A branched-chain aminotransferase may regulate hormone levels by affecting KNOX genes in plants

Abstract

Branched-chain amino acid transaminases (BCATs) play a crucial role in the metabolic pathway of leucine, isoleucine and valine by catalyzing the last step of synthesis and/or the initial step of degradation of these amino acids. In this study, we characterized a new BCAT from Nicotiana benthamiana (NbBCAT, GeneBank accession No. EU194916), the deduced amino acid sequence of which exhibits a very high percentage of identity to the homologous enzymes from Solanum tuberosum (StBCAT-2, 91.5%) and Arabidopsis thaliana (AtBCAT1-6, 56.4-68.6%). Complementation experiment using a Deltabat1/Deltabat2 double knockout yeast strain system demonstrated enzymatic activities for NbBCAT. Ectopically expressed NbBCAT::green fluorescence fusion protein was targeted predominantly to the chloroplasts in tobacco protoplasts. The highest levels of NbBCAT transcripts were found in open flowers as well as in young leaves. Virus-induced gene silencing of NbBCAT resulted in abnormal leaf development and loss of apical dominance. In NbBCAT-silenced plants, two KNOTTED1-type genes, NTH15 and NTH23, were upregulated. This was accompanied by various hormone changes, as a result of transcriptional regulation of gibberellin 20-oxidase (Ntc12) and adenosine phosphate isopentenyltransferase. The transcript levels of NbBCAT could also be repressed by hormone treatment. These results suggest that NbBCAT, an enzyme in the branched-chain amino acid metabolic pathway, may be involved in the regulation of endogenous hormones by its effect on KNOX genes.

Characterization of the Branched-Chain Amino Acid Aminotransferase Enzyme Family in Tomato

“…Alternatively, it may have a specific role beyond primary metabolism, as has been suggested for Arabidopsis AtBCAT3 and AtBCAT4 (Schuster et al, 2006;Knill et al, 2008). The NbBCAT, recently found to have a role in hormonal regulation, is also expressed highly in flowers and localized to chloroplasts (Gao et al, 2009). …”

“…In another example of diverse BCAT function, a Nicotiana benthamiana chloroplastic BCAT was implicated in transcriptional regulation of KNOX genes that affect levels of gibberellins. This enzyme was also able to restore the growth of a BCAT-deficient yeast and was expressed highly in young leaves, suggesting that it has a primary role in BCAA synthesis (Gao et al, 2009). Together, these studies show that BCATs have functions beyond amino acid metabolism, making it important to understand the characteristics of each enzyme form.…”

Genome-Wide Analysis of Branched-Chain Amino Acid Levels inArabidopsisSeeds

“…BCATs catalyze reversible reactions that convert 2-oxo acids into their corresponding amino acids and vice versa (Diebold et al, 2002;Schuster and Binder, 2005;Binder, 2010), and this class of enzymes has been studied biochemically and through transgenic approaches in several plant species (Campbell et al, 2001;Diebold et al, 2002;Schuster and Binder, 2005;Knill et al, 2008;Gao et al, 2009;Binder, 2010;Chen et al, 2010;Gonda et al, 2010;Maloney et al, 2010;Fernie, 2011, 2012;Kochevenko et al, 2012a,b). These studies have defined roles for individual BCAT family members in amino acid and glucosinolate metabolism, hormone signaling, and volatile production (Binder et al, 2007;Knill et al, 2008;Gao et al, 2009;Binder, 2010).…”

“…These studies have defined roles for individual BCAT family members in amino acid and glucosinolate metabolism, hormone signaling, and volatile production (Binder et al, 2007;Knill et al, 2008;Gao et al, 2009;Binder, 2010). Linkage analysis was performed on BCAAs from tomato fruit pericarp using a population of the wild relative Solanum pennellii introgressed into the cultivated tomato (Solanum lycopersicum; Schauer et al, 2006Schauer et al, , 2008Fernie, 2011, 2012).…”

Molecular identification of a further branched-chain aminotransferase 7 (BCAT7) in tomato plants

“…Despite their obvious importance, the functions of the individual members of the family are not well understood. This fact notwithstanding, BCATs have been partially characterized in a number of species, including spinach, potato, barley, Nicotiana sylvestris, and melon (Hagelstein et al, 1997;Campbell et al, 2001;Malatrasi et al, 2006;Gao et al, 2009;Gonda et al, 2010). In Arabidopsis thaliana, seven BCAT genes have been identified, six of them being transcribed and found in either plastids or mitochondria (Diebold et al, 2002).…”