Two evolutionarily duplicated domains individually and post‐transcriptionally control SWEET expression for phloem transport

Abstract

Summary Cell type‐specific gene expression is critical for the specialized functions within multicellular organisms. In Arabidopsis, SWEET11 and SWEET12 sugar transporters are specifically expressed in phloem parenchyma (PP) cells and are responsible for sucrose efflux from the PP, the first step of a two‐step apoplasmic phloem‐loading strategy that initiates the long‐distance transport of sugar from leaves to nonphotosynthetic sink tissues. However, we know nothing about what determines the PP cell‐specific expression of these SWEETs. Sequence deletions, histochemical β‐glucuronidase (GUS) analysis, cross‐sectioning, live‐cell imaging, and evolutionary analysis were used to elucidate domains responsible for PP specificity, while a Förster resonance energy transfer (FRET) sensor‐based transport assay was used to determine whether substrate specificity coevolved with PP specificity. We identified two domains in the Arabidopsis SWEET11 coding sequence that, along with its promoter (including 5′ UTR), regulate PP‐specific expression at the post‐transcriptional level, probably involving RNA‐binding proteins. This mechanism is conserved among vascular plants but independent of transport substrate specificity. We conclude that two evolutionarily duplicated coding sequence domains are essential and individually sufficient for PP‐specific expression of SWEET11. We also provide a crucial experimental tool to study PP physiology and development.

SWEET13 transport of sucrose, but not gibberellin, restores male fertility in Arabidopsis sweet13;14

“…GA and sucrose functions are tightly interrelated and both are required for growth, therefore one may hypothesize that GA supplementation rescued the fertility of the sweet13;14 double mutant, though an indirect effect. Of note, many physiological studies have implied that Clade 3 SWEETs function in sucrose transport processes, and in all cases starch levels were impacted, including phloem loading in Arabidopsis and maize, seed filling in Arabidopsis and rice, and, notably, nectar secretion in Arabidopsis (28)(29)(30)(31)(32). In the case of phloem loading, the presence of SWEET11, 12 and 13 in the phloem parenchyma -the cell predicted to supply sucrose to the adjacent sieve element companion cell complex, which imports sucrose via the SUT H + /sucrose symporter -provides additional support for the physiological function in sucrose transport (33).…”

SWEET11 and SWEET12 transporters function in tandem to modulate sugar flux in Arabidopsis: An account of the underlying unique structure–function relationship

“…The CTD was highly non-conserved for AtSWEET11 and AtSWEET12 orthologs from different plant species ( Supplementary Figure 11 ). Recently, it has been shown that phloem parenchyma cell-specific expression of AtSWEET11 is regulated at the post-transcriptional level by the two evolutionary duplicated domains in the AtSWEET11 coding sequence along with its promoter including 5′ UTR through RNA-binding protein (Zhang et al, 2021).…”

“…The differential expression of AtSWEET11 and AtSWEET12 orthologs also support their specific roles in different plant species ( Supplementary Figure 8 ). Moreover, it has been shown that the phloem-specific expression of AtSWEET11 is controlled at post-transcriptional level (Zhang et al, 2021). More studies are needed to understand the regulation of these two transporters in controlling sugar availability to pathogens and in restricting disease development.…”

Sucrose rather than GA transported by AtSWEET13 and AtSWEET14 supports pollen fitness at late anther development stages

“…Paraffin-embedded cross-sectioning was performed as described with minor modifications (Zhang et al ., 2021). GUS-stained samples of entire inflorescence from pSWEET13:SWEET13-GUS and pSWEET14:SWEET14-GUS plants were washed three times by PBS (pH 7.2) and fixed with cold 4% paraformaldehyde and 2.5 % sucrose buffer overnight.…”

“…More than 6 flowers at each stage were assayed for one repeat, and four independent repeats were conducted. For starch staining on wax embedded cross-sections, after stage 12 flower buds were fixed in Carnoy fixative overnight before moving to 70% ethanol, samples were dehydrated and embedded in wax as previously described (Zhang et al, 2021).…”