Abscisic Acid Coordinates Nod Factor and Cytokinin Signaling during the Regulation of Nodulation in Medicago truncatula

Abstract

Nodulation is tightly regulated in legumes to ensure appropriate levels of nitrogen fixation without excessive depletion of carbon reserves. This balance is maintained by intimately linking nodulation and its regulation with plant hormones. It has previously been shown that ethylene and jasmonic acid (JA) are able to regulate nodulation and Nod factor signal transduction. Here, we characterize the nature of abscisic acid (ABA) regulation of nodulation. We show that application of ABA inhibits nodulation, bacterial infection, and nodulin gene expression in Medicago truncatula. ABA acts in a similar manner as JA and ethylene, regulating Nod factor signaling and affecting the nature of Nod factor-induced calcium spiking. However, this action is independent of the ethylene signal transduction pathway. We show that genetic inhibition of ABA signaling through the use of a dominant-negative allele of ABSCISIC ACID INSENSITIVE1 leads to a hypernodulation phenotype. In addition, we characterize a novel locus of M. truncatula, SENSITIVITY TO ABA, that dictates the sensitivity of the plant to ABA and, as such, impacts the regulation of nodulation. We show that ABA can suppress Nod factor signal transduction in the epidermis and can regulate cytokinin induction of the nodule primordium in the root cortex. Therefore, ABA is capable of coordinately regulating the diverse developmental pathways associated with nodule formation and can intimately dictate the nature of the plants' response to the symbiotic bacteria.

Control of root architecture and nodulation by theLATD/NIPtransporter

“…The inhibitory effect of ABA modulates the frequency of calcium spiking, such that high levels of ABA can counteract the effect of Nod factor, inhibiting nodule initiation. 35 The role of ABA in nodule initiation was further confirmed by overexpressing the dominant abi1-1 allele of the Arabidopsis (ABI1) ABSCISIC ACID INSENSITIVE1 gene in Medicago, which interferes with ABA signaling and confers a hypernodulation phenotype. 35 The phenotype of the latd mutant, in which nodules arrest shortly after emerging from the root, suggests a second role for ABA in nodule development at a later stage than initiation, probably via a stimulatory effect on meristem function.…”

“…35 The role of ABA in nodule initiation was further confirmed by overexpressing the dominant abi1-1 allele of the Arabidopsis (ABI1) ABSCISIC ACID INSENSITIVE1 gene in Medicago, which interferes with ABA signaling and confers a hypernodulation phenotype. 35 The phenotype of the latd mutant, in which nodules arrest shortly after emerging from the root, suggests a second role for ABA in nodule development at a later stage than initiation, probably via a stimulatory effect on meristem function. 26 Phenotypes of other legume ABA signaling mutants suggest a complex pathway with at least one branch regulating nodulation, and with a second branch not affecting nodulation.…”

Deep Sequencing of the Medicago truncatula Root Transcriptome Reveals a Massive and Early Interaction between Nodulation Factor and Ethylene Signals

“…S5). ABA suppresses calcium spiking, nodulin expression, and bacterial infection and nodule development (Ding et al, 2008). In addition to ABA biosynthesis, Nod factors also stimulated the transcription of ABA hydroxylase, a negative regulator of ABA signaling.…”

Environmental nitrate signals through abscisic acid in the root tip

“…Classic examples include ABA regulation of stomatal closing, calcium spiking during initiation of legume nodulation and elongation of both primary and lateral roots. [38][39][40][41] Thus, the careful control of ABA levels can result in a more nuanced response to environmental signals.…”