Recent studies have demonstrated in leaves of maize (Zea mays L.) plants submitted to a moderate
water stress an early enhancement of vacuolar
invertase activity that paralleled the expression of the vacuolar
invertase Ivr2 gene and the accumulation of
hexoses. In this paper, the direct role of
abscisic acid (ABA) was checked by providing this
hormone to the root medium of hydroponically grown maize plantlets. ABA supplied to 10-day-old seedlings appeared to enhance the vacuolar
invertase activity within 1 h in roots and 2 h in leaves, the maximum being reached at 4 and 8 h, respectively. The Ivr2 gene expression varied accordingly, except that the maximum values were earlier. During the first 8 h of activity enhancement,
hexose and
sucrose concentrations were not significantly affected by ABA. The changes in activity were correlated to leaf and root ABA concentrations and they were concentration dependent in roots and leaves. In contrast, the addition of 1%
glucose or
polyethylene glycol, at the same osmotic potential, was ineffective on
invertase activity, but
glucose supply enhanced Ivr2 transcript levels, after 18 h, in a concentration-dependent manner in the leaf, whereas they were repressed at higher concentrations in intact roots. The latter result appeared specific to intact roots since similar treatments performed using excised leaf or root pieces confirmed a previous report on the enhancement of Ivr2 and Ivr1 transcript levels by
glucose in roots [J. Xu et al. (1996) Plant Cell 8:1209-1220]. Therefore, ABA appears to be a strong inducer of Ivr2-invertase expression in roots and leaves.