A male gametophyte‐specific monosaccharide transporter inArabidopsis

Abstract

SummaryThe AtSTP2 gene (sugar transport protein 2) of Arabidopsis thaliana encodes a high affinity, low specificity monosaccharide carrier that can transport a number of hexoses and pentoses at similar rates. AtSTP2 has 12 putative transmembrane helices and a molecular mass of 55.0 kDa. AtSTP2 expression was localized in AtSTP2 promoter-β-glucuronidase (GUS) Arabidopsis plants showing AtSTP2-driven GUS activity during pollen maturation and also in germinating pollen. Immunohistochemical studies with anti-AtSTP2 antiserum as well as RNA in situ hybridization analyses modified these results and showed that AtSTP2 expression is confined to the early stages of gametophyte development. Both AtSTP2 mRNA and AtSTP2 protein are first seen at the time of beginning callose degradation and microspore release from the tetrades. AtSTP2 mRNA and AtSTP2 protein are no longer detected after the mitotic divisions and the formation of the trinucleate gametophyte. No AtSTP2 mRNA or AtSTP2 protein is seen in fully developed or germinating pollen. The putative role of AtSTP2 in the uptake of glucose units resulting from callose degradation during pollen maturation is discussed.

Identification of a pollen‐specific sucrose transporter‐like proteinNtSUT3 from tobacco

“…The identification of NtSUT3 as a putative pollen-specific sucrose carrier is in accordance with the description of numerous pollen-specific genes that encode transport proteins having homologs in the sporophyte [19,20,31,32]. Nevertheless, this is the first report of a sucrose transporter-like protein specifically expressed in one single sink organ.…”

“…A proton/sucrose cotransport mechanism has been proposed in lily pollen tubes [18], whereas hexose carriers have recently been cloned and found to be specifically expressed in the pollen of petunia [20] and ArabMopsis [19]. Expression of AtSTP2 is limited to early stages of gametophyte development at the beginning of callose degradation and microspore release [19] and AtSTP2 would transport callose-derived monosaccharides. Pmtl is expressed specifically in the male gametophyte, and high levels of m R N A accumulate in mature pollen and in pollen germinated for 13 h [20].…”

“…NtSUT3 is expressed when pollen grains mature in the thecal fluid of the loculus and take up sugar from this fluid [30]. No data on the exact nature of sugars taken up during pollen maturation are available except for the cloning of hexose transporters in petunia and Arabidopsis [19,20]. NtSUT3 could thus mediate the uptake of sucrose in the pollen grain during maturation, particularly in tobacco pollen where sucrose represents by far the major soluble sugar (sucrose content is approx.…”

“…Surprisingly, the mechanisms of nutrient uptake is not known at the present time. Based on physiological data, the presence of a proton/sucrose mechanism in the pollen tube of the lily was suggested [18] but some recent data report on the cloning of pollen-specific hexose transporters in Arabidopsis (AtSTP2 [19]) and petunia (pmtl [20]). Therefore the form of carbon uptake (sucrose vs. hexose) in the pollen is not clear at the present time.…”

Analysis of Transport Activity of Arabidopsis Sugar Alcohol Permease Homolog AtPLT5

“…In contrast, AtSTP1 transports hexoses only in the CI conformation (23). For STPtype transporters, with the exception of STP6 in pollen, transport rates for fructose were typically less than 6% of the rate for glucose (22,24,25,28). This is interesting because the main carbohydrate delivered to sink cells via the phloem in most plants is sucrose, which is either transported directly into sink cells or is hydrolyzed in the wall space to yield fructose and glucose.…”