| Summary: | The effects of a decrease in red to far-red (R/FR) ratio on shoot growth of two-week-old Arabidopsis thaliana (L.) Heynh. seedlings were examined in the context of possible causal involvement of key plant growth hormones. Decreasing the R/FR ratio significantly increased petiole elongation and leaf area expansion of the Columbia (Col) line seedlings. In contrast, seedlings of the Landsberg erecta (Ler) line showed no significant change in leaf area and only a marginal increase in petiole growth. This low R/FR ratio-induced growth was accompanied by significant increases in concentrations of the growth "effector" gibberellin (GA4) and an auxin (indole-3-acetic acid (IAA)) in shoot tissues of Col. However, cytokinins (CKs) in Col shoot tissues were decreased and ethylene evolution was reduced when the R/FR ratio was decreased from that of normal sunlight to a low R/FR ratio. Several A. thaliana genotypes with plant hormone-related mutations were also assessed, including auxin resistant, axr2-1; GA insensitive, gai-1; and ethylene over-producing, eto2. None of these increased their petiole length or leaf area growth in response to lowering the R/FR ratio. We thus conclude that both GA4 and IAA are causally involved in the increased shoot growth of A. thaliana Col seedlings that occurs in response to a lower than normal R/FR ratio.
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