Role of cytokinin and salicylic acid in plant growth at low temperatures

Low temperature restrains plant growth by inhibiting the cell cycle, and phytohormones play important roles in this case; however, the molecular mechanisms whereby phytohormones affect growth at low temperature are largely unknown. When grown at 23, 16, 10, and 4°C, we found that Arabidopsis thaliana could develop with normal morphology, but needed a prolonged period of cultivation. By screening mutants, we could implicate cytokinin and salicylic acid. At 4°C, both amp1 plants, which have an increased level of cytokinin, and wild-type plants treated with exogenous cytokinin, displayed relative growth rates greater than control by increasing total cell number. Additionally, transgenic NahG plants, which have lower salicylic acid content, grew faster than wild-type accompanied by larger cells. Expression of C-repeat binding transcription factors (CBFs), that mediate cold acclimation by stimulation of the expression of cold-inducible genes, was similar in all tested genotypes. Thus CBF expression did not correlate with the observed enhanced growth in mutants. The improved growth coincided with elevated expression of CYCD3;1, especially in NahG plants. At 4°C, enhanced endoreduplication took responsibility for larger cells in NahG plants, while enhanced cell division was observed in amp1 plants.