Minisymposium 4: Oxidative Mechanisms

Abs # 14004: Maintenance of root growth by ABA accumulation under water deficits: prevention of high levels of reactive oxygen species in the root growth zone

Presenter:	Sharp, Robert E, SharpR@missouri.edu
Authors	Sharp, Robert E (A)   Cho, In-Jeong  (A)   Sivaguru, Mayandi  (B)
Affiliations:	(A): Dept. Agronomy, Univ. Missouri-Columbia (B): Molecular Cytology Core Facility, Univ. Missouri-Columbia

Abscisic acid (ABA) accumulates to high concentrations in plants under water deficits, and this response has generally been considered to cause growth inhibition. This view has arisen largely from studies of the effects of exogenous ABA on well-watered plants. However, the use of ABA-deficient mutants to manipulate endogenous ABA levels has demonstrated that (a) the normal ABA levels in well-watered plants are required to maintain shoot growth in tomato and Arabidopsis, and (b) the accumulation of ABA under water deficits is required for root growth maintenance in maize seedlings. In all cases, the action of ABA involves suppression of ethylene production. Taken together, these studies indicate that water-deficient compared to well-watered plants require increased levels of ABA to prevent ethylene-induced growth inhibition. This difference may be related to a role of ABA accumulation in regulating the antioxidative system to maintain reactive oxygen species (ROS) at non-damaging levels during water deficits. Levels of ROS in the root apical region (encompassing the growth zone) were studied with the fluorescent dye Carboxy-H₂DCFDA, and imaged using confocal microscopy. The effect of ABA deficiency under water stress was studied using the vp14 mutant, in which ABA levels are deficient in water-stressed but not well-watered roots. Under well-watered conditions, ROS levels were low in roots of both wild-type and vp14 seedlings. Under water deficits, ROS levels were slightly greater in wild-type roots and increased dramatically in vp14. The increased ROS levels in vp14 were prevented when ABA was restored to the wild-type level by exogenous application. Interestingly, the effect of ABA-deficiency on ROS levels occurred specifically in the region 1-3 mm from the root apex where cell elongation is normally maintained under water deficits but is inhibited by ABA deficiency. Imaging with the dye propidium iodide provided evidence of loss of plasma membrane integrity in the same region of the ABA-deficient roots. The relation of the increase in ROS to the increase in ethylene production and inhibition of elongation in ABA-deficient roots under water deficits is under investigation.