Environmental Nitrate Stimulates Abscisic Acid Accumulation in Arabidopsis Root Tips by Releasing It from Inactive Stores

Abstract

Abscisic acid (ABA) signaling plays a major role in root system development, regulating growth and root architecture. However, the precise localization of ABA remains undetermined. Here, we present a mechanism in which nitrate signaling stimulates the release of bioactive ABA from the inactive storage form, ABA-glucose ester (ABA-GE). We found that ABA accumulated in the endodermis and quiescent center of Arabidopsis thaliana root tips, mimicking the pattern of SCARECROW expression, and (to lower levels) in the vascular cylinder. Nitrate treatment increased ABA levels in root tips; this stimulation requires the activity of the endoplasmic reticulum-localized, ABA-GE-deconjugating enzyme b-GLUCOSIDASE1, but not de novo ABA biosynthesis. Immunogold labeling demonstrated that ABA is associated with cytoplasmic structures near, but not within, the endoplasmic reticulum. These findings demonstrate a mechanism for nitrate-regulated root growth via regulation of ABA accumulation in the root tip, providing insight into the environmental regulation of root growth.

Environmental nitrate signals through abscisic acid in the root tip

“…1). 33 This ABA signaling module is activated by stimulation of b-GLUCOSIDASE 1 (AtBG1) expression within 2 hours. 33 AtBG1 encodes an ER-localized b-glucosidase that cleaves the inactive ABA conjugate, ABA glucose ester (ABA-GE), releasing ABA, which is transported to the cytoplasm, where it accumulates.…”

“…33 AtBG1 encodes an ER-localized b-glucosidase that cleaves the inactive ABA conjugate, ABA glucose ester (ABA-GE), releasing ABA, which is transported to the cytoplasm, where it accumulates. 33,34 Subsequently, expression of both ABA-responsive and nitrate-responsive genes is induced.…”

“…Although nitrate-induced stimulation of AtBG1 expression is quite rapid, and exogenous ABA induces the expression of both nitrate-responsive and ABA-responsive genes within 2 hours, nonetheless, nitrate induction of these same genes is much slower, and takes approximately 2 d to show a significant difference. 33 This could be simply the result of nitrate-induced changes in gene expression that are restricted to a small zone of the root, say, the root tip, and are thus swamped out when the entire root is used to analyze gene expression, as compared with ABA, which may induce the expression of these genes throughout the root. However, ABA accumulation itself appears to be delayed.…”

“…33 In addition, analysis of a ProRAB18: GFP reporter gene fusion indicates that the response is largely constrained to the root tip both after ABA and after nitrate treatment and yet the nitrate-induced ProRAB18:GFP expression features a similar 2-day lag. 33 These observations suggest that the interval between induction of AtBG1 gene expression and ABA responses may be due to some post-transcriptional regulation of BG1 function. Lee and colleagues have shown that polymerization of the BG1 enzyme results in a 4-fold increase in enzymatic activity.…”

“…Nitrate also stimulates expression of the AtBG1 gene, encoding an ER-localized b-glucosidase, that cleaves the inactive ABA conjugate, ABA-glucose ester (ABA-GE), releasing bioactive ABA. 33 Although AtBG1 gene expression is strongly induced within 2 hours of adding nitrate, the ABA levels gradually rise over the course of 2 d, 33 binding the intracellular PYR/PYL/RCAR receptor, stimulating expression of NIA1, NIA2 and NIR1, and inactivating the ABA co-receptor, ABI2. 13,63 Once ABI2 is inactivated, the CIPK23/CBL1 complex is free to act, phosphorylating AtNPF6.3 and inhibiting its ability to transport.…”