Incipient stem cell niche conversion in tissue culture: using a systems approach to probe early events in WUSCHEL-dependent conversion of lateral root primordia into shoot meristems.

Adventitious shoot organogenesis contributes to the fitness of diverse plant species, and control of this process is a vital step in plant transformation and in vitro propagation. New shoot meristems (SMs) can be induced by the conversion of lateral root primorida/meristems (LRP/LRMs) or callus expressing markers for this identity. To study this important and fascinating process we developed a high-throughput methodology for the synchronous initiation of LRP by auxin, and subsequent cytokinin-induced conversion of these LRP to SMs. Cytokinin treatment induces the expression of the shoot meristematic gene WUSCHEL (WUS) in converting LRP (cLRP) within 24-30 h, and WUS is required for LRP → SM conversion. Subsequently, a transcriptional reporter for CLAVATA3 (CLV3) appeared 32-48 h after transfer to cytokinin, marking presumptive shoot stem cells at the apex of cLRP. Thus the spatial expression of these two components (WUS and CLV3) of a regulatory network maintaining SM stem cells already resembles that seen in a vegetative shoot apical meristem (SAM), suggesting the very rapid initiation and establishment of the new SMs. Our high-throughput methodology enabled us to successfully apply a systems approach to the study of plant regeneration. Herein we characterize transcriptional reporter expression and global gene expression changes during LRP → SM conversion, elaborate the role of WUS and WUS-responsive genes in the conversion process, identify and test putative functional targets, perform a comparative analysis of domain-specific expression in cLRP and SM tissue, and develop a bioinformatic tool for examining gene expression in diverse regeneration systems.