Abstract
Although thigmotropism is an important avoidance behavior of plants, the molecular mechanism remains unclear. Using the two-layer medium test with ethylene biosynthesis modulators or mutants, we have suggested that ethylene is involved in root bending responses to physical hardness in Arabidopsis thaliana seedlings. Our recent study showed that analysis of ethylene by headspace gas chromatography identified a clear inverse relationship between ethylene production and root-bending response. QRT-PCR also showed that, among nine ACC synthase genes (ACS) examined, ACS-7 expression exclusively decreased in the bending roots, while expression of ACS-4, 9, and 11 increased in both bending and non-bending roots. Decreased and increased amounts of ethylene attenuated and enhanced root hardening. These results suggest that roots modulate the differential transcription of ACS family members catalyzing the rate-limiting step in ethylene biosynthesis, reduce or increase subsequent production of ethylene which regulates root rigidity, and consequently bend laterally or grow downward.
