RCN1-regulated phosphatase activity and EIN2 modulate hypocotyl gravitropism by a mechanism that does not require ethylene signaling - PubMed (original) (raw)

RCN1-regulated phosphatase activity and EIN2 modulate hypocotyl gravitropism by a mechanism that does not require ethylene signaling

Gloria K Muday et al. Plant Physiol. 2006 Aug.

Abstract

The roots curl in naphthylphthalamic acid1 (rcn1) mutant of Arabidopsis (Arabidopsis thaliana) has altered auxin transport, gravitropism, and ethylene response, providing an opportunity to analyze the interplay between ethylene and auxin in control of seedling growth. Roots of rcn1 seedlings were previously shown to have altered auxin transport, growth, and gravitropism, while rcn1 hypocotyl elongation exhibited enhanced ethylene response. We have characterized auxin transport and gravitropism phenotypes of rcn1 hypocotyls and have explored the roles of auxin and ethylene in controlling these phenotypes. As in roots, auxin transport is increased in etiolated rcn1 hypocotyls. Hypocotyl gravity response is accelerated, although overall elongation is reduced, in etiolated rcn1 hypocotyls. Etiolated, but not light grown, rcn1 seedlings also overproduce ethylene, and mutations conferring ethylene insensitivity restore normal hypocotyl elongation to rcn1. Auxin transport is unaffected by treatment with the ethylene precursor 1-aminocyclopropane carboxylic acid in etiolated hypocotyls of wild-type and rcn1 seedlings. Surprisingly, the ethylene insensitive2-1 (ein2-1) and ein2-5 mutations dramatically reduce gravitropic bending in hypocotyls. However, the ethylene resistant1-3 (etr1-3) mutation does not significantly affect hypocotyl gravity response. Furthermore, neither the etr1 nor the ein2 mutation abrogates the accelerated gravitropism observed in rcn1 hypocotyls, indicating that both wild-type gravity response and enhanced gravity response in rcn1 do not require an intact ethylene-signaling pathway. We therefore conclude that the RCN1 protein affects overall hypocotyl elongation via negative regulation of ethylene synthesis in etiolated seedlings, and that RCN1 and EIN2 modulate hypocotyl gravitropism and ethylene responses through independent pathways.

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Figures

Figure 1.

Figure 1.

The rcn1 mutant phenotype is attenuated in light-grown seedlings. Photographs show 5-d-old wild-type (A, C, and E) and rcn1 mutant (B, D, and F) seedlings that were grown in darkness (A and B), low light (8 _μ_mol m−2 s−1; C and D), or high light (100 _μ_mol m−2 s−1; E and F). G, The lengths of 5-d-old hypocotyls grown in the dark and low light were quantified by analysis of digital images of hypocotyls. The values shown are averages (±

se

) for 20 seedlings from two separate experiments, and length for Ws and rcn1 were compared by Student's t test. ***, P < 0.0005. Scale bar = 2 mm.

Figure 2.

Figure 2.

Ethylene synthesis is elevated in dark-grown rcn1 seedlings but not in light-grown seedlings. Ethylene levels were determined by gas chromatography for Ws and rcn1 seedlings grown in high light or darkness. There is significantly less ethylene synthesis (P < 0.05) in Ws in the dark than in either Ws in the light or rcn1 in the dark.

Figure 3.

Figure 3.

IAA transport is elevated in dark-grown rcn1 seedlings. A, IAA transport was compared in dark-grown seedlings matched for age or for size. B, IAA transport was compared for dark- and low light-grown seedlings. Values shown represent averages (±

se

) for 30 to 40 seedlings from four separate experiments, and the differences between Ws and rcn1 under each condition were compared by Student's t test. ***, P < 0.0005.

Figure 4.

Figure 4.

Hypocotyls of the rcn1 mutant exhibit enhanced gravity response in the absence or presence of ACC. Etiolated seedlings were transferred under a dim green safelight to agar plates containing 0 or 10 μ

m

ACC at 5 d after germination. The plates were immediately reoriented by 90° relative to gravity and digital photographs were taken each hour for 6 h. Representative assay results 6 h after reorientation on medium without ACC are shown (A). Curvature (B) and elongation (C) were measured on sequential digital photographs. Each data point represents the average (±

se

) for 10 plants. Scale bar = 2 mm.

Figure 5.

Figure 5.

Reduced ethylene perception enhances growth but does not impair gravitropic response. Seedlings were grown for 4 d in the dark and then transferred to fresh control media or media supplemented with 100 n

m

silver nitrate. After 18 h, seedlings were reoriented 90° relative to gravity and the amount of growth (A) and the gravitropic response (B) was quantified 6 h after reorientation. The average and

se

of 25 to 35 seedlings from three separate experiments are reported. C, Representative images of the seedlings after 6 h are shown. Student's t tests were used to determine statistical differences between growth and gravity response between untreated and silver-treated seedlings within genotypes (white bars). *, P < 0.05. Scale bar = 5 mm.

Figure 6.

Figure 6.

Ethylene resistance does not impede rcn1 hypocotyl gravity response. Seedlings of the indicated genotypes were grown in the dark for 5 d and transferred to media with or without ACC. Each value represents the average (±

se

) for growth and gravity response 6 h after transfer and reorientation with two or three separate experiments averaged (n = 17 to 30 seedlings).

Figure 7.

Figure 7.

Roles of RCN1-regulated PP2A activity in etiolated hypocotyls. RCN1-regulated PP2A activity controls hypocotyl elongation and gravity response through genetically separable pathways in dark-grown seedlings. PP2A activity controls elongation through a pathway requiring regulated ethylene biosynthesis and response, while PP2A control of gravity response involves regulation of auxin transport but is independent of ethylene response.

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