Analysis of phloem function as a signaling center

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Plant body plan is regulated by the tight network of cell-cell communication. Vascular plants possess a unique long-distance transport system of nutrients and signals, named the vascular system. The vascular system consists of xylem, phloem and procambium/cambium, which gives rise to xylem and phloem cells. We have identified TDIF, a dodeca-peptide that regulates vascular development as an intercellular signal secreted from phloem. In this study, therefore, we analyzed the function of phloem-related signals involved in vascular development. For this purpose we focused on the following three themes. 1) To elucidate the function of TDIF as a phloem signal and consequently to understand of the status of phloem. 2) To analyze the function of COE1 receptor kinase, whose gene may be expressed in phloem cells and function in vascular pattern formation of rice. 3) To identify new transcription factors that regulate TDIF expression in phloem. As a result, we succeeded in demonstrating that phloem cells functions as niche cells in the maintenance of vascular stem cells such as procambial and cambial cells by providing TDIF as a intercellular factor promoting self-renewal of stem cells and inhibiting xylem cell differentiation. We also indicated that COE1 functions in spatial regulation of vascular tissue differentiation in concert with auxin and brassinosteroid signals. Finally we isolated transcription factors regulating the CLE41 gene encoding TDIF, using a new comprehensive transcription factor screening method. These findings casted a new light on function of the phloem cells as a signal center.