Osmotic stress- and salt stress-inhibition and gibberellin-mitigation of leaf elongation associated with up-regulation of genes controlling cell expansion

• Salt and osmotic stress inhibited leaf elongation of tall fescue.
• Gibberellic acid promoted leaf elongation under osmotic or salt stress.
• GA effects on leaf elongation was due to its up-regulation of cell-wall expansion genes.

Leaf elongation rates (LER) is sensitive to salt stress and osmotic stress, and gibberellic acid (GA) may mediate stress-inhibition of leaf elongation by affecting genes controlling cell expansion. The objectives of this study were to examine the differential inhibitory effects of osmotic stress and salt stress on LER for fast- and slow-growing genotypes of tall fescue (Festuca arundinacea) and to determine whether GA could mitigate stress-induced decline in LER. Plants of fast-growing ‘K-31′ and slow-growing ‘Bonsai’ were grown hydroponically and treated with GA under either osmotic or salt stress. LER of both genotypes were inhibited by osmotic and salt stress (46–86%) due to reduced cell elongation and production rate. ‘Bonsai’ LER was reduced to a greater extent than that of ‘K-31′ due to greater reduction in cell production rate. Exogenous GA application significantly enhanced LER regardless of stress for both genotypes, which was more pronounced in ‘Bonsai’ than in ‘K-31′ caused by greater cell elongation and production rates. Quantitative PCR analysis revealed that four expansin genes (EXPA4, EXPA5, EXPA7 and EXPB7) and one XET gene (XET2) were down-regulated by osmotic stress and EXPA7 was down-regulated by salt stress in both genotypes. Three expansin genes (EXPA4, EXPA5 and EXPA7) were up-regulated by GA treatment in both genotypes under non-stress conditions, in which EXPA4 was also up-regulated by GA treatment in both genotypes under osmotic stress. The expression level of XET1 and XET3 increased in ‘K-31′ and ‘Bonsai’, respectively, with exogenous GA treatment under non-stress and salt stress. This study demonstrated the roles of GA regulation of genetic variations in leaf differential responses to osmotic and salt stress involving different expansins and XET genes.