Regulation of Water Deficit-Induced Abscisic Acid Accumulation by Apoplastic Ascorbic Acid in Maize

Water deficit-induced abscisic acid (ABA) accumulation is one of the most important stress signaling pathways in plant cells. Redox regulation of cellular signaling has currently attracted particular attention, but much less is known about its roles and mechanisms in plant signaling. Herein, we report that water deficit-induced ABA accumulation could be regulated by ascorbic acid (AA)-controlled redox status in leave apoplast. The AA content in non-stressed leaves was approximately 3 μmol/g FW, corresponding to a mean concentration of 3 mmol/L in a whole cell. Because AA is mainly localized in the cytosol and chloroplasts, the volume of which is much smaller than that of the whole cell, AA content in cytosolic and chloroplast compartments should be much higher than 3 mmol/L. Water deficit-induced ABA accumulation in both leaf and root tissues of maize seedlings was significantly inhibited by AA and reduced glutathione (GSH) at concentrations of 500 μmol/L and was completely blocked by 50 mmol/L AA and GSH. These results suggest that the AA-induced inhibition of ABA accumulation should not occur at sites where AA exists in high concentrations. Although water deficit led to a small increase in the dehydroascorbic acid (DHA) content, no significant changes in AA content were observed in either leaf or root tissues. When compared with the whole leaf cell, the AA content in the apoplastic compartment was much lower (i.e.approximately 70 nmol/g FW, corresponding to 0.7 mmol/L). Water deficit induced a significant decrease (approximately 2.5-fold) in the AA content and an increase (approximately 3.4-fold) in the DHA content in the apoplastic compartment, thus leading to a considerably decreased redox status there, which may have contributed to the relief of AA-induced inhibition of ABA accumulation, alternatively, promoting water deficit-induced ABA accumulation. Reactive oxygen species (ROS) could not mimic water deficit in inducing ABA accumulation, suggesting that the inhibition of ABA accumulation by AA or GSH was not related to their ROS-scavenging ability. The results of the present study suggest that the redox status in the apoplastic compartment, as determined by AA and DHA, may play a vital role in the regulation of the signaling process for water deficit-induced ABA accumulation.