Enhancement of Abscisic Acid Sensitivity and Reduction of Water Consumption in Arabidopsis by Combined Inactivation of the Protein Phosphatases Type 2C ABI1 and HAB1  

Abstract

Abscisic acid (ABA) plays a key role in plant responses to abiotic stress, particularly drought stress. A wide number of ABAhypersensitive mutants is known, however, only a few of them resist/avoid drought stress. In this work we have generated ABA-hypersensitive drought-avoidant mutants by simultaneous inactivation of two negative regulators of ABA signaling, i.e. the protein phosphatases type 2C (PP2Cs) ABA-INSENSITIVE1 (ABI1) and HYPERSENSITIVE TO ABA1 (HAB1). Two new recessive loss-of-function alleles of ABI1, abi1-2 and abi1-3, were identified in an Arabidopsis (Arabidopsis thaliana) T-DNA collection. These mutants showed enhanced responses to ABA both in seed and vegetative tissues, but only a limited effect on plant drought avoidance. In contrast, generation of double hab1-1 abi1-2 and hab1-1 abi1-3 mutants strongly increased plant responsiveness to ABA. Thus, both hab1-1 abi1-2 and hab1-1 abi1-3 were particularly sensitive to ABA-mediated inhibition of seed germination. Additionally, vegetative responses to ABA were reinforced in the double mutants, which showed a strong hypersensitivity to ABA in growth assays, stomatal closure, and induction of ABA-responsive genes. Transpirational water loss under drought conditions was noticeably reduced in the double mutants as compared to single parental mutants, which resulted in reduced water consumption of whole plants. Taken together, these results reveal cooperative negative regulation of ABA signaling by ABI1 and HAB1 and suggest that fine tuning of ABA signaling can be attained through combined action of PP2Cs. Finally, these results suggest that combined inactivation of specific PP2Cs involved in ABA signaling could provide an approach for improving crop performance under drought stress conditions.

ESCRT-I Component VPS23A Affects ABA Signaling by Recognizing ABA Receptors for Endosomal Degradation

“…The data imply that VPS23A also acts upstream of ABI1. abi1-2 is a loss-of-function mutant of ABI1, and hab1-1 is the mutant in hypersensitive to ABA1 (HAB1), another PP2C that cooperates with ABI1 to negatively regulate ABA signaling (Saez et al, 2006). The double mutant abi1-2 hab1-1 exhibited strong hypersensitivity to ABA (Saez et al, 2006).…”

“…abi1-2 is a loss-of-function mutant of ABI1, and hab1-1 is the mutant in hypersensitive to ABA1 (HAB1), another PP2C that cooperates with ABI1 to negatively regulate ABA signaling (Saez et al, 2006). The double mutant abi1-2 hab1-1 exhibited strong hypersensitivity to ABA (Saez et al, 2006). Interestingly, the triple mutant vps23a abi1-2 hab1-1 exhibited a more ABA-hypersensitive phenotype than the single mutant vps23a or the double mutant abi1-2 hab1-1 (Figures S2C and S2E), implying that VPS23A might directly or indirectly affect ABI1 and HAB1.…”

Structural insights into PYR/PYL/RCAR ABA receptors and PP2Cs

“…These mutations were isolated 25-years ago by genetic screenings in Arabidopsis, aimed to the identification of ABAinsensitive mutants [27] and the cloning of the mutant loci provided pioneering insights into ABA signaling [28,29,30,31]. Both abi1-1D and abi2-1D are strong ABA-insensitive mutants that show diminished response to ABA in seeds and vegetative tissues, however, analysis of lossof-function alleles indicates that ABI1 and ABI2 gene products, as well as other clade A PP2Cs, are negative regulators of ABA signaling [32,33,34,35]. Since the phenotype of the abi1-1D and abi2-1D dominant alleles is just the opposite of loss-of-function alleles, they can´t represent dominant negative alleles, as it was accurately noted by Robert et al, [36].…”

ABA signalling is fine-tuned by antagonistic HAB1 variants

“…HAB1 binds to and dephosphorylates SnRK2s, especially SnRK2.6 (OST1), and negatively regulates ABA signalling 17,18 . Genetic analyses have suggested that HAB1 is functionally redundant with ABI1 and ABI2 in the regulation of post-germination development and stomatal movement under drought conditions 19,20 . It is apparent that HAB1 is essential in ABA signalling.…”