Induction of HDMBOA-Glc accumulation and DIMBOA-Glc 4-O-methyltransferase by jasmonic acid in poaceous plants

Induction of the accumulation of 2-(2-hydroxy-4,7-dimethoxy-1,4-benzoxazin-3-one)-beta-D-glucopyranose (HDMBOA-Glc) by jasmonic acid (JA) was investigated in wheat, Job's tears (Coix lacryma-jobi), and rye. An increase in HDMBOA-Glc and a corresponding decrease in 2-(2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one)-beta-D-glucopyranose (DIMBOA-Glc) were found in wheat and Job's tears, whereas no such changes were observed in rye. The activity of S-adenosyl-L-methionine:DIMBOA-Glc 4-O-methyltransferase which catalyzes the conversion of DIMBOA-Glc to HDMBOA-Glc was detected in wheat leaves treated with 50 micro M JA. The activity started to increase 3 h after treatment with JA, reached a maximum after 9 h, and then decreased gradually. This mode of induction was well correlated with that for the accumulation of HDMBOA-Glc, indicating the induction of enzyme activity was responsible for the accumulation of HDMBOA-Glc. The enzyme was purified from JA-treated wheat leaves by three steps of chromatography, resulting in 95-fold purification. The enzyme showed strict substrate specificity for DIMBOA-Glc with a K(m) value of 0.12 mM. DIBOA-Glc was also accepted as substrate, but the K(m) value was 10 times larger than that for DIMBOA-Glc. The aglycones, DIMBOA and DIBOA, were not methylated by the enzyme. The K(m) value for S-adenosyl-L-methionine was 0.06 mM. The optimum pH and temperature were 7.5 and 35 degrees C, respectively. The activity was slightly enhanced by the presence of 1 mM EDTA, while heavy metal ions at 5 mM completely inhibited the activity.