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An auxin response element is a cis-regulatory element involved in auxin-responsive transcription. It is a DNA sequence recognized by regulatory proteins, including auxin response factors. The element, its binding protein and the plant hormone are different participants in the response.^[ref]

A motivating experiment compared the sequence requirements for factor binding in vitro with those associated with a transcriptional response in living cells. Agreement linked molecular recognition to regulation, rather than merely identifying a sequence that happened to lie near an expressed gene.^[ref]

The synthetic element used in the DR5 auxin response reporter supplies a concrete example. It can drive a measurable reporter gene response, but its output also depends on the regulatory state of the cell. In the original experiments, Aux/IAA proteins could repress reporter expression. Thus a response element is part of a control system: the same regulatory DNA does not guarantee the same output under every cellular condition.^[ref]

The DR5 auxin response element is a synthetic regulatory DNA arrangement containing repeated auxin response elements. When used to control a reporter gene, it contributes the responsive regulatory part of a DR5 auxin response reporter. The DNA element and the measurable reporter product are different parts of that system.^[ref]

The original study compared DR5 with natural regulatory sequences and found strong auxin-responsive expression in the tested plant systems. It also connected binding by auxin response factor 1 with regulatory effects involving Aux/IAA proteins. This made DR5 useful for studying responsive transcription, rather than establishing a direct physical measurement of auxin concentration.^[ref]

An illuminating experimental distinction is between asking whether a protein recognizes this DNA arrangement and asking whether a reporter is expressed in a particular cell. The first concerns molecular binding; the second also depends on the surrounding regulatory system and reporter behavior. A repeated regulatory element can therefore be a sensitive response tool without being a universal hormone-concentration scale.

Phtheirospermum japonicum YUC3 is a gene encoding a YUCCA flavin monooxygenase involved in auxin biosynthesis in the parasitic plant Phtheirospermum japonicum. Its role connects local gene expression with development of a haustorium, the attachment and invasion organ used in interactions with a host plant.^[ref]

The2016 study localized Phtheirospermum japonicum YUC3 expression to epidermal cells near the host-contact site during early haustorium formation. The pattern coincided with the DR5 auxin response reporter. Reducing Phtheirospermum japonicum YUC3 expression by RNA interference decreased haustorium formation, while expression at other epidermal positions could induce haustorium-like structures.^[ref]

The measurements asked complementary questions. Gene expression localized the candidate mechanism; perturbation tested its developmental contribution; a biochemical assay found approximately threefold greater free indole-3-acetic acid in roots overexpressing Phtheirospermum japonicum YUC3 than in empty-vector controls. A reporter gene pattern alone is not a direct chemical measurement of auxin, and a haustorium-like structure alone does not establish a successful host connection.^[ref]