4-Chloroindole-3-acetic acid (4-Cl-IAA) is a potent auxin in various auxin bioassays. Researchers have used 4-Cl-IAA as well as other halogenated auxins in biological assays to understand the structural features of auxins required to induce auxin mediated growth in plants. 4-Cl-IAA is a naturally occurring auxin in plants from the Vicieae tribe of the Fabaceae family; and 4-Cl-IAA has also been identified in one species outside the Vicieae tribe, Pinus sylvestris. The apparent function of the unique auxin 4-Cl-IAA in normal plant growth and development will be discussed with a focus on Pisum sativum and Vicia faba.
4-Chloroindole-3-acetic acid (4-Cl-IAA) and indole-3-aldehyde (IAId) have been characterized as endogenous constituents in seeds of Pinus sylvestris L. by gas chromatography-mass spectrometry. Quantitative estimates indicate that immature seeds contained 640 pg 4-Cl-IAA (g fresh weight)-1 while mature seeds contained 340 pg (g dry weight)-1. 4-Cl-IAA could not be detected in seeds five days after germination. The content of IAld increased from 127 pg (g dry weight)-1 in mature seeds to 315 pg (g dry weight)-1 after five days of germination.
Auxins are a class of plant growth hormones naturally present in all plants. They have been implicated in many processes of plant growth and development including stem elongation, root initiation, ethylene biosynthesis, and tissue vascularization . Auxins occurring naturally in plants include indole-3-acetic acid (IAA; Figure 1a), indole-3-butyric acid, phenylacetic acid (a weak auxin) and 4-chloroindole-3-acetic acid. The review will discuss the occurrence and activity of 4-Cl-IAA in plant tissues. A review by Engvild is recommended for other related discussions and references on halogenated auxins beyond the scope of this review.
4-Cl-IAA is a potent auxin generally showing more activity than IAA in standard biological assays. 4-Cl-IAA has been tested in many different bioassays and has been reported to be 1.3 to 50 times more activethan IAA (relative activity equals the concentration of IAA giving half maximal response divided by the concentration of 4-Cl-IAA giving the same response). Stimulation of growth of excised tissues is the standard biological assay for auxins including 4-Cl-IAA and IAA (4-Cl-IAA examples: pea stem split curvature growth, pea stem straight growth,oat and wheat coleoptile growth, and mung bean hypocotyl growth . 4-Cl-IAA treatment also increases root initiation and ethylene evolution in pea shoot cuttings . Reports on 4-Cl-IAA’s effects on the growth of intact plant organs include wheat and cucumber root inhibition, tomato epinasty, tomato parthenocarpy, mung bean growth inhibition and root initiation, and pea pericarp growth .
4-Cl-IAA ismore biologically active than IAAin these assays; however, Stenlid and Engvild reported that wheat roots were inhibitedmore by IAA treatment than 4-Cl-IAA.4-Cl-IAA’s strong auxin activity has been postulated to occur via reduced metabolism of 4-Cl-IAA,or a receptor and signal transduction pathway unique frond production, IAA and 4-Cl-IAA at 10 _M were only weakly inhibitory. Of the halogenated auxins tested, 5-Br-IAA was proposed for mutant selection studies since it was phytotoxic when tested with Lemna gibba and Zea mays.
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