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

Abstract

Suc transporters (SUTs) play a key role in the allocation and partitioning of photosynthetically fixed carbon in plants. While a function could be assigned to many members of the SUT family, almost no information is available on their regulation. Here, the transcriptional regulation of SUTs in response to various environmental stimuli in the leaves of five dicots (Arabidopsis Extensive data on expression of SUTs in relation to changes of environmental conditions were obtained through a global analysis of 168 transcriptomics data sets. Results were validated by quantitative PCR measurements and extended by the measurement of photosynthesis rate and phloem sugar content to draw insight on the correlation of SUT expression and sugar export from leaves. For the apoplasmic phloem loaders, a clear difference in transcriptional regulation in response to different environmental stimuli was observed. The consistent patterns of SUT expression under abiotic stress indicates which types of SUTs are involved in the regulation of leaf sugar status and in stress signaling. Furthermore, it is shown that down-regulation of phloem loading is likely to be caused by transcriptional regulation of SUTs, while up-regulation depends on post-transcriptional regulation. In poplar, expression of PtaSUT4 was found to consistently respond to environmental stimuli, suggesting a significant role in the regulation of sugar export from leaves in this passive symplasmic phloem loader.

Expression of Sucrose Transporters from Vitis vinifera Confer High Yield and Enhances Drought Resistance in Arabidopsis

“…SUTs play a key role in the allocation and partitioning of photosynthetically fixed carbon in plants and therefore, if sucrose transport efficacy is promoted, the yield of plants could be further optimized. Moreover, high light could enhance photosynthesis, leaf sugar levels, and phloem loading; however, these factors do not influence SUT expression directly [48]. Thus, SUT overexpression under high light conditions could be an important component of increased plant yield phenotypes.…”

Cold acclimation has a differential effect on leaf vascular bundle structure and carbon export rates in natural Arabidopsis accessions originating from southern and northern Europe

“…Further, previous studies have shown that regulation of SUTs is part of abiotic stress response (see e.g. (Gong et al, 2015)) and that both transcriptional and posttranscriptional mechanisms are involved in regulation of Suc export (Xu et al, 2018).…”

Carbon export from leaves is controlled via ubiquitination and phosphorylation of sucrose transporter SUC2

“…Gene expression was found to scale with phloem loading in response to certain abiotic stresses (5-7), but did not scale with phloem loading in response to increased light levels (7). In tomato, increased phloem loading under high-light conditions coincided with increased protein abundance of the phloem-loading SUC instead, indicating posttranscriptional regulation (7). Transcriptional and posttranscriptional regulation of the phloem-loading SUC was also observed in sugar beet in response to exogenously fed sucrose (8,9).…”

“…This arrangement makes SUCs the most likely target of regulation. Indeed, recent analysis of phloem transport in response to environmental cues suggested that carbon export from leaves is directly linked to SUC activity (5)(6)(7).…”

“…Despite their strategic role in whole-plant carbon partitioning, relatively little is known about the mechanism of regulation of SUCs responsible for phloem loading. Gene expression was found to scale with phloem loading in response to certain abiotic stresses (5-7), but did not scale with phloem loading in response to increased light levels (7). In tomato, increased phloem loading under high-light conditions coincided with increased protein abundance of the phloem-loading SUC instead, indicating posttranscriptional regulation (7).…”