Control of cell pluripotency in plants
Mingzhu Fan, Chongyi Xu, Ke Xu, Wei Xin, Jing Liu and Yuxin Hu* 

Key Laboratory of Plant Molecular Physiology, Institute of Botany, Chinese Academy of Sciences, Beijing 100093.
*Corresponding Author: Tel: (010) 62836650; E-mail: huyuxin@incas.ac.cn.

Abstract
Plant cells have long been considered to be pluripotent because most of cells can regenerate a whole plant under appropriate culture conditions. Although the remarkable regeneration capability of plant tissues or organs under the culture conditions has underlain an extensive practice for decades, the early molecular mechanism underlying callus initiation is largely elusive. We analyzed the dynamic transcriptome profiling of callus initiation in Arabidopsis aerial and root explants, and identified 1,342 differentially expressed genes in both explants after incubation on callus inducing medium (CIM). These genes were mainly related to hormone homeostasis and signaling, transcriptional and post transcriptional regulations, protein phosphorelay cascades and DNA- or chromatin-modification. We further showed that four LATERAL ORGAN BOUNDARIES DOMAIN (LBD)/ASYMMETRIC LEAVES2-LIKE (ASL) downstream of AUXIN RESPONSE FACTORs (ARFs), LBD16, LBD17, LBD18 and LBD29, were rapidly and dramatically induced by CIM in multiple organs. Ectopic expression of each of the four LBD genes in Arabidopsis was sufficient to trigger the spontaneous callus formation without exogenous phytohormone, whereas suppression of LBD function inhibited the callus formation induced by CIM. We also provided the evidence that the callus directed by LBD resembled that induced by CIM by characteristics of ectopically activated root meristem genes and efficient regeneration capacity. Our work defines LBD transcription factors as key regulators in callus induction process, thereby establishing a molecular link between auxin signaling and cell pluripotency control in plants.

Key words: LBD; Callus formation, Auxin; Cell pluripotency; Arabidopsis.