Year-2002 reviews on the hormonal signaling network and the hormonal regulation of seed dormancy and germination:

Comment: Plant hormones are of utmost importance in the regulation of the genetical, physiological and biochemical properties of seeds. Only little is known about the interconnected molecular key processes regulating seed dormancy and germination in response to plant hormones and environmental cues. The dry dormant seed is well equipped to survive extended periods of unfavorable conditions. Seed dormancy can be 'coat-imposed' and/or determined by the embryo itself and is a temporary failure or block of a viable seed to complete germination under physical conditions that normally favor the process (Koornneef et al. 2002). There is considerable genetic variation of seed dormancy within species, accessions of wild type and varieties of cultivated plants. The substantial influence of environmental effects on the expression of germination characteristics and the involvement of many genes make dormancy a typical quantitative trait, which is subject to QTL (quantitative trait loci) analysis (Koornneef et al. 2002). In many plant species with coat-imposed dormancy the seed envelope imposes a physical constraint to radicle protrusion, which has to be overcome by the growth potential of the embryo. In cases of endosperm-limited germination, weakening of the micropylar endosperm surrounding the radicle tip seems to be required for radicle protrusion and is likely to involve cell-wall hydrolysis by the action of hydrolytic enzymes. Major recent contributions to the issue of the hormonal regulation of endosperm-limited germination were achieved by using tomato and Nicotiana species as suitable model systems for endospermic seeds (e.g. Grappin et al., 2000; Koornneef et al. 2002; Leubner-Metzger 2000; Toorop et al,2000; Wu et al., 2000). However, recent progress in the field of seed biology was especially achieved by molecular genetic studies with Arabidopsis thaliana hormone mutants. Several recent reviews focus on the interactions between abscisic acid (ABA), gibberellins (GA), and brassinosteroids (BR) in regulating the interconnected molecular key processes that determine dormancy and germination. These reviews are listed here and the reading of these year-2002 highlights is highly recommended.

(1) Of special interest for seed biologists:
• Koornneef, M., Bentsink, L. and Hilhorst, H. (2002) Seed dormancy and germination.
  Current Opinion in Plant Biology 5, 33-36 (hyperlink to abstract).

(2) Gibberellins (GA):
• Peng, J. and Harberd, N.P. (2002) The role of GA-mediated signalling in the control of seed germination.
  Current Opinion in Plant Biology 5, 376-381 (hyperlink to abstract).
• Olszewski, N., Sun, T.P. and Gubler, F. (2002) Gibberellin signaling:
   Biosynthesis, catabolism, and response pathways.
  The Plant Cell 14, S61-S80 (no abstract, hyperlink to supplement issue).

(3) Brassinosteroids (BR):
• Bishop, G.J. and Koncz, C. (2002) Brassinosteroids and plant steroid hormone signaling.
   The Plant Cell 14, S97-S110 (no abstract, hyperlink to supplemenr issue).

(4) Abscisic acid (ABA):
• Finkelstein, R.R., Gampala, S.S.L. and Rock, C.D. (2002) Abscisic acid signaling in seeds and seedlings.
   The Plant Cell 14, S15-S45 (no abstract, hyperlink to supplement issue).

Camille M. Steber and Peter McCourt
A role for brassinosteroids in germination in Arabidopsis
Plant Physiology 125: 763-769 (2001)
and
Leubner-Metzger G
Brassinosteroids and gibberellins promote tobacco seed germination by distinct pathways
Planta in press (2001)

Comment: These two paper present evidence that plant brassinosteroid (BR) hormones play a role in promoting seed germination. Abscisic acid (ABA) inhibits and gibberellins (GA) promote germination of Arabidopsis thaliana and Nicotiana tabacum, but only little was known about the effect of BR on seeds. In their work with Arabidopsis Camille M. Steber (United States Department of Agriculture-Agricultural Research Service, Washington State University, Pullman, Washington) and Peter McCourt (Department of Botany, University of Toronto, Ontario, Canada) show that BR rescues the germination phenotype of severe GA-biosynthesis mutants and of the GA-insensitive mutant sleepy1. In my work with non-photodormant tobacco seeds I show that while GA can replace light in promoting dark germination, BR promotes tobbacco germination in the light and in the dark. GA, but not BR, can break photodormancy of photodormant tobacco seeds. In both species BR counteracts the inhibitory effect of ABA on seed germination in an GA-independent manner. Germination of the BR-deficient det2-1 and the BR-insensitive bri1-1 mutant is more strongly inhibited by ABA compared to Arabidopsis wild-type. Taken together these results suggest that BR and GA promote seed germination by distinct signalling pathways and that BR might act by direct promotion of the embryo's growth potential.

Beaudoin N, Serizet C, Gosti F, Giraudat J. Interactions between abscisic acid and ethylene signaling cascades.
The Plant Cell 12: 1103-1115 (2000)
Ghassemian M, Nambara E, Cutler S, Kawaide H, Kamiya Y, McCourt P. Regulation of abscisic acid signaling by the ethylene response pathway in Arabidopsis.
The Plant Cell 12: 1107-1126 (2000)

Comment: Seed germination is inhibited by abscisic acid (ABA) and in many species promoted by ethylene (reviewed by Kepczynski and Kepczynska, Physiol. Plant. 101: 720-726, 1997). The work presented in the two Plant Cell papers demonstrate with Arabidopsis mutants that the two signaling pathways interact during germination. Ethylene appears to promote germination by decreasing the sensitivity to endogenous ABA and therefore seems to be a negative reulator of ABA action during seed germination. ABA also inhibits the transcipt expression of ethylene-responsive element binding proteins (EREBPs) in germinating tobacco seeds (Leubner-Metzger et al., 1998). The ethylene-inducible EREBPs seem to be another seed-associated step where the ABA and ethylene signaling pathways interact.

Weber H, Rolletschek H, Heim U, Golombek S, Gubatz S, Wobus U. Antisense-inhibition of ADP-glucose pyrophosphorylase in developing seeds of Vicia narbonensis moderately decreases starch but increases protein content and affects seed maturation.
The Plant Journal 24: 33-43 (2000)

Comment: During seed development, processes like cell elongation, differentiation and storage activities appear to be subject to metabolic control (reviewed by Wobus and Weber, Biol Chem 380: 937-944, 1999). The group of Hans Weber and Ulrich Wobus from the Institute for Plant Genetics and Crop Plant Research (IPK) in Gatersleben (Germany) showed that reduction of the small subunit of the ADP-glucose pyrophosphorylase by antisense transformation not only affected starch content, seed filling and other storage activities, but most interestingly extended the seed maturation phase. They conclude that alteration in carbon metabolism can induce temporal changes in seed development.

Nonogaki H, Gee OH, Bradford KJ. A germination-specific endo-ß-mannanase gene is expressed in the micropylar endosperm cap of tomato seeds
Plant Physiology 123: 1235-1245 (2000)
Toorop PE, van Aelst AC, Hilhorst HWM. The second step of the biphasic endosperm cap weakening that mediates tomato (Lycopersicon esculentum) seed germination is under control of ABA.
Journal of Experimental Botany 51: 1371-1379 (2000)

Comment: These papers represent recent progress concerning the role of hydrolases during endosperm weakening of germinating tomato seeds. The group of Henk Hilhorst from Wageningen University (The Netherlands) proposed that tomato endosperm cap weakening is a biphasic precess. The first phase is characterized by major ABA-independent weakening, whereas the second phase just prior to radicle emergence is under the control of ABA. The group of Kent Bradford from the University of Califormia, Davis (California, USA) cloned a cDNA (LeMAN2) encoding a tomato endo-ß-mannanase. LeMAN2 expression is confined to the endosperm cap tissue of seeds prior to radicle emergence, is induced by GA in gib-1 mutant seeeds, but is not inhibited in wild-type seeds by ABA. These results support the view that the LeMAN2 germinative mannanase is one component among several that are required for cell wall disassembly and tissue weakening in the endosperm cap to allow radicle emergence. The final ABA-dependent step of radicle emergence might involve the expression of additional ABA-controlled hydrolase genes. The role of endo-ß-mannanase during seed germination is reviewed by J. Derek Bewley (Trends in Plant Science 2: 464-469, 1997).

Grappin P, Bouinot D, Sotta B, Miginiac E, Jullien M. Control of seed dormancy in Nicotiana plumbaginifolia: post-imbibition abscisic acid synthesis imposes dormancy maintenance.
Planta 210: 279-285 (2000)

Comment: Abscisic acid (ABA) is needed to establish and maintain primary dormancy in many species including the genus Nicotiana. The onset of dormancy in Nicotiana seeds is correlated with a peak in ABA content, which declines rapidely during further seed maturation (DAP 15-25 in tobacco). The work of the group Marc Jullien at the INRA in Versailles (France) demonstrates that afterripening of Nicotiana seeds is correlated with a further decline in ABA content and decreased sensitivity to ABA. Dry dormant (fresh) seeds contain more ABA than dry non-dormant (afterripened) seeds. Interestingly de novo ABA synthesis was found in imbibed dormant seed, but not in non-dormant seed. Gibberellin (GA) treatment inhibited the accumulation of ABA in dormant seeds. This work reveals an important role for ABA synthesis in dormancy maintenance in imbibed seeds.