The AtProT Family. Compatible Solute Transporters with Similar Substrate Specificity But Differential Expression Patterns

Abstract

Proline transporters (ProTs) mediate transport of the compatible solutes Pro, glycine betaine, and the stress-induced compound g-aminobutyric acid. A new member of this gene family, AtProT3, was isolated from Arabidopsis (Arabidopsis thaliana), and its properties were compared to AtProT1 and AtProT2. Transient expression of fusions of AtProT and the green fluorescent protein in tobacco (Nicotiana tabacum) protoplasts revealed that all three AtProTs were localized at the plasma membrane. Expression in a yeast (Saccharomyces cerevisiae) mutant demonstrated that the affinity of all three AtProTs was highest for glycine betaine (K m 5 0.1-0.3 mM), lower for Pro (K m 5 0.4-1 mM), and lowest for g-aminobutyric acid (K m 5 4-5 mM). Relative quantification of the mRNA level using real-time PCR and analyses of transgenic plants expressing the b-glucuronidase (uidA) gene under control of individual AtProT promoters showed that the expression pattern of AtProTs are complementary. AtProT1 expression was found in the phloem or phloem parenchyma cells throughout the whole plant, indicative of a role in long-distance transport of compatible solutes. b-Glucuronidase activity under the control of the AtProT2 promoter was restricted to the epidermis and the cortex cells in roots, whereas in leaves, staining could be demonstrated only after wounding. In contrast, AtProT3 expression was restricted to the above-ground parts of the plant and could be localized to the epidermal cells in leaves. These results showed that, although intracellular localization, substrate specificity, and affinity are very similar, the transporters fulfill different roles in planta.

Alteration of plant primary metabolism in response to insect herbivory

“…Although expression of these transporters is required for herbivore-induced carbohydrate reallocation (Ferrieri et al, 2012), there is so far little evidence showing that their expression is directly influenced by herbivore attack. In contrast, genes encoding transporters for both inorganic nitrogen and specific amino acids have been shown to be differentially expressed in response to wounding (Grallath et al, 2005;Meyer et al, 2006), jasmonic acid treatment (Okamoto et al, 2006;Camañes et al, 2012), and insect herbivory (Voelckel et al, 2004;Coppola et al, 2013). Thus, these herbivore-induced changes in gene expression could provide a gateway for future efforts to incorporate resource reallocation into the current signaling and molecular framework of herbivore-induced responses.…”

Proline Metabolism and Its Implications for Plant-Environment Interaction

“…Such observations suggest the need for plasma membrane transporters that move proline. Arabidopsis Proline Transporter1 (ProT1), ProT2 and ProT3 were identified based on a combination of yeast complementation and sequence comparison (Rentsch et al, 1996;Grallath et al, 2005) and shown to be plasma membrane localized. The ProTs transport not only proline but also GABA and glycine betaine, two additional compatible solutes, but with varying affinities (Rentsch et al, 1996;Grallath et al, 2005).…”

“…Arabidopsis Proline Transporter1 (ProT1), ProT2 and ProT3 were identified based on a combination of yeast complementation and sequence comparison (Rentsch et al, 1996;Grallath et al, 2005) and shown to be plasma membrane localized. The ProTs transport not only proline but also GABA and glycine betaine, two additional compatible solutes, but with varying affinities (Rentsch et al, 1996;Grallath et al, 2005). The three ProTs also have differing patterns of tissue-specific expression, with ProT2 being highest in roots and ProT1 being higher in stems and flowers (Grallath et al, 2005).…”

“…The ProTs transport not only proline but also GABA and glycine betaine, two additional compatible solutes, but with varying affinities (Rentsch et al, 1996;Grallath et al, 2005). The three ProTs also have differing patterns of tissue-specific expression, with ProT2 being highest in roots and ProT1 being higher in stems and flowers (Grallath et al, 2005). It might be expected that a proline transporter would be induced by stress when proline levels are also increasing.…”

Isolation and characterisation of the ADP-glucose pyrophosphorylase small subunit gene (AgpS1) promoter in tomato (Solanum lycopersicum L.)

“…Indeed, a phloemspecific proline transporter (AtProT1) was reported in Arabidopsis, and its role in long-distance phloem transport via the vascular system was suggested (Grallath et al 2005). Considering the phloem-specific expression of the AgpS1 gene, it is likely that proline transported via phloem is involved in the regulation of AgpS1 expression.…”