Effects of light on the hormonal regulation of VR-ACS6 truncated promoter in transgenic tobacco

Promoter of a mungbean (Vigna radiata L.) ACC synthase gene (VR-ACS6) was specifically activated by auxin and the response was modulated by several plant hormones. A 400 bp truncated promoter region of VR-ACS6/GUS chimeric construct was tested for auxin inducibility and hormonal responses in transgenic tobacco (Nicotiana tabacum cv. Samsun). The truncated promoter (-214 to +186) of VR-ACS6 gene showed similar GUS expression level as the full-length promoter (-1531 to +186) without indole-3-acetic acid (IAA) treatment. The truncated promoter had the same positive response to IAA as the full-length promoter and a negative response to methyl jasmonate and abscissic acid. Interestingly, the brassinosteroid (BR) effect on the promoter was remarkably different between etiolated and green seedlings. Maximum GUS activity was found at 10 μM IAA in etiolated seedlings, whereas maximum GUS activity was detected at 0.1 μM IAA in the IAA dose-response curve for the green seedlings. When the etiolated seedlings were de-etiolated, the IAA dose-response curve changed completely. Furthermore, the enhancer effect of BR shown in etiolated seedlings was eliminated in green seedlings. These results suggest that light signals might modulate the promoter activity of VR-ACS6 possibly via interaction with other hormonal signals.