RESUMO
BACKGROUND AND AIMS: Coffee seed germination represents an interplay between the embryo and the surrounding endosperm. A sequence of events in both parts of the seed determines whether germination will be successful or not. Following previous studies, the aim here was to further characterize the morphology of endosperm degradation and embryo growth with respect to morphology and cell cycle, and the influence of abscisic acid on these processes. METHODS: Growth of cells in a fixed region of the axis was quantified from light micrographs. Cell cycle events were measured by flow cytometry and by immunocytochemistry, using antibodies against beta-tubulin. Aspects of the endosperm were visualized by light and scanning electron microscopy. KEY RESULTS: The embryonic axis cells grew initially by isodiametric expansion. This event coincided with reorientation and increase in abundance of microtubules and with accumulation of beta-tubulin. Radicle protrusion was characterized by a shift from isodiametric expansion to elongation of radicle cells and further accumulation of beta-tubulin. Early cell division events started prior to radicle protrusion. Abscisic acid decreased the abundance of microtubules and inhibited the growth of the embryo cells, the reorganization of the microtubules, DNA replication in the embryonic axis, the formation of a protuberance and the completion of germination. The endosperm cap cells had smaller and thinner cell walls than the rest of the endosperm. Cells in the endosperm cap displayed compression followed by loss of cell integrity and the appearance of a protuberance prior to radicle protrusion. CONCLUSIONS: Coffee seed germination is the result of isodiametric growth of the embryo followed by elongation, at the expense of integrity of endosperm cap cells. The cell cycle, including cell division, is initiated prior to radicle protrusion. ABA inhibits expansion of the embryo, and hence subsequent events, including germination.
Assuntos
Ácido Abscísico/farmacologia , Divisão Celular , Coffea/efeitos dos fármacos , Coffea/embriologia , Germinação/efeitos dos fármacos , Sementes/efeitos dos fármacos , Análise de Variância , Coffea/citologia , Coffea/fisiologia , Microscopia Crioeletrônica , Citometria de Fluxo , Microtúbulos/fisiologia , Sementes/citologia , Tubulina (Proteína)/metabolismoRESUMO
The mechanism and regulation of coffee seed germination were studied in Coffea arabica L. cv. Rubi. The coffee embryo grew inside the endosperm prior to radicle protrusion and abscisic acid (ABA) inhibited the increase in its pressure potential. There were two steps of endosperm cap weakening. An increase in cellulase activity coincided with the first step and an increase in endo-beta-mannanase (EBM) activity with the second step. ABA inhibited the second step of endosperm cap weakening, presumably by inhibiting the activities of at least two EBM isoforms and/or, indirectly, by inhibiting the pressure force of the radicle. The increase in the activities of EBM and cellulase coincided with the decrease in the force required to puncture the endosperm and with the appearance of porosity in the cell walls as observed by low-temperature scanning electronic microscopy. Tissue printing showed that EBM activity was spatially regulated in the endosperm. Activity was initiated in the endosperm cap whereas later during germination it could also be detected in the remainder of the endosperm. Tissue printing revealed that ABA inhibited most of the EBM activity in the endosperm cap, but not in the remainder of the endosperm. ABA did not inhibit cellulase activity. There was a transient rise in ABA content in the embryo during imbibition, which was likely to be responsible for slow germination, suggesting that endogenous ABA also may control embryo growth potential and the second step of endosperm cap weakening during coffee seed germination.