Plant cells respond to low concentrations of auxin by cell expansion, and at a slightly higher concentration, these cells divide. Previous work revealed that null mutants of the ␣ -subunit of a putative heterotrimeric G protein ( GPA1 ) have reduced cell division. Here, we show that this prototypical G protein complex acts mechanistically by controlling auxin sensitivity toward cell division. Loss-of-function G protein mutants have altered auxin-mediated cell division throughout development, especially during the auxin-induced formation of lateral and adventitious root primordia. Ectopic expression of the wild-type G ␣ -subunit phenocopies the G ␤ mutants (auxin hypersensitivity), probably by sequestering the G ␤␥ -subunits, whereas overexpression of G ␤ reduces auxin sensitivity and a constitutively active (Q222L) mutant G ␣ behaves like the wild type. These data are consistent with a model in which G ␤␥ acts as a negative regulator of auxin-induced cell division. Accordingly, basal repression of approximately one-third of the identified auxin-regulated genes (47 of 150 upregulated genes among 8300 quantitated) is lost in the G ␤ transcript-null mutant. Included among these are genes that encode proteins proposed to control cell division in root primordia formation as well as several novel genes. These results suggest that although auxin-regulated cell division is not coupled directly by a G protein, the G ␤ -subunit attenuates this auxin pathway upstream of the control of mRNA steady state levels.

The phytohormone abscisic acid (ABA) promotes plant water conservation by decreasing the apertures of stomatal pores in the epidermis through which water loss occurs. We found that Arabidopsis thaliana plants harboring transferred DNA insertional mutations in the sole prototypical heterotrimeric GTP-binding (G) protein alpha subunit gene, GPA1, lack both ABA inhibition of guard cell inward K(+) channels and pH-independent ABA activation of anion channels. Stomatal opening in gpa1 plants is insensitive to inhibition by ABA, and the rate of water loss from gpa1 mutants is greater than that from wild-type plants. Manipulation of G protein status in guard cells may provide a mechanism for controlling plant water balance.

The alpha subunit of a prototypical heterotrimeric GTP-binding protein (G protein), which is encoded by a single gene (GPA1) in Arabidopsis, is a modulator of plant cell proliferation. gpa1 null mutants have reduced cell division in aerial tissues throughout development. Inducible overexpression of GPA1 in Arabidopsis confers inducible ectopic cell division. GPA1 overexpression in synchronized BY-2 cells causes premature advance of the nuclear cycle and the premature appearance of a division wall. Results from loss of function and ectopic expression and activation of GPA1 indicate that this subunit is a positive modulator of cell division in plants.

To directly address the function of a putative auxin receptor designated ABP1, a reverse genetic approach was taken to identify and characterize ABP1 mutant alleles in Arabidopsis. A homozygous null mutation in ABP1 confers embryo lethality. Null mutant embryos develop normally until the early stages of the globular embryo but are unable to make the transition to a bilaterally symmetrical structure because cells fail to elongate. Cell division was also aberrant both in the suspensor and embryo proper. Antisense suppression of ABP1 in tobacco cells causes slow proliferation and eliminates auxin-induced cell elongation and reduces cell division. The complete lack of auxin-inducible elongation in individual cells confirms the results observed in embryos, indicates a cell autonomous function, and, taken together with biochemical evidence that ABP1 binds auxins, suggests that ABP1 mediates auxin-induced cell elongation and, directly or indirectly, cell division.

Seed germination is regulated by many signals. We investigated the possible involvement of a heterotrimeric G protein complex in this signal regulation. Seeds that carry a protein null mutation in the gene encoding the alpha subunit of the G protein in Arabidopsis (GPA1) are 100-fold less responsive to gibberellic acid (GA), have increased sensitivity to high levels of Glc, and have a near-wild-type germination response to abscisic acid and ethylene, indicating that GPA1 does not directly couple these signals in germination control. Seeds ectopically expressing GPA1 are at least a million-fold more responsive to GA, yet still require GA for germination. We conclude that the GPA1 indirectly operates on the GA pathway to control germination by potentiation. We propose that this potentiation is directly mediated by brassinosteroids (BR) because the BR response and synthesis mutants, bri1-5 and det2-1, respectively, share the same GA sensitivity as gpa1 seeds. Furthermore, gpa1 seeds are completely insensitive to brassinolide rescue of germination when the level of GA in seeds is reduced. A lack of BR responsiveness is also apparent in gpa1 roots and hypocotyls suggesting that BR signal transduction is likely coupled by a heterotrimeric G protein at various points in plant development.Seeds integrate many intrinsic signals to control germination (Koornneef et al., 2002). For example, since the original observation by Chrispeels and Varner (1966), it has been repeatedly shown that GA induces germination and that abscisic acid (ABA) antagonizes the GA effect (Koornneef and Van der Veen, 1980;Karssen et al., 1989; Gilroy and Jones, 1994;Ritchie and Gilroy, 1998;Lovegrove and Hooley, 2000). Seed germination of GA synthesis mutants can be rescued by the application of GA and ABA synthesis and insensitive mutants lack the ABA inhibition of GA-induced germination as well as being viviparous (Koornneef et al., 1982(Koornneef et al., , 1984 Finkelstein and Somerville, 1990;Koornneef and Karssen, 1994;Leon-Kloosterziel et al., 1996). Brassinosteroid (BR) probably acts downstream of GA because BR is able to rescue germination of GA-deficient (Steber and McCourt, 2001) and GA response (Steber et al., 1998) mutant seeds. These authors argue that the BR input is likely to reside upstream of ABA's attenuating effect on GA-induced germination because BR synthesis and response mutants have slightly altered ABA sensitivity.The inhibitory effect of high concentrations of sugars on germination may occur via ABA. The evidence supporting this comes from measurements of ABA in Glc-treated seedlings (Arenas-Huertero et al., 2000) and from the observation that ABA synthesis and response mutants are insensitive to Glc (Laby et al., 2000;Rook et al., 2001). Ethylene controls the Glc inhibition of germination (Zhou et al., 1998). Evidence supporting this includes the observations that high concentrations of ethylene antagonize the Glc repression of germination (Ghassemian et al., 2000), the ctr1 (constitutive ethylene response mutant; Gibs...