Turnover of LeACS2, a wound-inducible 1-aminocyclopropane-1-carboxylic acid synthase in tomato, is regulated by phosphorylation/dephosphorylation.

1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS) is the rate-limiting enzyme of the ethylene biosynthesis pathway. ACS is regulated both transcriptionally and post-translationally. We previously reported that LeACS2, a wound-inducible ACS in tomato (Solanum lycopersicum), is phosphorylated in vivo, and suggested that phosphorylation regulates protein stability rather than enzymatic activity. In this report, we demonstrate that phosphorylation/dephosphorylation of LeACS2 regulates its turnover upstream of the ubiquitin-26S-proteasome degradation pathway. Pulse-chase experiments coupled with treatment with protein kinase/phosphatase inhibitors demonstrated that LeACS2 is stabilized by phosphorylation and degraded after dephosphorylation. The amount of LeACS2 affected by the protein kinase/phosphatase inhibitors significantly influenced cellular ACS activity, ACC content, and ethylene production levels in tomato fruit tissue, suggesting that post-translational regulation by phosphorylation plays an important role in the control of ethylene production. A calcium-dependent protein kinase (CDPK), LeCDPK2, was isolated as one of the protein kinases that are able to phosphorylate LeACS2 at Ser-460. LeACS2 was immediately phosphorylated after translation by CDPK and mitogen-activated protein kinase at different sites in response to wound signaling and almost all functional LeACS2 molecules are phosphorylated in the cell. Phosphorylation at both sites was required for LeACS2 stability.

Enhancement of cold tolerance and inhibition of lipid peroxidation by citrus dehydrin in transgenic tobacco.

Citrus ( Citrus unshiu Marcov.) dehydrin in response to chilling stress was overexpressed in tobacco ( Nicotiana tabacum L.), and the cold stress tolerance of transgenics at low temperature was analyzed. The freezing at -4 degrees C for 3 h of 24 independent lines indicated that a phenotype expressing citrus dehydrin showed less electrolyte leakage than the control. Dehydrin protein content was correlated with freezing tolerance in transgenics. Dehydrin-expressing tobacco exhibited earlier germination and better seedling growth than the control at 15 degrees C. Cell fractionation experiments suggested that the protein was predominantly expressed in mitochondria and the soluble fraction. Malondialdehyde production enhanced by chilling stress was lower in tobacco plants expressing citrus dehydrin than in control phenotypes. Dehydrin protein, purified from Escherichia coli expressing citrus dehydrin cDNA, prevented peroxidation of soybean ( Glycine max L.) liposomes in vitro. The inhibitory activity of dehydrin against liposome oxidation was stronger than that of albumin, glutathione, proline, glycine betaine, and sucrose. These results suggest that dehydrin facilitates plant cold acclimation by acting as a radical-scavenging protein to protect membrane systems under cold stress.