Salinity Tolerance of Two Potato Cultivars (Solanum tuberosum) Correlates With Differences in Vacuolar Transport Activity.

Potato is an important cultivated crop species and since it is moderately salt sensitive there is a need to develop more salt tolerant cultivars. A high activity of Na+ transport across the tonoplast in exchange for H+ is essential to reduce Na+ toxicity. The proton motive force (PMF) generated by the V-H+-ATPase and the V-H+-PPase energizes the Na+(K+)/H+ antiport. We compared the activity, gene expression, and protein levels of the vacuolar proton pumps and the Na+/H+ antiporters in two potato cultivars (Solanum tuberosum) contrasting in their salt tolerance (cv. Desiree; tolerant and Mozart; sensitive) grown at 0 and 60 mM NaCl. Tonoplast-enriched vesicles were used to study the pump activity and protein levels of the V-H+-ATPase and the V-H+-PPase and the activity of the Na+/H+ antiporter. Although salt stress reduced the V-H+-ATPase and the V-H+-PPase activity in both cultivars, the decline in H+ pump activity was more severe in the salt-sensitive cultivar Mozart. After salt treatment, protein amounts of the vacuolar H+ pumps decreased in Mozart but remained unchanged in the cultivar Desiree. Decreased protein amounts of the V-H+-PPase found in Mozart may explain the reduced V-H+-PPase activity found for Mozart after salt stress. Under non-stress conditions, protein amounts of V-H+-PPase were equal in both cultivars while the V-H+-PPase activity was already twice as high and remained higher after salt treatment in the cultivar Desiree as compared to Mozart. This cultivar-dependent V-H+-PPase activity may explain the higher salt tolerance of Desiree. Moreover, combined with reduced vacuolar H+ pump activity, Mozart showed a lower Na+/H+ exchange activity and the Km for Na+ is at least twofold lower in tonoplast vesicles from Desiree, what suggests that NHXs from Desiree have a higher affinity for Na+ as compared to Mozart. From these results, we conclude that the higher capacity in combination with the higher affinity for Na+ uptake can be an important factor to explain the differences in salt tolerance of these two potato cultivars.

Differences in proton pumping and Na/H exchange at the leaf cell tonoplast between a halophyte and a glycophyte.

The tonoplast Na(+)/H(+) antiporter and tonoplast H(+) pumps are essential components of salt tolerance in plants. The objective of this study was to investigate the transport activity of the tonoplast Na(+)/H(+) antiporter and the tonoplast V-H(+)-ATPase and V-H(+)-PPase in a highly tolerant salt-accumulating halophyte, Salicornia dolichostachya, and to compare these transport activities with activities in the related glycophyte Spinacia oleracea. Vacuolar membrane vesicles were isolated by density gradient centrifugation, and the proton transport and hydrolytic activity of both H(+) pumps were studied. Furthermore, the Na(+)/H(+)-exchange capacity of the vesicles was investigated by 9-amino-6-chloro-2-methoxyacridine fluorescence. Salt treatment induced V-H(+)-ATPase and V-H(+)-PPase activity in vesicles derived from S. oleracea, whereas V-H(+)-ATPase and V-H(+)-PPase activity in S. dolichostachya was not affected by salt treatment. Na(+)/H(+)-exchange capacity followed the same pattern, i.e. induced in response to salt treatment (0 and 200 mM NaCl) in S. oleracea and not influenced by salt treatment (10 and 200 mM NaCl) in S. dolichostachya. Our results suggest that S. dolichostachya already generates a high tonoplast H(+) gradient at low external salinities, which is likely to contribute to the high cellular salt accumulation of this species at low external salinities. At high external salinities, S. dolichostachya showed improved growth compared with S. oleracea, but V-H(+)-ATPase, V-H(+)-PPase and Na(+)/H(+)-exchange activities were comparable between the species, which might imply that S. dolichostachya more efficiently retains Na(+) in the vacuole.

Hyperacidification of vacuoles by the combined action of two different P-ATPases in the tonoplast determines flower color.

The acidification of endomembrane compartments is essential for enzyme activities, sorting, trafficking, and trans-membrane transport of various compounds. Vacuoles are mildly acidic in most plant cells because of the action of V-ATPase and/or pyrophosphatase proton pumps but are hyperacidified in specific cells by mechanisms that remained unclear. Here, we show that the blue petal color of petunia ph mutants is due to a failure to hyperacidify vacuoles. We report that PH1 encodes a P3B-ATPase, hitherto known as Mg2(+) transporters in bacteria only, that resides in the vacuolar membrane (tonoplast). In vivo nuclear magnetic resonance and genetic data show that PH1 is required and, together with the tonoplast H(+) P3A-ATPase PH5, sufficient to hyperacidify vacuoles. PH1 has no H(+) transport activity on its own but can physically interact with PH5 and boost PH5 H(+) transport activity. Hence, the hyperacidification of vacuoles in petals, and possibly other tissues, relies on a heteromeric P-ATPase pump.

Effect of salt stress on growth, Na+ accumulation and proline metabolism in potato (Solanum tuberosum) cultivars.

Potato (Solanum tuberosum) is a major crop world-wide and the productivity of currently used cultivars is strongly reduced at high soil salt levels. We compared the response of six potato cultivars to increased root NaCl concentrations. Cuttings were grown hydroponically and treated with 0 mM, 60 mM and 180 mM NaCl for one week. Growth reduction on salt was strongest for the cultivars Mozart and Mona Lisa with a severe senescence response at 180 mM NaCl and Mozart barely survived the treatment. The cultivars Desiree and Russett Burbank were more tolerant showing no senescence after salt treatment. A clear difference in Na(+) homeostasis was observed between sensitive and tolerant cultivars. The salt sensitive cultivar Mozart combined low Na(+) levels in root and stem with the highest leaf Na(+) concentration of all cultivars, resulting in a high Na(+) shoot distribution index (SDI) for Mozart as compared to Desiree. Overall, a positive correlation between salt tolerance and stem Na(+) accumulation was found and the SDI for Na(+) points to a role of stem Na(+) accumulation in tolerance. In stem tissue, Mozart accumulated more H2O2 and less proline compared to the tolerant cultivars. Analysis of the expression of proline biosynthesis genes in Mozart and Desiree showed a clear reduction in proline dehydrogenase (PDH) expression in both cultivars and an increase in pyrroline-5-carboxylate synthetase 1 (P5CS1) gene expression in Desiree, but not in Mozart. Taken together, current day commercial cultivars show promising differences in salt tolerance and the results suggest that mechanisms of tolerance reside in the capacity of Na(+) accumulation in stem tissue, resulting in reduced Na(+) transport to the leaves.