Drought Stress Reduces Stem Elongation and Down-regulates Gibberellin Biosynthesis in Tomatoes during Vegetative Growth

Drought stress can reduce whole plant and cell elongation. Since gibberellins play an important role in controlling cell elongation, our objective was to quantify the effects of drought stress on elongation and gibberellin homeostasis. We exposed ‘Moneymaker’ tomatoes to drought stress to observe the effect on internode elongation and gibberellin metabolism- and signaling-related gene expression. Plants were grown from seed in 15-cm pots filled with a peat–perlite substrate in a greenhouse for 25 days. Irrigation was automated using a datalogger, which maintained substrate moisture levels of 35% and 15% (v/v) for well-watered and drought stressed conditions, respectively. Drought stress reduced total plant height (P = 0.0203), internode length (P < 0.0001), and cell size (P = 0.002) compared to well-watered conditions. To further investigate the effect of gibberellins on elongation, paclobutrazol was applied as a second treatment level to reduce gibberellin biosynthesis. We analyzed the transcription levels of SlGA20ox1, -2, -3, and -4, SlGA3ox2, and SlGA2ox2, -4, and -5, genes coding for enzymes catalyzing later steps of gibberellin biosynthesis. Transcript levels of LeEXP1, and -2, encoding for expansins related to the loosening of the cell wall necessary for cell expansion, was also analyzed. Down regulation of transcript levels due to drought stress was observed for SlGA20ox4, SlGA2ox5, and LeEXP1, but not for any of the other genes. Paclobutrazol upregulated the expression levels of SlGA20ox1, and -3, and SlGA3ox2. Application of paclobutrazol reduced elongation and the up-regulation of these genes suggests altered regulation of gibberellin biosynthesis to compensate for their lowered production. These findings emphasize the inhibiting effect drought can have on elongation and gibberellin metabolism during vegetative growth. Understanding the impact of drought stress on plant growth may help growers better plan for water shortage periods.