Unique and overlapping expression patterns among the Arabidopsis 1-amino-cyclopropane-1-carboxylate synthase gene family members

Abstract

1-Aminocyclopropane-1-carboxylate synthase (ACS) catalyzes the rate-limiting step in the ethylene biosynthetic pathway in plants. The Arabidopsis genome encodes nine ACS polypeptides that form eight functional (ACS2, ACS4-9, and ACS11) homodimers and one nonfunctional (ACS1) homodimer. Transgenic Arabidopsis lines were constructed expressing the beta-glucuronidase (GUS) and green fluorescence protein (GFP) reporter genes from the promoter of each of the gene family members to determine their patterns of expression during plant development. All genes, except ACS9, are expressed in 5-d-old etiolated or light-grown seedlings yielding distinct patterns of GUS staining. ACS9 expression is detected later in development. Unique and overlapping expression patterns were detected for all the family members in various organs of adult plants. ACS11 is uniquely expressed in the trichomes of sepals and ACS1 in the replum. Overlapping expression was observed in hypocotyl, roots, various parts of the flower (sepals, pedicle, style, etc.) and in the stigmatic and abscission zones of the silique. Exogenous indole-3-acetic acid (IAA) enhances the constitutive expression of ACS2, 4, 5, 6, 7, 8, and 11 in the root. Wounding of hypocotyl tissue inhibits the constitutive expression of ACS1 and ACS5 and induces the expression of ACS2, 4, 6, 7, 8, and 11. Inducers of ethylene production such as cold, heat, anaerobiosis, and Li(+) ions enhance or suppress the expression of various members of the gene family in the root of light-grown seedlings. Examination of GUS expression in transverse sections of cotyledons reveals that all ACS genes, except ACS9, are expressed in the epidermis cell layer, guard cells, and vascular tissue. Similar analysis with root tip tissue treated with IAA reveals unique and overlapping expression patterns in the various cell types of the lateral root cap, cell division, and cell expansion zones. IAA inducibility is gene-specific and cell type-dependent across the root tip zone. This limited comparative exploration of ACS gene family expression reveals constitutive spatial and temporal expression patterns of all gene family members throughout the growth period examined. The unique and overlapping gene activity pattern detected reveals a combinatorial code of spatio-temporal coexpression among the various gene family members during plant development. This raises the prospect that functional ACS heterodimers may be formed in planta.

Figure 1.

Expression of the ACS gene family members in 5-d-old etiolated seedlings.

Figure 2.

Expression of the ACS gene family members in 5-d-old light-grown seedlings. The white lines indicate the location of the transverse sectioning for obtaining the results shown in Figure 9. A magnified view of the shoot apex region is shown in Figure 3. Summary Table of Figure 2, in which ○ denotes expression. [Table: see text]

Figure 3.

Expression of the ACS gene family members in the shoot apex regions of Figure 2.

Figure 4.

Expression of ACS1, ACS6, ACS9, and ACS11 gene family members in 1-month-old plants.

Figure 5.

Expression of ACS gene family members in the flowers of 1-month-old transgenic plants. Summary Table of Figure 5, in which ○ denotes expression. [Table: see text]

Figure 6.

Expression of ACS gene family members in the siliques of 1-month-old transgenic plants. Summary Table of Figure 6, in which ○ denotes expression. [Table: see text]

Figure 7.

Effect of IAA on the expression of the ACS gene family members in the root tip of 5-d-old light-grown seedlings. IAA concentration, 20 μm; duration of treatment, 24 h. The white lines indicate the location of the transverse sectioning for obtaining the results shown in Figure 11.

Figure 8.

Effect of cutting on the expression of ACS1, ACS5, ACS6, and ACS9 in 5-d-old light-grown seedlings. Each seedling was cut with a blade in the middle of the hypocotyl (see white line in A) and incubated for 4 h on Murashige and Skoog media prior to staining. A, Intact seedlings. B, Upper part of seedlings after 4 h from cutting. C, Magnified view of B. D, Lower part of seedlings after 4 h from cutting. E, Magnified view of D.

Figure 9.

Expression of ACS gene family members observed in transverse sections of the cotyledons (left column) and hypocotyls (right column) of 5-d-old light-grown seedlings. Tissue sectioning was carried out after 2 h of GUS staining. Section thickness, cotyledon, 20 μm; hypocotyl, 8 μm.

Figure 10.

Expression of the ACS gene family members in the vascular tissue (A) and guard cells (B) observed in transverse sections of cotyledons of 5-d-old light-grown transgenic seedlings. Tissue sectioning was performed after 2 h of GUS staining. Section thickness, 8 μm.

Figure 11.

Expression of ACS gene family members detected by transverse sectioning in various zones of the primary root tip in 5-d-old light-grown transgenic seedlings treated with 20 μm IAA for 24 h. GUS staining, 2 h. Section thickness, 8 μm.Summary Table of Figure 11, in which denotes −IAA and • denotes +IAA. Arrows (↑) denote enhanced expression in response to auxin. [Table: see text]

Figure 12.

Expression of ACS8 in the lateral root cap zone of the primary root. GUS activity staining in several transverse sections (8 μm) of the primary root tip is shown for a 5-d-old light-grown transgenic for ACS8-GUS/GFP. The numbers show the order of sectioning away from the root tip.