AtSWEET4, a hexose facilitator, mediates sugar transport to axial sinks and affects plant development

Abstract

Plants transport photoassimilates from source organs to sink tissues through the phloem translocation pathway. In the transport phloem, sugars that escape from the sieve tubes are released into the apoplasmic space between the sieve element/companion cell complex (SE/CC) and phloem parenchyma cells (PPCs) during the process of long-distance transport. The competition for sugar acquisition between SE/CC and adjoining PPCs is mediated by plasma membrane translocators. YFP-tagged AtSWEET4 protein is localized in the plasma membrane, and PromoterAtSWEET4-GUS analysis showed that AtSWEET4 is expressed in the stele of roots and veins of leaves and flowers. Overexpression of AtSWEET4 in Arabidopsis increases plant size and accumulates more glucose and fructose. By contrast, knock-down of AtSWEET4 by RNA-interference leads to small plant size, reduction in glucose and fructose contents, chlorosis in the leaf vein network, and reduction in chlorophyll content in leaves. Yeast assays demonstrated that AtSWEET4 is able to complement both fructose and glucose transport deficiency. Transgenic plants of AtSWEET4 overexpression exhibit higher freezing tolerance and support more growth of bacterium Pseudomonas syringae pv. phaseolicola NPS3121. We conclude that AtSWEET4 plays an important role in mediating sugar transport in axial tissues during plant growth and development.

Regulatory roles of sugars in plant growth and development

“…However, other reports support the previous opinion that sucrose is a ubiquitous transport sugar, and hexoses are usually absent in the phloem stream [54]. Recently, new membrane transporters of sugar, named SWEET (sugars will eventually be exported transporters), have been discovered, which, among others, are involved in the process of phloem loading, together with H + /sucrose symporter [55,56]. It was shown that AtSWEET4 overexpression increased the size of A. thaliana rosette leaves.…”

“…It was shown that AtSWEET4 overexpression increased the size of A. thaliana rosette leaves. In contrast, knock-down of AtSWEET4 (by RNAi) reduced plant growth and decreased chlorophyll content in the leaves [56]. Sugars (sucrose) have been found to affect the activity (and expression) of both the sucrose transporters, which are essential for the efficient transport of sugar over long distances.…”

“…The following strategies were used in order to select the appropriate mutants: (i) if high sugar concentrations in the medium inhibited germination and seedling development, then the seedlings showing development were nonsusceptible to sugars (for example, gin -glucose insensitive, rsr -reduced sugar response, sis -sucrose insensitive, or migmannose insensitive germination); (ii) seeds incapable of germinating (and growing) on media containing sugar concentrations that did not inhibit the development of other plants were mutants that were excessively sensitive to sugars (for example, gss -glucose super sensitive, sss -sucrose super sensitive, or hsr -high sugar-response). During the last decades, a huge progress has been made in understanding the physiological roles of sugar-metabolizing enzymes or sugar transporters, mainly by using transgenic/ mutational approaches [2,9,18,55,56,75,76]. The use of novel mutants (and transgenic plants) that specifically react to different sugars, as well as hormonal mutants, will certainly be helpful in subsequent studies on the regulatory role of sugars throughout plant development and in the elucidation of crosstalk with other signaling pathways.…”

Cytokinins and Expression of SWEET, SUT, CWINV and AAP Genes Increase as Pea Seeds Germinate

“…Transgenic work has implicated both a Clade II and a Clade IV SWEET in the germination of Arabidopsis seeds [31,32]. We show here that several PsSWEET gene family members are strongly expressed in germinating pea seeds with PsSWEET5b (a Clade II SWEET ) very strongly expressed within 4 hpi and PsSW17 (a Clade IV SWEET ) strongly up-regulated during germination.…”

“…Transgenic analysis has indicated that SWEET s may be activated during seed germination. Seeds over-expressing AtSWEET16 (a Clade IV SWEET ) germinated faster than controls [31] as did those over-expressing AtSWEET4 (a Clade II SWEET ) [32]. However, the expression of endogenous SWEET s during germination has yet to be shown.…”