Abstract
The phytohormone abscisic acid (ABA) is a key regulator of plant growth and development as well as plant responses to situations of decreased water availability. Protein phosphatases type 2C (PP2Cs) from group A, which includes the ABI1/HAB1 and PP2CA branches, are key negative regulators of ABA signaling. Specifically, HAB1, ABI1, ABI2, and PP2CA have been shown to affect both seed and vegetative responses to ABA. To further understand their contribution to ABA signaling and to unravel possible genetic interactions and functional redundancy among them, we have generated different combinations of double and triple mutants impaired in these PP2Cs. Interestingly, hab1-1pp2ca-1 and abi1-2pp2ca-1 double mutants showed reduced water loss and enhanced resistance to drought stress, which further supports the role of PP2CA in vegetative responses to ABA. Two triple hab1-1abi1-2abi2-2 and hab1-1abi1-2pp2ca-1 mutants were generated, which showed an extreme response to exogenous ABA, impaired growth, and partial constitutive response to endogenous ABA. Thus, transcriptomic analysis revealed a partial up-regulation/down-regulation of a subset of ABA-responsive genes in both triple mutants in the absence of exogenous ABA. Comparison of ABA responses in the different pp2c mutants showed that a progressive increase in ABA sensitivity could be obtained through combined inactivation of these PP2Cs. These results indicate that ABA response is finely tuned by the integrated action of these genes, which is required to prevent a constitutive response to endogenous ABA that might have a deleterious effect on growth and development in the absence of environmental stress.
Highlights
The phytohormone abscisic acid (ABA) is a key regulator of plant growth and development as well as plant responses to situations of decreased water availability
We wondered whether combination of the pp2ca-1 mutation with either hab1-1 or abi1-2 might reinforce ABA responses as it was described previously for the double hab1-1abi1-2 mutant (Saez et al, 2006)
ABA-mediated inhibition of seed germination was stronger in the pp2ca-1 single mutant than in hab1-1 and abi1-2 single mutants (Fig. 1A), which is in agreement with previous results from Yoshida et al (2006)
Summary
The phytohormone abscisic acid (ABA) is a key regulator of plant growth and development as well as plant responses to situations of decreased water availability. HAB1, ABI1, ABI2, and PP2CA have been shown to affect both seed and vegetative responses to ABA To further understand their contribution to ABA signaling and to unravel possible genetic interactions and functional redundancy among them, we have generated different combinations of double and triple mutants impaired in these PP2Cs. Interestingly, hab1-1pp2ca-1 and abi1-2pp2ca-1 double mutants showed reduced water loss and enhanced resistance to drought stress, which further supports the role of PP2CA in vegetative responses to ABA. Combined inactivation of close members of a gene family is usually required to unravel possible functional genetic redundancy and to establish a functional hierarchy among them This fact has been evident in hormonal signaling pathways (Hua and Meyerowitz, 1998; Kwak et al, 2003; Higuchi et al, 2004; Achard et al, 2006; Iuchi et al, 2007). Gain-of-function approaches can circumvent genetic redundancy, as deduced from the global ABA-insensitive phenotype found in the dominant mutants abi1-1D and abi2-1D, as well as the transgenic lines 35S:HAB1, 35S:PP2CA, and 35S:hab1Gly-246Asp (Koornneef et al, 1984; Saez et al, 2004; Kuhn et al, 2006; Robert et al, 2006)
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