Auxin Amidohydrolases – From Structure to Function: Revisited

Abstract

The control of plant growth and development is a well-coordinated process between exogenous and endogenous signals. Auxins are plant hormones belonging to the endogenous signals, which control a vast array of different processes. While auxins are growth promoting at low concentrations, higher levels are often inhibitory. Therefore, the tight control of auxin concentrations in a given plant tissue is essential. Among several processes that participate in auxin homeostasis, we focused herein on the process of reversible auxin conjugation that considers the synthesis of inactive auxin conjugates, which can be hydrolyzed back to the active form by so called auxin conjugate hydrolases. Although these proteins have been known for quite some time, their role in plants is still not clear, especially since novel hydrolases with different substrate specificities have been isolated. Thus, we have revisited the knowledge about auxin hydrolases, from their structure and biochemistry to the role in plant development and in dealing with unfavorable climate conditions.

Physcomitrium patens Mutants in Auxin Conjugating GH3 Proteins Show Salt Stress Tolerance but Auxin Homeostasis Is Not Involved in Regulation of Oxidative Stress Factors

“…While the GH3 proteins of P. patens are similar in sequence and function to the higher plant enzymes [ 15 ], the hydrolysis pathway was thought not to take place at all, since the typical plant sequences for conjugate hydrolases were missing [ 75 ]. However, recent work provided experimental evidence for the possible hydrolysis of IAA amino acid conjugates in P. patens as well, since it was found that the moss contained sequences similar to bacterial auxin amidohydrolases [ 24 , 76 ] and its capacity to cleave some conjugates in vivo and in vitro [ 24 ]. These sequences were possibly acquired by horizontal gene transfer [ 76 ].…”

“…However, recent work provided experimental evidence for the possible hydrolysis of IAA amino acid conjugates in P. patens as well, since it was found that the moss contained sequences similar to bacterial auxin amidohydrolases [ 24 , 76 ] and its capacity to cleave some conjugates in vivo and in vitro [ 24 ]. These sequences were possibly acquired by horizontal gene transfer [ 76 ]. Modeling of one protein sequence showed similarity to the respective proteins from A. thaliana [ 76 ].…”

“…These sequences were possibly acquired by horizontal gene transfer [ 76 ]. Modeling of one protein sequence showed similarity to the respective proteins from A. thaliana [ 76 ]. Although as far as we know no one has examined P. patens to determine which auxin conjugate is the native product so far, the most likely of these compounds to be present in vivo may be those described for other moss species [ 76 ].…”

“…Modeling of one protein sequence showed similarity to the respective proteins from A. thaliana [ 76 ]. Although as far as we know no one has examined P. patens to determine which auxin conjugate is the native product so far, the most likely of these compounds to be present in vivo may be those described for other moss species [ 76 ]. The authors found that the most common moss conjugate was IAA-Aspartate at low concentrations.…”

Altered Root Growth, Auxin Metabolism and Distribution in Arabidopsis thaliana Exposed to Salt and Osmotic Stress

“…Auxin metabolism and homeostasis have been extensively studied and numerous genes have been identified to date. A review of auxin homeostasis has been updated [ 7 , 8 , 9 , 10 , 11 , 12 ] and is summarized in Figure 1 .…”