Targets of the StBEL5 Transcription Factor Include the FT Ortholog StSP6A

Abstract

ORCID IDs: 0000-0001-7693-8503 (P.S.); 0000-0001-8195-9202 (D.J.H.).The BEL1-like family of transcription factors is ubiquitous in plants and plays important roles in regulating development. They function in tandem with KNOTTED1 types to bind to a double TTGAC motif in the upstream sequence of target genes. StBEL5 of potato (Solanum tuberosum) functions as a mobile RNA signal that is transcribed in leaves, moves down into stolons in response to short days, and induces tuber formation. Despite their importance, however, very little is known about the targets of BEL1-like transcription factors. To better understand this network, we made use of a phloem-mobile BEL5 induction model, an ethanol-inducible system coupled with RNA sequencing analysis, and a screen for tandem TTGAC ciselements in the upstream sequence to catalog StBEL5 target genes. Induction of StBEL5 activated several genes that are also induced by StSP6A (S. tuberosum SELF-PRUNING 6A), a FLOWERING LOCUS T coregulator that functions as a signal for tuberization. Both enhancement and suppression of StBEL5 expression were also closely linked to StSP6A transcriptional activity. Site mutagenesis in tandem TTGAC motifs located in the upstream sequence of StSP6A suppressed the short dayinduced activity of its promoter in both young tubers and leaves. The expression profile of StBEL5 induced in stolons from plants grown under long-day conditions revealed almost 10,000 differentially expressed genes, including important tuber marker genes and genes involved in cell growth, transcription, floral development, and hormone metabolism. In a random screen of 200 differentially expressed targets of StBEL5, 92% contained tandem TTGAC motifs in the upstream sequence within 3 kb of the transcription start site.

The Multiple Signals That Control Tuber Formation

“…FDL proteins normally activate floral genes like AP1 or LFY. In contrast to StBEL5 targets (Sharma et al, 2016), however, no downstream tuber marker genes (e.g. StGA20OX1, StGA2OX1, or StLOG) putatively regulated by the TAC have been identified.…”

“…Overexpression and enhanced accumulation led to early, increased tuberization, whereas RNA interference suppression of StBEL5 significantly reduced tuber yields in several independent lines (Sharma et al, 2016). Overexpression lines of full-length StBEL5 RNA tuberize under noninductive LD, whereas constructs that contain only the coding sequence of StBEL5 minus both UTRs do not (Banerjee et al, 2006).…”

“…POTLX1), SCARECROW, SHORT-ROOT, CLAVATA1, and flowering genes like EARLY FLOWERING4, CONSTANS-LIKE, and APETALA2. In a screen of 200 differentially expressed targets of StBEL5, 92% contained the tandem TTGAC motifs in their upstream sequence, some with as many as nine copies (Sharma et al, 2016). Including its first intron, StGA2OX1 contains six of these tandem motifs.…”

“…Using an ethanol-inducible promoter and RNA profiling, thousands of BEL5 target genes were identified in stolon tips after just 6 h of induction, many involved in cell growth, signal transduction, transcription, and hormone metabolism (Sharma et al, 2016). A few examples include StSP5G, BRASSINOSTEROID INSENSITIVE1 and BRASSI-NOSTEROID INSENSITIVE1-ASSOCIATED RECEP-TOR KINASE, numerous lipoxygenases (e.g.…”

“…Using whole-plant induction assays, numerous genes highlighted for their involvement in tuberization also have been identified as targets of StBEL5. These include GA2OX1 and GA20OX1, AGL8, three members of the LOG family, several PINs and AUX/IAA genes, and seven StBEL genes (Sharma et al, 2016), including two that suppress tuberization, StBEL11 and StBEL29 (Ghate et al, 2017). Only those target genes that contain the tandem TTGAC elements present in their upstream promoter sequence were considered for further study.…”