Abstract
Blue light (BL) induces many adaptive responses in plants, including shoot growth inhibition and phototropism. To clarify the mechanism of growth inhibition in response to unilateral BL illumination, the isolation and identification of various growth inhibitors has been attempted in several plant species. In maize coleoptiles, benzoxazinoids (MBOA and its precursor, DIMBOA), were isolated and identified as candidate inhibitors. Previously, we found in the early stage of phototropism that unilateral BL illumination promotes P-glucosidase (DIMBOA-glucosidase) activity on the illuminated side of maize coleoptiles, releasing DIMBOA and simultaneously producing MBOA. We found that BL induces H_2O_2 accumulation via the up-regulation of DIMBOA and probably of MBOA. We also found that these compounds contribute to their growth inhibitory activity by promoting H_2O_2 synthesis and subsequent lignification (cell-wall stiffness). These results suggest a close association between an oxidative burst and phototropism. In our previous report, the unilateral application of these compounds suppressed the growth of coleoptiles on the treated side and causing them to bend towards the side of application. Furthermore, BL up-regulated the transcriptional level of DIMBOA-glucosidase. Thus, it is concluded that phototropism is triggered by a local gradient, induced at the site of illumination, of BL-induced growth inhibitors interfering with the action of the evenly distributed auxin.
