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IN BRIEF
Indolebutyric Acid–Derived Auxin and Plant Development
Auxins play a critical role in plant cell
growth and are involved in a wide variety
of developmental processes, including
initiation of leaf primordia, apical dominance, phototropism, fruit development,
and lateral root production. Although over
70 years has passed since the pioneering
work of F.W. Went leading to the identification and isolation of the auxin indole3-acetic acid (IAA; Went, 1935), the
biosynthesis and storage forms of IAA in
the plant are still not completely characterized (Ludwig-Müller, 2000; Zhao, 2010).
Indolebutyric acid (IBA) is a closely related
auxin that differs from IAA only in the length
of its side chain, which contains two
additional CH2 groups. IBA can be converted to IAA in a peroxisome-dependent
reaction, and several IBA-resistant (ibr)
response mutants have been shown to be
defective in peroxisomal enzymes (Zolman
et al., 2008).
Application of supraoptimal levels of
exogenous IAA or IBA to germinating
wild-type Arabidopsis thaliana seedlings
inhibits hypocotyl elongation. Screens for
auxin response mutants in light-grown
seedlings have identified mutants resistant
to the inhibitory effect of exogenous IAA
and IBA. Because some IBA-resistant
mutants isolated in these assays are not
IBA-resistant in the dark, Strader et al.
(pages nnn) devised a screen for IBA
resistance in dark-grown Arabidopsis
seedlings to isolate additional genes required for the IBA response. Subsequently,
they isolated a mutant that is insensitive to
IBA-induced inhibition of hypocotyl elongation in the dark (see figure, top panel).
Mapping, sequencing, and complementation experiments showed this mutant to
have a lesion in ENOYL-COA HYDRATASE2 (ECH2), an enzyme involved in the
removal of two-carbon units from fatty
acids during peroxisomal b-oxidation. Localization studies with a fluorescent ECH2
fusion protein showed punctate fluorescence colocalizing with a dye that stains
peroxisomes. Because germinating Arabi-
dopsis seeds metabolize stored fatty acids
by peroxisomal b-oxidation, the authors
tested ech2 seedlings for sucrose dependence to determine whether they were
defective in b-oxidation. Dark-grown ech2
seedlings elongated normally with or without sucrose, demonstrating normal fatty
acid mobilization and suggesting that
ECH2 functions specifically in the conversion of IBA to IAA.
In addition to IBA-resistant hypocotyl
elongation, ech2 mutants also showed decreased apical hook formation, lessened hypocotyl elongation in response to elevated
temperature, and altered root morphology
compared with wild-type seedlings. These
phenotypes were enhanced when combined with other ibr mutations, indicating
that ECH2 functions nonredundantly with
previously characterized IBR proteins.
Studies using an auxin-responsive DR5GUS reporter construct showed that ech2
plants were highly resistant to IBA-induced
stimulation of lateral root production (see
figure, bottom panel). As with the ech2
phenotypes noted above, this phenotype
was enhanced when combined with other
ibr mutations.
The authors outline a proposed pathway
for peroxisomal conversion of IBA to IAA
and discuss this pathway in the context of
various known peroxisomal enzyme and
transport mutants. They also consider the
possibility that conversion of IBA to IAA is
part of a positive feedback loop to maintain
endogenous auxin levels. In addition to its
possible role as an auxin storage form, the
authors speculate that IBA may be an
intermediate in a de novo IAA synthesis
pathway that has yet to be fully elucidated.
Clearly, there is still much to learn about
this important plant hormone.
Gregory Bertoni
Science Editor
[email protected]
REFERENCES
Dark-grown ech2 seedlings are resistant to the
IBA-induced inhibition of hypocotyl elongation
observed in wild-type (Wt) seedlings (top). These
mutant seedlings also are resistant to IBAstimulated production of lateral root primordia
(arrowheads) and auxin-responsive reporter
gene expression (blue; bottom). (Reprinted from
Strader et al. [2011].)
Ludwig-Müller, J. (2000). Indole-3-butyric acid
in plant growth and development. Plant
Growth Regul. 32: 219–230.
Strader, L.C., Wheeler, D.L., Christensen,
S.E., Berens, J.C., Cohen, J.D., Rampey,
R.A., and Bartel, B. (2011). Multiple facets
of Arabidopsis seedling development require
indole-3-butyric acid–derived auxin. Plant Cell
23: nnn.
Went, F. (1935). Auxin, the plant growth hormone. Bot. Rev. 1: 162–182.
Zhao, Y. (2010). Auxin biosynthesis and its role
in plant development. Annu. Rev. Plant Biol.
61: 49–64.
Zolman, B.K., Martinez, N., Millius, A., Adham,
A.R., and Bartel, B. (2008). Identification and
characterization of Arabidopsis indole-3-butyric
acid response mutants defective in novel
peroxisomal enzymes. Genetics 180: 237–251.
www.plantcell.org/cgi/doi/10.1105/tpc.111.230312
The Plant Cell Preview, www.aspb.org ã 2011 American Society of Plant Biologists
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Indolebutyric Acid−Derived Auxin and Plant Development
Gregory Bertoni
Plant Cell; originally published online March 29, 2011;
DOI 10.1105/tpc.111.230312
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