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

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 K_m 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.