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Roles for Arabidopsis ALDH10 enzymes in γ-butyrobetaine synthesis, seed development, germination and salt tolerance.

Plant genomes generally contain two aldehyde dehydrogenase 10 genes (ALDH10), which encode NAD+-dependent enzymes. These oxidize various aminoaldehydes that are produced by the catabolism of amino acids and polyamines. ALDH10s are closely related to the animal and fungal trimethylaminobutyraldehyde dehydrogenases (TMABADH) that are involved in the synthesis of γ butyrobetaine, which is the precursor of carnitine. Here we explore the ability of the Arabidopsis thaliana proteins AtALDH10A8 and AtALDH10A9 to oxidize aminoaldehydes. We demonstrate that these enzymes display high TMABADH activities in vitro. Moreover, they can complement the Candida albicans tmabadhΔ/Δ mutant. These findings illustrate the link between AtALDH10A8 and AtALDH10A9 and γ-butyrobetaine synthesis. An analysis of single and double knockout Arabidopsis mutant lines revealed that the double mutants had reduced γ-butyrobetaine levels. However, there were no changes in the carnitine contents of these mutants. The double mutants were more sensitive to salt stress. In addition, the siliques of the double mutants had a significant proportion of seeds that failed to mature. The mature seeds contained higher amounts of triacylglycerol, facilitating accelerated germination. Taken together, these results show that ALDH10 enzymes are involved in γ butyrobetaine synthesis. Furthermore, γ-butyrobetaine fulfils a range of physiological roles, in addition to those related to carnitine biosynthesis.