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The BEL1 family of potato: a model for RNA transport

Project leader: Tian Lin (lintian@iastate.edu)

Update on the StBEL gene family of potato

Tuber formation in potatoes (Solanum tuberosum L.) is a complex developmental process involving a number of important biological systems. It has long been known that under short-day photoperiodic conditions receptors in the source leaves perceive the light cue and activate a transmissible signal that moves from the leaf through the phloem down to the stolon to induce growth in the apical stem of the stolon. This graft-transmissible signal initiates processes that lead to cell division and expansion as well as a change in the orientation of cell growth in the subapical region of the stolon tip, leading to starch accumulation and formation of the tuber.

As one approach to identifying this mobile signal, research efforts have focused on the role of transcription factors in regulating tuber formation. One important group of transcription factors includes the TALE superfamily. In plants, these include members from the BEL1- and the KNOTTED1-like families: both are involved in pattern formation and maintenance of the shoot apical meristem (Reiser et al. 2000; Kanrar et al. 2008). In potato, seven BEL1-like transcription factors have been identified (Chen et al. 2003). All seven of the StBEL RNAs have been confirmed to be present in phloem cells (Campbell et al., 2008). This family of transcription factors is ubiquitous among plant species and these proteins interact with KNOTTED1-like transcription factors (see below) to regulate a range of developmental processes (Müller et al. 2001; Chen et al. 2003; Smith and Hake 2003; Kumar et al. 2007; Pagnussat et al. 2007; Kanrar et al. 2008; Ragni et al. 2008).

In potato, the BEL1-like transcription factor, StBEL5 and its Knox protein partner, POTH1, regulate tuber formation by mediating hormone levels in the stolon tip (Rosin et al. 2003; Chen et al. 2004). This change in hormone level leads to the establishment of a strong sink organ, the tuber, by activating cell division and enlargement in specific cells of the stolon tip (Xu et al. 1998; Hannapel et al. 2004). Upon this activation, abundant amounts of sucrose are transported to the stolon tip for the synthesis of starch in the newly formed tuber. The focus of this project is to identify the function of other BEL1-like transcription factors of potato and to verify the long-distance mobility of their transcripts.