ABA Metabolism and Homeostasis in Seed Dormancy and Germination

Abstract

Abscisic acid (ABA) is a key hormone that promotes dormancy during seed development on the mother plant and after seed dispersal participates in the control of dormancy release and germination in response to environmental signals. The modulation of ABA endogenous levels is largely achieved by fine-tuning, in the different seed tissues, hormone synthesis by cleavage of carotenoid precursors and inactivation by 8'-hydroxylation. In this review, we provide an overview of the current knowledge on ABA metabolism in developing and germinating seeds; notably, how environmental signals such as light, temperature and nitrate control seed dormancy through the adjustment of hormone levels. A number of regulatory factors have been recently identified which functional relationships with major transcription factors, such as ABA INSENSITIVE3 (ABI3), ABI4 and ABI5, have an essential role in the control of seed ABA levels. The increasing importance of epigenetic mechanisms in the regulation of ABA metabolism gene expression is also described. In the last section, we give an overview of natural variations of ABA metabolism genes and their effects on seed germination, which could be useful both in future studies to better understand the regulation of ABA metabolism and to identify candidates as breeding materials for improving germination properties.

Keywords: abscisic acid; biosynthesis; catabolism; dormancy; germination; natural variation; seed; transcription factor.

Figure 1

The ABA biosynthesis pathway from zeaxanthin. Synthesis of violaxanthin is catalyzed by zeaxanthin epoxidase (ZEP). A reverse reaction catalyzed by violaxanthin de-epoxidase (VDE) contributes to energy dissipation in chloroplasts under high light. The formation of cis-isomers of violaxanthin and neoxanthin remains elusive. Two proteins, ABA-DEFICIENT4 and NEOXANTHIN-DEFICIENT1, are necessary for neoxanthin synthase (NSY) activity. Recent evidence suggests that ABA4 is also involved in ABA synthesis from 9-cis-violaxanthin. Cleavage of cis-xanthophylls is catalysed by a family of 9-cis-epoxycarotenoid dioxygenases (NCED). Xanthoxin is then converted by a xanthoxin dehydrogenase (XD) into abscisic aldehyde, which is oxidized into ABA by an abscisic aldehyde oxidase (ABAO). ABAO contains a molydenum cofactor activated by a MoCo sulfurase (MOCOS). Arabidopsis mutants are indicated for each enzymatic step.

Figure 2

ABA catabolic pathways. ABA 8′-hydroxylase is encoded by the CYP707A gene family and converts ABA into 8′-hydroxy-ABA, which undergoes a spontaneous isomerization to give phaseic acid (PA). PA reductase (PAR) then converts PA into dihydrophaseic acid (DPA). 7′ and 9′ hydroxylations are minor catabolic routes, 9′-hydroxy-ABA is produced by CYP707A and converted into neoPA, but the enzyme responsible for 7′-hydroxylation remains unknown. ABA is also inactivated to ABA glucose ester (ABA-GE) by UDP-glucosyltransferases (UGT), ABA-GE is then converted to free ABA by β-glucosidases (BG).

Figure 3

Transcriptional regulation of ABA metabolism in dormancy induction. The transcription factors ABI4, MYB96 and bHLH57 have been described to directly bind promoters of either NCED or CYP707A genes and regulate ABA levels and dormancy depth. (A) ODR1 interaction with bHLH57 prevents binding to NCED6 and NCED9 promoters and activation of ABA biosynthesis. ODR1 may also regulate ABA levels by decreasing the transcription of ABI4. (B) During seed maturation, ABI3 binding to the ODR1 promoter represses its expression and releases bHLH57 inhibition thus promoting ABA biosynthesis and seed dormancy. This regulation may be amplified by the stimulation of ABI3 by ABA. ABI4 has been shown to bind CYP707A1/2 promoters and inhibit ABA catabolism, leading to higher ABA levels and seed dormancy. MYB96 would have a dual function in the regulation ABA metabolism by directly activating NCED2 and NCED6 and indirectly repressing CYP707A2 through its positive effect on ABI4 expression. Dashed lines indicate either an indirect effect of the signaling factor, or a reduced expression of the downstream gene.

Figure 4

Transcriptional regulation of ABA metabolism in germination responses to light, high temperature and nitrate. Light-activated phytochromes interact with PIF1 and promote its degradation. In the dark, PIF1 interacts with ABI3 and both bind to the promoter of SOM gene, which indirectly regulates ABA metabolism genes, resulting in an increase in ABA levels and inhibition of germination. Similarly, SOM is also indirectly involved in germination thermoinhibition, ABI3 and ABI5 form a complex with DELLA proteins and bind to the SOM promoter. In response to heat, DREBC has been shown to directly binds to NCED9 promoter and upregulates ABA accumulation. Moreover DREBC expression would be indirectly subject to a negative regulation by ODR1, which role in temperature response has not been investigated. Nitrate promotes germination through NLP8 binding to CYP707A2 promoter and activation of its expression, thus reducing ABA levels upon imbibition. Dashed lines indicate either an indirect effect of the signaling factor, or a reduced expression of the downstream gene.