Conserved cis-regulatory elements for DNA-binding-with-one-finger and homeo-domain-leucine-zipper transcription factors regulate companion cell-specific expression of the Arabidopsis thaliana SUCROSE TRANSPORTER 2 gene

Abstract

The transition from young carbon-importing sink leaves of higher plants to mature carbon-exporting source leaves is paralleled by a complete reversal of phloem function. While sink-leaf phloem mediates the influx of reduced carbon from older source leaves and the release of this imported carbon to the sink-leaf mesophyll, source-leaf phloem catalyzes the uptake of photoassimilates into companion cells (CCs) and sieve elements (SEs) and the net carbon export from the leaf. Phloem loading in source leaves with sucrose, the main or exclusive transport form for fixed carbon in most higher plants, is catalyzed by plasma membrane-localized sucrose transporters. Consistent with the described physiological switch from sink to source, the promoter of the Arabidopsis AtSUC2 gene is active only in source-leaf CCs of Arabidopsis or of transgenic tobacco (Nicotiana tabacum). For the identification of regulatory elements involved in this companion cell-specific and source-specific gene expression, we performed detailed analyses of the AtSUC2 promoter by truncation and mutagenesis. A 126-bp promoter fragment was identified, which seems to contain these fragments and which drives AtSUC2-typical expression when combined with a 35S minimal promoter. Within this fragment, linker-scanning analyses revealed two cis-regulatory elements that were further characterized as putative binding sites for transcription factors of the DNA-binding-with-one-finger or the homeo-domain-leucine-zipper families. Similar or identical binding sites are found in other genes and in different plant species, suggesting an ancient regulatory mechanism for this important physiological switch.

Arabidopsis Type I Proton-Pumping Pyrophosphatase Expresses Strongly in Phloem, Where It Is Required for Pyrophosphate Metabolism and Photosynthate Partitioning

“…Of note, leaves of 15-d-old seedlings (Fig. 1) show a distinct sink-to-source transition pattern that mimics that of the PM-localized Suc-H + symporter, SUC2 (Truernit and Sauer, 1995;Imlau et al, 1999;Schneidereit et al, 2008). The sink-to-source transition starts at the tip of expanding young leaves and proceeds toward the base (Turgeon and Webb, 1973;Wright et al, 2003).…”

“…The sink-to-source transition starts at the tip of expanding young leaves and proceeds toward the base (Turgeon and Webb, 1973;Wright et al, 2003). SUC2 and orthologs are essential transporters required for phloem loading of Suc in apoplasmic loaders like Arabidopsis, and they are commonly used markers of the sink-to-source transition in leaves (Wright et al, 2003;Schneidereit et al, 2008). The closely correlated expression patterns of SUC2 and AVP1 further suggest that AVP1 is involved in the phloem-loading process.…”

“…DOF2 elements (AAAG or CTTT) are the core binding sites for DOF transcription factors (Noguero et al, 2013) that are positive regulators required in the formation and functioning of vascular tissues, including those of Arabidopsis (Ward et al, 1998;Zhao et al, 2005;Le Hir and Bellini, 2013). DOF2 elements are also present in the essential 241-bp SUC2 promoter fragment that drives phloem-specific SUC2 expression in Arabidopsis (Schneidereit et al, 2008), and the 244-bp promoter is sufficient to drive AVP1 expression in the phloem (Fig. 6, C-E).…”

Potassium (K + ) gradients serve as a mobile energy source in plant vascular tissues

“…The akt2-1 line was also rescued when AKT2 was expressed under the control of the phloem companion cell-specific AtSUC2 promoter (19) (Fig. S3, D and E).…”

Genome-wide identification and characterization of the Dof gene family in Medicago truncatula

“…In cereal seeds, most Dof genes are expressed during both seed germination and maturation, recognizing the 5'-TGTAAAG-3' motif, and are termed Prolamin-box binding factors (PBF). These Dof genes have a high degree of sequence similarity to other Dof genes, including ZmPBF from maize (Schneidereit et al, 2008), OsPBF from rice (Yamamoto et al, 2006), WPBF from wheat (Mena et al, 1998;Dong et al, 2007), BPBF from barley (Mena et al, 1998), and FMPBF from finger millet . These genes have similar expression profiles with similar functions in the development of the endosperm during the seed-filling phase.…”