Abstract
Amidases [EC 3.5.1.4] capable of converting indole-3-acetamide (IAM) into the major plant growth hormone indole-3-acetic acid (IAA) are assumed to be involved in auxin de novo biosynthesis. With the emerging amount of genomics data, it was possible to identify over forty proteins with substantial homology to the already characterized amidases from Arabidopsis and tobacco. The observed high conservation of amidase-like proteins throughout the plant kingdom may suggest an important role of theses enzymes in plant development. Here, we report cloning and functional analysis of four, thus far, uncharacterized plant amidases from Oryza sativa, Sorghum bicolor, Medicago truncatula, and Populus trichocarpa. Intriguingly, we were able to demonstrate that the examined amidases are also capable of converting phenyl-2-acetamide (PAM) into phenyl-2-acetic acid (PAA), an auxin endogenous to several plant species including Arabidopsis. Furthermore, we compared the subcellular localization of the enzymes to that of Arabidopsis AMI1, providing further evidence for similar enzymatic functions. Our results point to the presence of a presumably conserved pathway of auxin biosynthesis via IAM, as amidases, both of monocot, and dicot origins, were analyzed.
Highlights
The collective term auxin refers to a class of compounds sharing similar physiological functions in plants
We show that phenyl-2-acetic acid (PAA) is endogenous to Arabidopsis, and that it exerts an auxin-like effect in Arabidopsis root growth bioassays
NIT1-3 enzymes seem to be restricted to the Brassicaceae [22,38,39], which contradicts a general importance of these enzymes in auxin biosynthesis
Summary
The collective term auxin refers to a class of compounds sharing similar physiological functions in plants. IAM to IAA in vitro, while refusing most other naturally occurring amides as substrates On this basis, a tentative involvement of AtAMI1 in auxin biosynthesis in A. thaliana has been suggested. A tentative involvement of AtAMI1 in auxin biosynthesis in A. thaliana has been suggested Due to these lines of evidence, implying a possibly broader importance of IAA biosynthesis through the IAM-pathway in plants, it appeared intriguing to us to investigate whether IAM-amidohydrolases are restricted to only very few plant families or if this enzyme class shows a broader distribution in the plant kingdom. The identification and functional analysis of comparable AMI1-like enzymes from various plant species would likewise highlight a broader presence of the IAM-dependent IAA production and point towards a general concept in auxin biosynthesis. We show that PAA is endogenous to Arabidopsis, and that it exerts an auxin-like effect in Arabidopsis root growth bioassays
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