associated with the Arabidopsis root stem cell niche that are sustained by the activity of PLETHORA (PLT) proteins. These members of the AP2 transcription factor family are the main regulators of primary root meristem maintenance and the position of the meristematic boundary, and have an intimate relationship with auxin. PLT3, PLT5 and PLT7 play an important role in lateral root development which is also a read-out of reorientation of division planes. Lateral root primordia (LRP) initiate from lateral root founder cells that undergo stereotypical asymmetric cell divisions, forming shorter central cells and longer flanking cells. Subsequent rounds of anticlinal, periclinal, and tangential cell divisions form a dome-shaped primordium that emerges through the overlaying primary root cell layers, possessing a fully functional meristem that is highly reminiscent of the primary root. plt3plt5plt7 knock-out mutants exhibit distorted division planes in LRP and thus play a role regulating rhizotaxis, but are also required for correct phyllotactic patterning of the above-ground rosette and the flowers Studying these processes outside of the context of Arabidopsis can also provide novel insights. Firstly, the moss Physcomitrium patens is a valuable model organism for studying the basic mechanisms of oriented cell divisions. Furthermore, phylogenomic analysis of the AP2 transcription factor family has also shown lineage-specific patterns in monocots. Monocot roots differ greatly from the primary roots of Arabidopsis, which makes studying PLTs in these species especially useful for understanding how these developmental networks regulate root architecture.

Taken together, these combined efforts will continue to elucidate the molecular mechanisms that drive plant architecture, which moves us closer to our ambition of architecturally optimized and sustainable crops.