Abstract
In this review, we summarize the different biosynthesis-related pathways that contribute to the regulation of endogenous auxin in plants. We demonstrate that all known genes involved in auxin biosynthesis also have a role in root formation, from the initiation of a root meristem during embryogenesis to the generation of a functional root system with a primary root, secondary lateral root branches and adventitious roots. Furthermore, the versatile adaptation of root development in response to environmental challenges is mediated by both local and distant control of auxin biosynthesis. In conclusion, auxin homeostasis mediated by spatial and temporal regulation of auxin biosynthesis plays a central role in determining root architecture.
Highlights
Flowering plants have evolved a high level of developmental and morphological plasticity to accommodate adaptive growth in response to diverse environmental stimuli [1]
We demonstrate that all known genes involved in auxin biosynthesis have a role in root formation, from the initiation of a root meristem during embryogenesis to the generation of a functional root system with a primary root, secondary lateral root branches and adventitious roots
A member of ethylene-responsive AP2 transcription factors; ETHYLENE RESPONSE FACTOR1 (ERF1) was recently shown to bind the promoter of ASA1 in order to regulate auxin biosynthesis and ethylene-induced root growth inhibition [194]. Another interaction between ethylene and auxin biosynthesis was discovered in a chemical genetic strategy, using L-kynurenine, a chemical that represses the nuclear accumulation of the ethylene insensitive 3 (EIN3) transcription factor and TAA1/TAR were identified as a target for L-kyn [195]
Summary
Flowering plants have evolved a high level of developmental and morphological plasticity to accommodate adaptive growth in response to diverse environmental stimuli [1]. Following the initiation of roots, features such as secondary growth, gravitropic responses and the development of lateral root branches are important additions that improved the function of the root system and required more complex cellular communication [5,6]. Since cell-division and -expansion requires to be spatiotemporally regulated, an appropriate auxin distribution across the tissue is highly important to coordinate growth and tissue development. Auxin signals are perceived intracellularly by a family of AUXIN SIGNALING F-BOX (AFB) receptors of which (TIR1) TRANSPORT INHIBITOR RESPONSE1 is the founder These F-box proteins are the substrate-binding subunit of SCF-type ubiquitin protein ligase complexes, named after their subunits Skp, Cullin and an F-box [38]. Different TIR1/AFB-Aux/IAA combinations may contribute to different transcriptional responses, depending on their presence in certain tissues or the physiological status of the plants. Besides the naturally occurring auxins, there are multiple synthetic auxin-derivatives, many of which were developed because of herbicide activity such as for example 2,4-dichlorophenoxyacetic acid (2,4-D), 1-naphthaleneacetic acid (NAA), dicamba and picloram [64]
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