The Impact of the Long-Distance Transport of aBEL1-Like Messenger RNA on Development    

Abstract

BEL1-and KNOTTED1-type proteins are transcription factors from the three-amino-loop-extension superclass that interact in a tandem complex to regulate the expression of target genes. In potato (Solanum tuberosum), StBEL5 and its Knox protein partner regulate tuberization by targeting genes that control growth. RNA movement assays demonstrated that StBEL5 transcripts move through the phloem to stolon tips, the site of tuber induction. StBEL5 messenger RNA originates in the leaf, and its movement to stolons is induced by a short-day photoperiod. Here, we report the movement of StBEL5 RNA to roots correlated with increased growth, changes in morphology, and accumulation of GA2-oxidase1, YUCCA1a, and ISOPENTENYL TRANSFERASE transcripts. Transcription of StBEL5 in leaves is induced by light but insensitive to photoperiod, whereas in stolon tips growing in the dark, promoter activity is enhanced by short days. The heterodimer of StBEL5 and POTH1, a KNOTTED1-type transcription factor, binds to a tandem TTGAC-TTGAC motif that is essential for regulating transcription. The discovery of an inverted tandem motif in the StBEL5 promoter with TTGAC motifs on opposite strands may explain the induction of StBEL5 promoter activity in stolon tips under short days. Using transgenic potato lines, deletion of one of the TTGAC motifs from the StBEL5 promoter results in the reduction of GUS activity in new tubers and roots. Gel-shift assays demonstrate BEL5/POTH1 binding specificity to the motifs present in the StBEL5 promoter and a double tandem motif present in the StGA2-oxidase1 promoter. These results suggest that, in addition to tuberization, the movement of StBEL5 messenger RNA regulates other aspects of vegetative development.

The Multiple Signals That Control Tuber Formation

“…In this way, the movement of StBEL5 RNA is induced by SD and the presence of its 39 UTR. Long-distance transport of StBEL5 mRNA has been verified using RNA movement assays in both whole plants and heterografts (Banerjee et al, 2006(Banerjee et al, , 2009Lin et al, 2013;Cho et al, 2015). Movement through the phloem also has been confirmed for a BEL5-like transcript in Arabidopsis (Notaguchi et al, 2015).…”

“…Numerous reports of the enhanced activity of a particular hormone or receptor have been consistently linked to tuber formation. Specific genes like LONELY GUY (LOG), AGAMOUS-LIKE8 (AGL8), ISO-PENTENYL TRANSFERASE, the PINs, AUX/IAA, and POTATO LIPOXYGENASE1 (POTLX1) have been correlated with growth processes associated with tuberization (Kolomiets et al, 2001;Rosin et al, 2003;Eviatar-Ribak et al, 2013;Lin et al, 2013;Roumeliotis et al, 2013b;Gao et al, 2016). These results alone, however, do not establish one hormone or another as the sole developmental switch for tuber formation.…”

“…This complex recognizes a specific cis-element, a double TTGAC motif present in StBEL5 target genes. Site mutagenesis and gel-shift assays have demonstrated the importance of the fidelity of both motifs in facilitating the binding of the BEL1/KNOX complex (Smith et al, 2002;Chen et al, 2004;Lin et al, 2013). This double motif ensures specificity during the regulation of BEL1/KNOX target genes.…”

“…2). This autoregulation was verified through site mutagenesis in an upstream sequence of the StBEL5 gene (Lin et al, 2013) and helps to explain the SD-induced activity of the StBEL5 promoter in stolons growing in the dark (Chatterjee et al, 2007).…”

“…The potato BEL1 gene, StBEL5, functions to regulate tuber and root growth (Chen et al, 2003;Lin et al, 2013). Operating as a mobile signal, its full-length transcript has the capacity to move long distances through the sieve element system (Banerjee et al, 2006).…”