Expression of Sucrose Transporter cDNAs Specifically in Companion Cells Enhances Phloem Loading and Long-Distance Transport of Sucrose but Leads to an Inhibition of Growth and the Perception of a Phosphate Limitation    

Abstract

Sucrose (Suc) is the predominant form of carbon transported through the phloem from source to sink organs and is also a prominent sugar for short-distance transport. In all streptophytes analyzed, Suc transporter genes (SUTs or SUCs) form small families, with different subgroups evolving distinct functions. To gain insight into their capacity for moving Suc in planta, representative members of each clade were first expressed specifically in companion cells of Arabidopsis (Arabidopsis thaliana) and tested for their ability to rescue the phloem-loading defect caused by the Suc transporter mutation, Atsuc2-4. Sequence similarity was a poor indicator of ability: Several genes with high homology to AtSUC2, some of which have phloem-loading functions in other eudicot species, did not rescue the Atsuc2-4 mutation, whereas a more distantly related gene, ZmSUT1 from the monocot Zea mays, did restore phloem loading. Transporter complementary DNAs were also expressed in the companion cells of wild-type Arabidopsis, with the aim of increasing productivity by enhancing Suc transport to growing sink organs and reducing Suc-mediated feedback inhibition on photosynthesis. Although enhanced Suc loading and long-distance transport was achieved, growth was diminished. This growth inhibition was accompanied by increased expression of phosphate (P) starvation-induced genes and was reversed by providing a higher supply of external P. These experiments suggest that efforts to increase productivity by enhancing sugar transport may disrupt the carbon-to-P homeostasis. A model for how the plant perceives and responds to changes in the carbon-to-P balance is presented.

Regulation of Sucrose Transporters and Phloem Loading in Response to Environmental Cues

“…Reduced expression or knock-out of the phloem-loading SUT leads to Suc accumulation, while overexpression can lead to increased source strength (Dasgupta et al, 2014). The notion of SUT expression regulating phloem loading is corroborated here, but only for down-regulation in response to reduced light levels.…”

“…Therefore, a direct link between SUT expression and phloem loading rate does not seem to exist for the up-regulation of phloem loading under these conditions. This contrasts with studies that artificially up-regulate a relevant SUT through overexpression, which, in some cases, did result in increased phloem loading (Dasgupta et al, 2014;Wang et al, 2015).…”

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

“…The pCoYMV promoter is from Commelina Yellow Mottle Virus and known to confer strong companion cell-specific expression (Medberry et al, 1992;Matsuda et al, 2002). It is also up-regulated by Suc, whereas another companion cell-specific promoter, Arabidopsis SUCROSE-PROTON SYMPORTER2 (pAtSUC2), is repressed by Suc (Dasgupta et al, 2014). As expected, the AVP1 mRNA expression levels in these transgenic lines are higher than in Col-0, and individual lines with each construct show differences in the AVP1 transcript levels (Fig.…”

“…5A; AVP1-1 background). We used the well-characterized promoter element from Commelina Yellow Mottle Virus (pCoYMV) that directs expression specifically in the companion cells (Medberry and Olszewski, 1993;Dasgupta et al, 2014). All three independent pCoYMV::AVP1 lines showed a Figure 7.…”

“…pMDC32 contains a duplicated 35S enhancer element (Curtis and Grossniklaus, 2003). pENTR::AVP1 was recombined with pGPTV::CoYMVp::cmr::ccdB (Dasgupta et al, 2014) to get pGPTV::CoYMVp:: AVP1. These vectors were electroporated into Agrobacterium tumefaciens strain GV3101mp90.…”