Gene Expression and K⁺ Uptake of Two Tomato Cultivars in Response to Sub-Optimal Temperature

Sub-optimal temperatures can adversely affect tomato (Solanum lycopersicum) growth, and K⁺ plays an important role in the cold tolerance of plants. However, gene expression and K⁺ uptake in tomato in response to sub-optimal temperatures are still not very clear. To address these questions, one cold-tolerant tomato cultivar, Dongnong 722 (T722), and one cold-sensitive cultivar, Dongnong 708 (S708), were exposed to sub-optimal (15/10 °C) and normal temperatures (25/18 °C), and the differences in growth, K⁺ uptake characteristics and global gene expressions were investigated. The results showed that compared to S708, T722 exhibited lower reduction in plant growth rate, the whole plant K⁺ amount and K⁺ net uptake rate, and T722 also had higher peroxidase activity and lower K⁺ efflux rate under sub-optimal temperature conditions. RNA-seq analysis showed that a total of 1476 and 2188 differentially expressed genes (DEGs) responding to sub-optimal temperature were identified in S708 and T722 roots, respectively. Functional classification revealed that most DEGs were involved in "plant hormone signal transduction", "phenylpropanoid biosynthesis", "sulfur metabolism" and "cytochrome P450". The genes that were significantly up-regulated only in T722 were involved in the "phenylpropanoid biosynthesis" and "plant hormone signal transduction" pathways. Moreover, we also found that sub-optimal temperature inhibited the expression of gene coding for K⁺ transporter SIHAK5 in both cultivars, but decreased the expression of gene coding for K⁺ channel AKT1 only in S708. Overall, our results revealed the cold response genes in tomato roots, and provided a foundation for further investigation of mechanism involved in K⁺ uptake in tomato under sub-optimal temperatures.