The manipulation of auxin in the abscission zone cells of Arabidopsis flowers reveals that indoleacetic acid signaling is a prerequisite for organ shedding

Abstract

A number of novel strategies were employed to examine the role of indoleacetic acid (IAA) in regulating floral organ abscission in Arabidopsis (Arabidopsis thaliana). Analysis of auxin influx facilitator expression in β-glucuronidase reporter plants revealed that AUXIN RESISTANT1, LIKE AUX1, and LAX3 were specifically up-regulated at the site of floral organ shedding. Flowers from mutants where individual family members were down-regulated exhibited a reduction in the force necessary to bring about petal separation; however, the effect was not additive in double or quadruple mutants. Using the promoter of a polygalacturonase (At2g41850), active primarily in cells undergoing separation, to drive expression of the bacterial genes iaaL and iaaM, we have shown that it is possible to manipulate auxin activity specifically within the floral organ abscission zone (AZ). Analysis of petal breakstrength reveals that if IAA AZ levels are reduced, shedding takes place prematurely, while if they are enhanced, organ loss is delayed. The At2g41850 promoter was also used to transactivate the gain-of-function AXR3-1 gene in order to disrupt auxin signaling specifically within the floral organ AZ cells. Flowers from transactivated lines failed to shed their sepals, petals, and anthers during pod expansion and maturity, and these organs frequently remained attached to the plant even after silique desiccation and dehiscence had taken place. These observations support a key role for IAA in the regulation of abscission in planta and reveal, to our knowledge for the first time, a requirement for a functional IAA signaling pathway in AZ cells for organ shedding to take place.

Figure 1.

Temporal and spatial expression of GUS in Arabidopsis floral AZs and developing siliques from Pro_AUX1::GUS (A), Pro_LAX1::GUS (B), and Pro_LAX3::GUS (C) plants. The time course of expression is shown from floral development positions 2 to 16, where position 1 is the first flower where petals are visible to the eye. Bars = 1 mm.

Figure 2.

Breakstrength analysis of petals from Col-0, aux1, lax1, lax3, aux1 lax3, and aux1 lax1 lax2 lax3 plants. Breakstrength was determined at flower positions 2, 3, and 4 (n = 3).

Figure 3.

QPCR expression of At2g41850 at flower positions 4 to 5, 6 to 7, and 8 to 9.

Figure 4.

Temporal and spatial expression of DR5::GUS in Arabidopsis floral AZs and developing siliques from Col-0 (A), Pro_At2g41850::iaaM (B), and Pro_At2g41850::iaaL (C) lines. The time course of expression is shown from floral development positions 2 to 18, where position 1 is the first flower where petals are visible to the eye. Bars = 1 mm.

Figure 5.

Breakstrength analysis of petals from Col-0, Pro_At2g41850::iaaM, and Pro_At2g41850::iaaL plants. Breakstrength was determined at flower positions 2 to 8.

Figure 6.

Flower and silique development in At2g41850_Pro>>AXR3-1 plants. The time course of floral development is shown from positions 2 to 16 and at silique dehiscence, where position 1 is the first flower where petals are visible to the eye. Floral organ abscission in wild-type plants normally takes place in the glasshouse at position 5 or 6. Bar = 1 mm. [See online article for color version of this figure.]

Figure 7.

A, GFP expression in M0233 GFP plants. Bar = 1 mm. B, M0233_Pro>>AXR3 plant showing retention of floral organs throughout inflorescence development. Bar = 1 mm. [See online article for color version of this figure.]