The role of chromatin-remodeling factor PKL in balancing osmotic stress responses during Arabidopsis seed germination.

Embryogenesis in Arabidopsis results in mature, osmotolerant embryos within dry seeds. Late-embryogenesis programs involve the transcription factors ABI3 and ABI5, which are necessary for osmotolerance. ABI3 and ABI5 are degraded in seeds initiating germination, abolishing their protected state. However, during an early stage of germination, strong osmotic stresses, or ABA exposure maintain ABI3 and ABI5 expression, leading to growth arrest, and osmotolerance. Mild stress stimuli delay ABI3 and ABI5 disappearance, retarding germination but not preventing eventual closure of embryogenesis programs. PICKLE (PKL), a putative chromatin modifier, is necessary to repress ABI3 and ABI5 expression during germination in response to ABA. We show that pkl mutants display persistent high expression of ABI3 and ABI5 upon ABA stimulation. In turn, maintenance of ABI5 expression leads to hypersensitive germination responses to ABA in pkl seeds. We provide evidence that ABI3 and ABI5 are less associated with repressed chromatin in pkl mutants. Our results provide evidence that PKL-dependent repression of embryonic gene expression extends to late-embryogenesis genes and is associated with changes in chromatin. We suggest that effective closure of embryogenesis omostolerance programs during germination prevents excessive plant reactions to stress.

A novel calmodulin-binding protein functions as a negative regulator of osmotic stress tolerance in Arabidopsis thaliana seedlings.

A clone for a novel Arabidopsisthaliana calmodulin (CaM)-binding protein of 25 kDa (AtCaMBP25) has been isolated by using a radiolabelled CaM probe to screen a cDNA expression library derived from A. thaliana cell suspension cultures challenged with osmotic stress. The deduced amino acid sequence of AtCaMBP25 contains putative nuclear localization sequences and shares significant degree of similarity with hypothetical plant proteins only. Fusion of the AtCaMBP25 coding sequence to reporter genes targets the hybrid protein to the nucleus. Bacterially expressed AtCaMBP25 binds, in a calcium-dependent manner, to a canonical CaM but not to a less conserved isoform of the calcium sensor. AtCaMBP25 is encoded by a single-copy gene, whose expression is induced in Arabidopsis seedlings exposed to dehydration, low temperature or high salinity. Transgenic plants overexpressing AtCaMBP25 exhibits an increased sensitivity to both ionic (NaCl) and non-ionic (mannitol) osmotic stress during seed germination and seedling growth. By contrast, transgenic lines expressing antisense AtCaMBP25 are significantly more tolerant to mannitol and NaCl stresses than the wild type. Thus, the AtCaMBP25 gene functions as a negative effector of osmotic stress tolerance and likely participates in stress signal transduction pathways.