Extragenic suppressors of the Arabidopsis gai mutation alter the dose-response relationship of diverse gibberellin responses

Abstract

Active gibberellins (GAs) are endogenous factors that regulate plant growth and development in a dose-dependent fashion. Mutant plants that are GA deficient, or exhibit reduced GA responses, display a characteristic dwarf phenotype. Extragenic suppressor analysis has resulted in the isolation of Arabidopsis mutations, which partially suppress the dwarf phenotype conferred by GA deficiency and reduced GA-response mutations. Here we describe detailed studies of the effects of two of these suppressors, spy-7 and gar2-1, on several different GA-responsive growth processes (seed germination, vegetative growth, stem elongation, chlorophyll accumulation, and flowering) and on the in planta amounts of active and inactive GA species. The results of these experiments show that spy-7 and gar2-1 affect the GA dose-response relationship for a wide range of GA responses and suggest that all GA-regulated processes are controlled through a negatively acting GA-signaling pathway.

Figure 1

Germination of wild-type (WT), gai, gai spy-7, gai gar2–1, and gai spy-7 gar2–1 seeds grown in the presence (gray bars) or absence (black bars) of 0.1 mm PAC. Germination was scored 7 d after moving the plates containing the seeds from the cold room. Results are presented as mean ± se of three separate experiments (for each sample, n = 10–22).

Figure 2

Comparison of GA response of the wild type (WT), gai, and gai gar2–1. Adult heights (47 d after sowing) of GA₃-treated (gray bars; 0.1 mm GA₃) and control (black bars) plants are shown. Wild-type and ga4 (GA-deficient mutant) plants were taller after GA treatments, whereas gai and gai gar2–1 were unaffected. Plants were grown in an 18-h photoperiod. Results are presented as means ± se (n = 18–30).

Figure 3

A, Photograph of (left to right) wild-type, gai, gai spy-7, gai gar2–1, and gai spy-7 gar2–1 plants. B, Comparison of adult plant heights (55 d after sowing). Plants were grown in an 18-h photoperiod. Results are presented as means ± se (n = 19–26).

Figure 4

Chlorophyll contents of leaves of wild type, gai, gai spy-7, gai gar2–1, and gai spy-7 gar2–1 (21-d-old plants, grown in an 18-h photoperiod). Results are presented as means ± se of three separate experiments.

Figure 5

A, Photograph of (left to right) wild type, gai, gai spy-7, gai gar2–1 and gai spy-7 gar2–1 grown (50 d after sowing) in SD. The wild-type and gai plants do not have open flowers, whereas suppressed gai plants, in particular gai gar2–1 and gai spy-7 gar2–1, have already flowered. B, Comparison of flowering times of the wild type (WT), gai, gai spy-7, gai gar2–1, and gai spy-7 gar2–1 under LD (black bars) and SD (gray bars). Numbers of rosette and cauline leaves that appeared prior to the opening of the first flower bud were counted. Results are presented as means ± se (n = 29–35).

Figure 6

Vegetative rosette radii of wild-type (WT), gai, gai spy-7, gai gar2–1, and gai spy-7 gar2–1 plants (30 d old) grown in the presence (gray bars) or absence (black bars) of 1 nm PAC. Results are presented as means ± se (n = 16–29).

Figure 7

Growth of wild-type, gai, and gai spy-7 gar2–1 plants on germination medium (Valvekens et al., 1988) containing different concentrations of PAC (A, 0 nm PAC; B, 0.5 nm PAC; and C, 5 nm PAC) and GA₃. Seeds (15–18 seeds for each sample) were sterilized, placed on appropriate medium, chilled for 7 d at 4°C, and then grown at 23°C with an 18-h photoperiod. For each of the three genotypes, three representative plants (22 d old) are shown for each treatment.