The regulation of Arabidopsis thaliana lateral root development by redundant PLETHORA transcription factors

In Arabidopsis thaliana, lateral roots (LRs) initiate acropetally and their dynamic development forms the basis on which root system architecture is elaborated. This thesis work focuses on revealing the underlying molecular network of how redundant PLETHORA (PLT) genes, encode APETALA2 (AP2)-domain transcription factors, jointly control rhizotaxis and LR outgrowth. Rhizotaxis represents the spatial distribution of lateral root primordia (LRP) and LRs along primary roots. LR outgrowth, which is staged according to the number of radial cell layers, is defined as a ‘primordium-intrinsic’ establishment of new organ tissues and meristem in an orthogonal axis to the parental roots. PLT3, PLT5 and PLT7 prevent the formation of LRP close to one another, because, in their absence, successive LRP are frequently grouped in close clusters. In addition, LR outgrowth is disrupted in plt3plt5plt7 LRP from the Stage I to II transition phase, when central cells fail to induce formative cell divisions to define the stage-specific layers and the expression of key root tissue-specific and meristematic genes is mis-regulated. Re-introduction of any PLT proteins in plt3plt5plt7 LRP from Stage I onward completely restores LR morphology, tissue specificity and meristem. Thus, PLT3, PLT5 and PLT7 genes are ‘bottleneck’ genes to activate the orthogonal program of de novo LR tissue pattern and meristem activity after LR initiation. Moreover, we have identified three LATERAL ORGAN BOUNDARIES DOMAIN (LBD) family members, LBD14, LBD18 and LBD29 as direct PLT promoter binding proteins that are associated with rhizotaxis and LR outgrowth.