G Protein Signaling in the Regulation of Arabidopsis Seed Germination

Description

This record contains information specific to the G Protein Signaling in the Regulation of Arabidopsis Seed Germination.

Overview of Seed Germination

Seed germination occurs when the quiescent seed takes up water and resumes metabolic activity and embryo growth. Germination is experimentally defined as protrusion of the radicle (the embryonic root) through the seed coat. Essential to the growth of the new seedling is mobilization of seed storage reserves, which can be primarily carbohydrates, as in the cereals, or primarily lipids, as in oil seeds, such as canola and Arabidopsis. In some dicotyledonous species, including Arabidopsis, these reserves are stored in the cotyledons of the embryo, whereas in other species, including all cereals, the reserves are stored in a specialized tissue, the endosperm (1).

The processes of germination and reserve mobilization are regulated by most of the plant hormones (2–4). Two of the most important are gibberellic acid (GA), which promotes these processes, and abscisic acid (ABA), which inhibits them. In addition, the plant steroid hormones, brassinosteroids (BRs), and the gaseous plant hormone, ethylene, are also positive regulators of germination. Sugars, such as glucose, negatively regulate germination, probably through induction of increased ABA synthesis. This Connections Map does not attempt to recapitulate our current extensive knowledge of these highly interwoven pathways (2, 3, 5), but rather focuses solely on those aspects of Arabidopsis seed biology that have been linked to the functioning of heterotrimeric guanine nucleotide-binding proteins (G proteins) (Fig. 1).

G Protein Components in Plants

The genome of the diploid plant species Arabidopsis contains a single canonical G protein α subunit gene (GPA1), a single canonical G protein β subunit gene (AGB1), and two probable G protein γ subunit genes (AGG1 and AGG2) (6). One regulator of G protein signaling gene (AtRGS1) and one gene (AtGCR1) encoding a protein that physically interacts with Gα and has limited sequence identity to the slime mold adenosine 3′,5′-monophosphate (cAMP) receptor, CAR1, have also been identified (6). The Arabidopsis genome also encodes a wealth of other predicted seven-transmembrane (7TM) domain proteins (7); because G protein–coupled receptors (GPCRs) exhibit structural rather than sequence conservation, the probability that plants have additional GPCRs is high (8, 9). Because, in contrast to the situation in mammals, the majority of plant G protein components are not represented by gene families, analysis of knockout mutants has proved fruitful in identifying signaling roles of plant G proteins. Based largely on such analyses (10), G protein components have emerged as important secondary messengers in seeds.

Promotion of Germination by Pathways Including Activation of the G Protein Heterotrimer

Seeds of the gpa1 mutant exhibit reduced sensitivity to induction of germination by the plant hormone GA and increased sensitivity to the GA synthesis inhibitor paclobutrazol, but they are not entirely GA insensitive. In addition, Arabidopsis seeds overexpressing GPA1 have enhanced sensitivity to GA, but still require GA for germination (11, 12). There are several possible interpretations of these results. One interpretation is that GA signals both through pathways that are dependent on G protein activation (as indicated in the Connections Map by the arrow between GA and the G protein complex) and through pathways that are independent of G protein activation (as indicated in the Connections Map by the arrow between GA and germination-promoting genes and the arrow between GA and GCR1).

A second interpretation is that GA signals only through pathways that are independent of G protein activation, but that the GA response is potentiated by a pathway that is dependent on activation of the heterotrimeric complex. Such potentiation could possibly originate with BR activation of G protein signaling (as indicated in the Connections Map by the subpathway originating with BR passing through activation of inactive G, the heterotrimer, to production of Gα, and from Gα to GA). Consistent with this hypothesis, BRs promote germination of wild-type seeds (13); gpa1-null seeds show reduced BR sensitivity (11, 12); and it has been suggested that BRs may stimulate GA biosynthesis (13, 14).

Promotion of Germination by Pathways Excluding Activation of the G protein Heterotrimer but Including Activation of GCR1

GCR1 has a predicted 7TM domain structure and physically interacts with GPA1 in Arabidopsis tissue (15), suggesting that it functions as a GPCR. Like gpa1-null mutants, gcr1-null mutants exhibit reduced sensitivity to the germination-promoting effects of GA and brassinolide, and GCR1 overexpressors exhibit reduced seed dormancy (16). However, the straightforward explanation of this effect—namely, that these hormones activate GCR1, which in turn activates the heterotrimer—is not supported by current data (12). Even less sensitivity to GA or BR is exhibited by gcr1 gpa1 double mutants than by the respective single mutants. This result implies that GA and BR signaling through GCR1 occurs at least in part by a mechanism that is functionally independent of the heterotrimer. Examples of this phenomenon, whimsically termed "signaling at zero G," have also been observed for the slime mold cAMP receptor, CAR1, to which GCR1 exhibits greatest sequence similarity, and for some pathways of signaling through human GPCRs (17).

Inhibition of Germination

ABA inhibits germination, and this effect is moderately enhanced in gpa1-null mutants (11, 18). One explanation for this phenomenon is that GPA1, through an unknown mechanism, normally inhibits the efficacy of ABA action, as indicated by the line between GPA1 and ABA in the Connections Map. In the absence of this negative regulation, the strength of ABA-mediated inhibition of germination, through up-regulated expression of genes encoding products that negatively affect germination, is increased.

Knockout mutants of the presumed transcriptional cofactor, AtPirin1, also exhibit moderate ABA hypersensitivity in seed germination assays (18). By yeast two-hybrid analysis and in vitro pulldown assays, AtPirin1 and GPA1 have been shown to physically interact. The ABA hypersensitivity in the atpirin1 mutants could be due either to loss of an activating signal from AtPirin1 to the positive regulator, GPA1, or to loss of an inhibitory signal from AtPirin1 back to ABA. Both possibilities are depicted in the Connections Map.

Conclusion

Like the Arabidopsis gpa1 mutant, seeds of rice Gα mutants also exhibit reduced GA sensitivity, as assayed by analysis of GA-dependent gene expression [see the G Protein Signaling in the Regulation of Rice Seed Germination pathway (http://stke.sciencemag.org/cgi/cm/stkecm;CMP_18492)]. However, a pharmacological and biochemical study of ABA signaling in barley aleurone implicated G proteins as positive components in the ABA signaling pathway (19), whereas, as described above, GPA1 appears to be a negative regulator of ABA signaling in Arabidopsis seeds. These results raise the possibility that G proteins play different roles in ABA signaling in dicot versus cereal seeds. In future research, it will be interesting to discern the extent to which G protein signaling in Arabidopsis and rice seeds represents a conserved mechanism of hormone perception and response.

Pathway Details

URL: http://stke.sciencemag.org/cgi/cm/stkecm;CMP_18476

Scope: Specific

Organism: plants: Arabidopsis

Tissue Cell: plant structures: organs: seed

Canonical Pathway: G alpha i Pathway (http://stke.sciencemag.org/cgi/cm/stkecm;CMP_7430)