Genetic analysis of growth-regulator-induced parthenocarpy in Arabidopsis

Abstract

In Arabidopsis, seedless silique development or parthenocarpy can be induced by the application of various plant growth regulators (PGRs) to unfertilized pistils. Ecotype-specific responses were observed in the Arabidopsis ecotypes Columbia and Landsberg relative to the type of PGR and level applied. The parthenocarpic response was greatest in ecotype Landsberg, and comparisons of fruit growth and morphology were studied primarily in this ecotype. Gibberellic acid application (10 micromol pistil(-1)) caused development similar to that in pollinated pistils, while benzyladenine (1 micromol pistil(-1)) and naphthylacetic acid (10 micromol pistil(-1)) treatment produced shorter siliques. Naphthylacetic acid primarily modified mesocarp cell expansion. Arabidopsis mutants were employed to examine potential dependencies on gibberellin biosynthesis (ga1-3, ga4-1, and ga5-1) and perception (spy-4 and gai) during parthenocarpic silique development. Emasculated spy-4 pistils were neither obviously parthenocarpic nor deficient in PGR perception. By contrast, emasculated gai mutants did not produce parthenocarpic siliques following gibberellic acid application, but silique development occurred following pollination or application of auxin and cytokinin. Pollinated gai siliques had decreased cell numbers and morphologically resembled auxin-induced parthenocarpic siliques. This shows that a number of independent and possibly redundant pathways can direct hormone-induced parthenocarpy, and that endogenous gibberellins play a role in regulating cell expansion and promoting cell division in carpels.

Figure 1

Top and middle, Silique elongation of emasculated anthesis pistils after pollination (P), without pollination (UP), or treated with GA₃ (10 μmol pistil⁻¹), NAA (10 μmol pistil⁻¹), or BA (1 μmol pistil⁻¹) in the Ler (top) and Col-1 (middle) ecotypes. Bottom, Silique elongation in the gai background after emasculation of anthesis-stage pistils left unpollinated (UP) or after cross-pollination (P) or NAA treatment (10 μmol pistil⁻¹). For estimates of error (±sd) refer to Table I.

Figure 2

Siliques 7 DPA after treatment with IAA, NAA, GA₃, or BA in the Ler background (top) or Columbia (bottom), with respective application levels in micromoles per pistil indicated in each panel. UP, Unpollinated; P, pollinated.

Figure 3

Receptivity period for pollination- and GA₃-induced silique elongation was determined by a single treatment of either GA₃ (▴; 10 μmol pistil⁻¹) or pollination (●) to emasculated pistils of Arabidopsis at various DPA in the Ler (top) and Col-1 (middle) ecotypes. Seed set (bottom) was also determined with respect to the DPA following pollination in Ler (black bars) and Col-1 (hatched bars).

Figure 4

Ws-O siliques pollinated (P), unpollinated (UP), and GA₃ treated (10 μmol pistil⁻¹) compared with spy-4 unpollinated (UP), pollinated (P), and GA₃-, NAA-, or BA-treated (10, 10, and 1 μmol pistil⁻¹, respectively; top) siliques. gai and ga5-1 unpollinated pistils and pollinated siliques (Ler; bottom) compared with GA₃, NAA, and BA treatment (10, 10, and 1 μmol pistil⁻¹, respectively), as described in “Materials and Methods.”

Figure 5

Carpel wall cross-sections illustrating the degree of carpel expansion and development from an anthesis-stage pistil compared with 7-DPA unpollinated, pollinated, and PGR-treated siliques from the gai and Ler backgrounds. a, Anthesis-stage Ler pistil; b, unpollinated Ler pistil; c, pollinated Ler silique; d, Ler silique induced with 10 μmol GA₃ pistil⁻¹; e, Ler silique induced with 10 μmol NAA pistil⁻¹; f, pollinated gai silique. Unmarked arrowheads indicate dehiscence zones; X, exocarp; M, mesocarp; T, supportive sclerenchyma; N, endocarp; S, seed; O, ovule; P, septum; F, funiculus; R, replum. Scale bar = 250 μm.

Figure 6

Longitudinal carpel wall sections of Ler at anthesis (a) and at 7 DPA for an unpollinated silique (b), a pollinated silique (c), and parthenocarpic siliques induced by 10 μmol GA₃ pistil⁻¹ (d), 1 μmol BA pistil⁻¹ (e), 10 μmol NAA pistil⁻¹ (f). g to l, Silique wall sections of the respective Ler treatments in the gai background. Carpel wall sections of spy-4 anthesis pistil (m) and 7 at DPA for an unpollinated pistil (n), pollinated silique (o), and emasculated spy-4 pistil induced to grow with 10 μmol GA₃ pistil⁻¹ (p). Rescue of carpel wall structure in the ga5-1 biosynthetic mutant, 7-d pollinated ga5-1 silique (q), ga5-1 parthenocarpic silique induced with 10 μmol GA₃ pistil⁻¹ (r). X, Exocarp; M, mesocarp; T, supportive sclerenchyma; E, endocarp; O, ovule; F, funiculus. Scale bar = 100 μm.

Figure 7

A model suggesting how GA biosynthesis and perception may determine silique structure in Arabidopsis. Signals from fertilization allow cell division, cell expansion, and differentiation during development. This may include activation of certain steps within the GA signal transduction cascade required for normal differentiation. Levels of active GAs (GA₁, GA₃, or GA₄) would specifically limit cell division and the biosynthetic mutants (italics) would block or alter this process. One exception occurs at GA4, where other known 3β-hydrolyases may allow synthesis of active GAs. Based on mutant analysis described here, GAI may participate in GA perception by transducing signals that regulate cell division. a, At high GA levels cell differentiation occurs as for normal pollinated siliques; b, at low levels of active GA an auxin-like effect dominates with limited cellular division; and c, at very low levels of GA pistils cannot differentiate into siliques. The steps in GA biosynthesis between ent-copalyl diphosphate and GA₁₉ or GA₂₄ are abbreviated for simplicity. Other steps are detailed in Hedden and Kamiya (1997) and Sponsel et al. (1997). X, Exocarp; M, mesocarp; E, endocarp.