Effects of Exogenous Spermidine on Carbon and Nitrogen Metabolism in Tomato Seedlings under High Temperature

We investigated the effects of exogenous spermidine (Spd) on the carbohydrate, nitrogen (N), and endogenous polyamine status of tomato (Solanum lycopersicum) seedlings exposed to high-temperature stress [38/28 degrees C (day/night)]. High-temperature stress reduced the contents of pyruvate and succinate and inhibited plant growth. The application of exogenous Spd alleviated the inhibition of plant growth induced by high temperature, and also led to an increase in pyruvate, citrate, and succinate levels. High temperature markedly increased the NH4+-N content and reduced the activities of nitrate reductase (NR), glutamine synthetase (GS), and glutamate dehydrogenase (GDH). Spd significantly alleviated the negative effects on NH4+-N assimilation induced by high-temperature stress. Moreover, Spd significantly increased the activities of NR and GDH in the high-temperature-stressed tomato leaves. In contrast, Spd application to high-temperature-stressed plant leaves counteracted high-temperature-induced mRNA expression changes in N metabolism. Spd significantly upregulated the transcriptional levels of NR, nitrite reductase, GS, GDH, and glutamate synthase (GOGAT). In addition, exogenous Spd significantly increased endogenous polyamines. These results suggest that Spd could improve carbohydrate and N status through regulating the gene expression and activity of key enzymes for N metabolism, thus confers the tolerance to high temperature on tomato seedlings.