The effect of light on the gravimorphogenesis of cucumber seedlings

Light affects the responses of plants to gravity. For example, light is inhibitory to the gravimorphogenesis of cucumber seedlings, peg formation. We have suggested that auxin regulates peg formation by stimulating the expression of auxin-inducible genes including CsACS1 that encodes key enzyme in ethylene synthesis. We also have hypothesized that ethylene promotes the development of peg in cucumber seedlings. To understand how light inhibits peg formation, we examined the effects of light on the auxin-induced peg formation, the expression of auxin-inducible genes and ethylene production in cucumber seedlings. Number of seedlings that produced a peg in response to applied IAA was significantly decreased by exposure to light compared with that in the dark. In addition, exposure of seedlings to light decreased mRNA accumulation of CsACS1 and ethylene production. However ethylene application did not compensate the inhibition of peg formation by light. These results suggest that light blocks auxin-mediated step(s) including CsACS1 expression for the initiation of peg formation.