Purine Nucleoside Transport in Petunia Pollen Is an Active, Carrier-Mediated System Not Sensitive to Nitrobenzylthioinosine and Not Renewed during Pollen Tube Growth

Abstract

Adenosine and guanosine are transported into Petunia hybrida pollen by a saturable, carrier-mediated mechanism. The energy poisons carbonylcyanide-m-chlorophenylhydrazone, 2,4-dinitrophenol, 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole, and N,N'-dicyclohexylcarbodiimide all inhibit uptake, suggesting an energy coupled (active) transport process. Transport takes place against a concentration gradient, strongly favoring an active transport mechanism. The purine nucleoside transport in Petunia pollen differs from that already reported for pyrimidine nucleosides in that it exhibits a significantly higher K. for nucleoside and is not so severely inhibited by the polyamine, spermine. Like that for the pyrimidine nucleosides uridine and cytosine, however, the system exhibits a broad pH optimum, is inhibited by sulfydryl-binding reagents, while the potent inhibitors of nucleoside transport in animal cells, nitrobenzylthioinosine and dipyridamole, have no effect. Transport of both purine and pyrimidine nucleosides in germinating pollen decreases steadily with time, a finding consistent with reports that RNA synthesis and DNA repair are early events of pollen germination and tube elongation. However, since these precursors are often used to demonstrate nucleic acid synthesis, it cannot be ruled out that the lack of precursor transport itself leads to scoring nucleic acid synthesis as negative. The results indicate that the newly synthesized pollen tube membranes contain little or no nucleoside transporters.In Petunia hybrida, when a pollen grain alights on the stigma, germination of the pollen grain takes place, pollen tube formation is initiated and the pollen tube elongates down through the female tissue of the stigma and then the style. During these processes there is a close interaction between pollen and style (3,14) and it is known that some metabolites are taken up from the style by the pollen and pollen tube (14). A study of the transport processes involved is therefore of relevance to our understanding of pollen-pistil interaction. Although DNA replication may not take place during pollen germination in many species (5), it is known that DNA repair (4-8) and RNA synthesis (17) occur at this time, implying a need for precursors ofnucleic acid synthesis for these processes. The most readily utilized precursors of nucleic acid biosynthesis, the purine and pyrimidine nucleosides, are known to accumulate in Petunia styles soon after pollination (24) and could therefore be available for germinating pollen depending on the transport mechanisms possessed by the pollen.Pyrimidine nucleoside transport systems have already been described for germinating Petunia pollen (1 1, 12). We have shown that pyrimidine nucleosides are taken up by an active, carriermediated mechanism except for thymidine which appears to be transported by a nonactive, carrier-mediated process. This communication describes the transport system for purine nucleosides in germinating Petunia pollen, and its relative activity, together with that for pyri...

Characterization of three novel members of the Arabidopsis thaliana equilibrative nucleoside transporter (ENT) family

“…Uptake studies using 32 P-labelled ATP and ADP confirmed that these metabolites are not substrates for AtENT4 and 7 (results not shown). This is in good agreement with our findings on Arabidopsis leaves (Figure 1), and also with import capacities of cotyledons from germinating castor beans, potato tuber discs, cultured cells of white spruce (Picea glauca) and petunia (Petunia hybrida) pollen [1,[23][24][25][26]. In this respect, the transport properties of AtENT6 and 7 resemble the biochemical features of AtENT1 [9].…”

Biochemical and genetic characterization of 5-fluoro-2?-deoxyuridine-resistant mutants of Arabidopsis thaliana

“…In Escherichia coli cells, however, more than one transport system has been demonstrated; these systems differ in their affinity for different nucleosides (Mygind and Munch-Petersen 1975). Kamboj and Jackson (1987) reported that some properties of purinenucleoside transport were different from those of pyrimidine-nucleoside transport in Petunia pollen. The finding that furl/furl plants had significantly decreased accumulation of pyrimidine nucleosides but not purine nucleosides provides preliminary evidence that purine and pyrimidine nucleosides might be transported by different systems in Arabidopsis.…”

“…Wasternack (1976) suggested that all pyrimidine nucleosides were transported by a simple diffusion mechanism in Euglena while Suss and Tupy (1982) showed that the uridine-uptake system was saturable in tobacco pollen. Kamboj and Jackson (1984, 1987 undertook the investigation of nucleoside transport in Petunia pollen. They Jackson 1984, 1985) showed that uridine and cytidine were actively transported, while thymidine transport had the properties of a nonactive, carrier-mediated system.…”

Pollen metabolome analysis reveals adenosine as a major regulator of dendritic cell–primed TH cell responses

“…The presence of nucleosides in pollen might result from active, carrier-dependent import, which is a prerequisite for RNA synthesis during early pollen germination. 26,27 APEs generated from freshly collected birch pollen contain adenosine in micromolar concentrations, whereas concentrations of other nucleosides are low. The unique accumulation of adenosine in fresh birch pollen grains might be a consequence of ATP breakdown.…”