Identification of novel inhibitors of 1-aminocyclopropane-1-carboxylic acid synthase by chemical screening in Arabidopsis thaliana.

Ethylene is a gaseous hormone important for adaptation and survival in plants. To further understand the signaling and regulatory network of ethylene, we used a phenotype-based screening strategy to identify chemical compounds interfering with the ethylene response in Arabidopsis thaliana. By screening a collection of 10,000 structurally diverse small molecules, we identified compounds suppressing the constitutive triple response phenotype in the ethylene overproducer mutant eto1-4. The compounds reduced the expression of a reporter gene responsive to ethylene and the otherwise elevated level of ethylene in eto1-4. Structure and function analysis revealed that the compounds contained a quinazolinone backbone. Further studies with genetic mutants and transgenic plants involved in the ethylene pathway showed that the compounds inhibited ethylene biosynthesis at the step of converting S-adenosylmethionine to 1-aminocyclopropane-1-carboxylic acid (ACC) by ACC synthase. Biochemical studies with in vitro activity assay and enzyme kinetics analysis indicated that a representative compound was an uncompetitive inhibitor of ACC synthase. Finally, global gene expression profiling uncovered a significant number of genes that were co-regulated by the compounds and aminoethoxyvinylglycine, a potent inhibitor of ACC synthase. The use of chemical screening is feasible in identifying small molecules modulating the ethylene response in Arabidopsis seedlings. The discovery of such chemical compounds will be useful in ethylene research and can offer potentially useful agrochemicals for quality improvement in post-harvest agriculture.

Alkaline alpha-galactosidase degrades thylakoid membranes in the chloroplast during leaf senescence in rice.

Here, we studied the functional role of a chloroplast alkaline alpha-galactosidase (OsAkalphaGal) in the breakdown of thylakoid membranes during rice (Oryza sativa) leaf senescence. We assayed the enzyme activity of recombinant OsAkalphaGal with different natural substrates and examined the effect of ectopic OsAkalphaGal expression in rice plants. Recombinant OsAkalphaGal showed at least a two-fold greater substrate-binding affinity and a 10-fold greater turnover rate to galactolipid digalactosyl diacylglycerol than the raffinose family of oligosaccharides (verbascose, stachyose, raffinose) and melibiose. The OsAkalphaGal null mutant (osakalphagal) displayed a delayed leaf senescence phenotype. OsAkalphaGal complementation in osakalphagal recovered OsAkalphaGal expression and showed a senescence phenotype similar to that of wild-type plants. Transgenic plants overexpressing OsAkalphaGal (UbiP-OsAkalphaGal) exhibited retarded plant growth and development, and showed a pale-green phenotype coupled with a reduced chlorophyll content to 42% in newly unfolded leaves. UbiP-OsAkalphaGal leaves also showed a 29-fold increase in alkaline alpha-galactosidase activity compared with wild-type leaves. An ultrastructural study of Ubi-OsAkalphaGal chloroplasts in newly unfolded leaves revealed abnormal grana organization. Our findings strongly suggest that OsAkalphaGal is a thylakoid membrane-degrading enzyme involved in the degradation of digalactosyl diacylglycerol during rice leaf senescence.