A Truncated Arabidopsis NUCLEOSOME ASSEMBLY PROTEIN 1, AtNAP1;3T, Alters Plant Growth Responses to Abscisic Acid and Salt in the Atnap1;3-2 Mutant

Chromatin remodeling is thought to have crucial roles in plant adaptive response to environmental stimulus. Here, we report that, in Arabidopsis, the evolutionarily conserved histone chaperone, NUCLEOSOME ASSEMBLY PROTEIN 1 (NAP1), is involved in plant response to abscisic acid (ABA), a phytohormone important in stress adaptation. We show that simultaneous loss-of-function of AtNAP1;1, AtNAP1;2, and AtNAP1;3 (the triple mutant m123-1) caused a slight hypersensitive response to ABA in seedling growth. Strikingly, the other triple mutant m123-2 containing a different mutant allele of AtNAP1;3, the Atnap1;3-2 allele, showed a hyposensitive response to ABA and a decreased tolerance to salt stress. This ABA-hyposensitive and salt response phenotype specifically associated with the Atnap1;3-2 mutant allele. We show that this mutant allele produced a truncated protein, AtNAP1;3T, which lacks 34 amino acids at the C-terminus compared to the wild-type protein AtNAP1;3. We further show that the heterozygous plants containing the Atnap1;3-2 mutant allele as well as transgenic plants overexpressing AtNAP1;3T exhibit ABA-hyposensitive phenotype. It thus indicates that AtNAP1;3T functions as a dominant negative factor in ABA response. The expression of some ABA-responsive genes, including genes encoding protein kinases and transcription regulators, was found perturbed in the mutant and in the AtNAP1;3T transgenic plants. Taken together, our study uncovered AtNAP1 proteins as positive regulators and AtNAP1;3T as a negative regulator in ABA signaling pathways, providing a novel link of chromatin remodeling to hormonal and stress responses.