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Abstract
Dihydroxyacid dehydratase (DHAD) catalyses a key step in the branched-chain amino acid (BCAA) biosynthetic pathway that exists in numerous organisms, including bacteria, fungi, and plants, but not humans. In Arabidopsis thaliana, DHAD is encoded by a single gene (AT3G23940), but its biological function in controlling plant development remains uncharacterized. In this study, we showed that DHAD is highly expressed in most vegetative and reproductive tissues. It is an essential gene, and complete disruption caused partial sterility in both male and female gametophyte phases. In addition, reduced expression of DHAD in knockdown mutants resulted in a reduction in the accumulation of all three BCAAs in roots and, as a consequence, led to a shorter root phenotype, which could be restored by an exogenous supplement of free BCAAs. Interestingly, the knockdown mutants became hypersensitive to salt stress, not to heavy metal stress, implying that BCAAs may act as osmolytes in salt tolerance. This would be the second amino acid shown to confer such a function in addition to the well-documented proline. Our results provide evidence that BCAA biosynthesis plays important roles in gametophyte and root development, and BCAA homeostasis contributes to the adaptation of Arabidopsis to salinity stress.
Highlights
Isoleucine (Ile), valine (Val), and leucine (Leu) are grouped as branched-chain amino acids (BCAAs), as they all contain short branched hydrocarbon residues
AtDHAD is encoded by a single gene that is constitutively expressed worth noting that Dihydroxyacid dehydratase (DHAD) seems to occur as a single-copy gene in all plant species investigated, implying that there is selection pressure to preserve DHAD genes as singletons following genome-wide or small-scale duplication events during evolution
As the essential components of protein synthesis and as important signalling molecules, BCAAs have been thought to regulate numerous aspects of plant growth required for plant survival
Summary
Isoleucine (Ile), valine (Val), and leucine (Leu) are grouped as branched-chain amino acids (BCAAs), as they all contain short branched hydrocarbon residues. Research interest in the biosynthetic pathway of BCAAs in plants was motivated by two main reasons. Like animals, human are unable to synthesize BCAAs de novo, and have to obtain them through diet or other sources (Jander and Joshi, 2009, 2010). Some enzymes involved in BCAA biosynthesis are targets of herbicide applications (Tan et al, 2006). The enzymes participating in BCAA biosynthesis are widely conserved in bacteria, fungi, and plants. Threonine deaminase (TD) catalyses the first and committed step
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