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.
Highlights
SWEETs are sugar transporters, which are present in plants as well as in animals and humans[15]
We investigated the expression levels of two Glutamate semialdehyde aminotransferase (GSA-AT) genes in AtSWEET4 transgenic, atsweet[] mutant and wild-type plants, quantitative RT-PCR (qRT-PCR) analysis showed that expression levels of GSA-AT1 and GSA-AT2 were reduced in RNAi4-8 and increased in OE4-4 when compared to their expression in Col-0, there was no significant difference in the expression of these two genes between atsweet[] and Col-0 (Fig. 2J,K)
Reduction in AtSWEET4 expression level leads to phenotypic alterations (Fig. 2), but this reduction has a critical threshold, changes of AtSWEET4 expression levels above or below this threshold lead to no visible phenotypic alterations (Fig. 2, Supplementary Fig. 2)
Summary
SWEETs are sugar transporters, which are present in plants as well as in animals and humans[15]. AtSWEET11 and 12 localize to the plasma membrane and mediate sucrose export from phloem parenchyma cells into the apoplasmic space to supply sucrose for the H+-coupled sugar transporter SUT1 in the SE/CC16. Arabidopsis transgenic plants overexpressing AtSWEET16 showed altered germination rate, growth phenotype, and stress tolerance[7]. AtSWEET17 is a vacuolar fructose exporter[18] It is highly expressed in the cortex of roots and functions as an energy-independent fructose carrier, overexpression of AtSWEET17 reduces the fructose content in leaves[19]. We generated AtSWEET4 overexpression and knock-down transgenic Arabidopsis lines and studied the phenotypic changes in transgenic plants with altered AtSWEET4 expression. Our results suggest that AtSWEET4 plays a key role in mediating sugar supply to the axial tissues, and that the tight control of AtSWEET4 expression is important for plant growth
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