Auxin transport is required for hypocotyl elongation in light-grown but not dark-grown Arabidopsis - PubMed (original) (raw)

Auxin transport is required for hypocotyl elongation in light-grown but not dark-grown Arabidopsis

P J Jensen et al. Plant Physiol. 1998 Feb.

Abstract

Many auxin responses are dependent on redistribution and/or polar transport of indoleacetic acid. Polar transport of auxin can be inhibited through the application of phytotropins such as 1-naphthylphthalamic acid (NPA). When Arabidopsis thaliana seedlings were grown in the light on medium containing 1.0 microM NPA, hypocotyl and root elongation and gravitropism were strongly inhibited. When grown in darkness, however, NPA disrupted the gravity response but did not affect elongation. The extent of inhibition of hypocotyl elongation by NPA increased in a fluence-rate-dependent manner to a maximum of about 75% inhibition at 50 mumol m-2 s-1 of white light. Plants grown under continuous blue or far-red light showed NPA-induced hypocotyl inhibition similar to that of white-light-grown plants. Plants grown under continuous red light showed less NPA-induced inhibition. Analysis of photoreceptor mutants indicates the involvement of phytochrome and cryptochrome in mediating this NPA response. Hypocotyls of some auxin-resistant mutants had decreased sensitivity to NPA in the light, but etiolated seedlings of these mutants were similar in length to the wild type. These results indicate that light has a significant effect on NPA-induced inhibition in Arabidopsis, and suggest that auxin has a more important role in elongation responses in light-grown than in dark-grown seedlings.

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Figures

Figure 1

Figure 1

The effect of light on NPA-induced inhibition of elongation of hypocotyls (A) and roots (B). Growth is normalized to that of seedlings grown without NPA. The fluence rate of the white light was 50 μmol m−2 s−1. Values are the means ± sd from three independent replicates, with at least 10 seedlings per replicate. conc., Concentration.

Figure 2

Figure 2

The effect of fluence rate on inhibition of elongation by 1.0 μm NPA. A, Comparison of hypocotyl lengths in 1.0 μm NPA-treated and untreated seedlings. B, Comparison of root lengths in 1.0 μm NPA-treated and untreated seedlings. C, Percentage inhibition of hypocotyl elongation calculated as growth on 1.0 μm NPA compared with untreated seedlings at the corresponding fluence rate. D, Percentage inhibition of root elongation calculated as growth on 1.0 μm NPA compared with untreated seedlings (see C). Values are the means ± sd from three independent replicates, with at least 10 seedlings per replicate.

Figure 3

Figure 3

The effect of 1.0 μm NPA on the gravity response of hypocotyls (A) and roots (B) of light- and dark-grown seedlings. Data are presented as the sd of the orientation angles of the hypocotyls and roots as a measure of the randomness of orientation.

Figure 4

Figure 4

The effect of light quality on the inhibition of hypocotyl elongation by 1.0 μm NPA. Seedlings were grown under continuous white light (80 μmol m−2 s−1), blue light (30 μmol m−2 s−1), far-red light (10 μmol m−2 s−1), or red light (25 μmol m−2 s−1) for 7 d. Values are the means ± sd from three independent replicates, with at least 10 seedlings per replicate.

Figure 5

Figure 5

The effect of blue and far-red light fluence rate on inhibition of hypocotyl elongation by 1.0 μm NPA. The percentage inhibition of elongation is calculated as growth on 1.0 μm NPA compared with untreated seedlings at the corresponding fluence rate. Values are the means ± sd from three independent replicates, with at least 10 seedlings per replicate.

Figure 6

Figure 6

The effect of 1.0 μm NPA on the hypocotyl elongation of photoreceptor mutants and wild types (Ler and Rld). Values are the means ± sd from three independent replicates, with at least 10 seedlings per replicate. The percentage-inhibition values were calculated by determining the percentage inhibition for each NPA-treated seedling using the average length of the appropriate control seedlings. The values for each seedling were then averaged to generate the data points ± sd. Seedlings were grown as described in Figure 4.

Figure 7

Figure 7

Inhibition of lateral root formation by 0.5 μm NPA at various fluence rates. Values are the means ± sd from three independent replicates, with at least 10 seedlings per replicate.

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