Plant & Cell Physiology 53: 81-95 (2012)

Myrigalone A inhibits Lepidium sativum seed germination by interference with gibberellin metabolism and apoplastic superoxide production required for embryo extension growth and endosperm rupture^[W]

Abstract
Myrica gale L. (sweet gale) fruit leachate contains myrigalone A (MyA), a rare C-methylated dihydrochalcone and putative allelochemical, which is known to be a phytotoxin impeding seedling growth. We found that MyA inhibited Lepidium sativum L. seed germination in a dose-dependent manner. MyA did not affect testa rupture, but inhibited endosperm rupture and the transition to subsequent seedling growth. MyA inhibited micropylar endosperm cap (CAP) weakening and the increase in the growth potential of the radical/hypocotyl region (RAD) of the embryo, both being key processes required for endosperm rupture. We compared the contents of abscisic acid (ABA) and gibberellins (GA) in the tissues and found that the major bioactive forms of GA in L. sativum seed tissues were GA4 and GA6, while GA8 and GA13 were abundant inactive metabolites. MyA did not appreciably affect the ABA contents, but severely interfered with GA metabolism and signalling by inhibiting important steps catalysed by GA3 oxidase, as well as by interfering with the GID1-type GA signalling pathway. The hormonally and developmentally regulated formation of apoplastic superoxide radical is important for embryo growth. Specific zones within the RAD were associated with accumulation of apoplastic superoxide radical and endoreduplication indicative of embryo cell extension. MyA negatively affected both of these processes and acted as a scavenger of apoplastic reactive oxygen species. We propose that MyA is an allelochemical with a novel mode of action on seed germination.

Key words: Embryo cell extension growth • Endoreduplication • Endosperm rupture • Gibberellin metabolism • Lepidium sativum • Myrica gale • Phytotoxicity • Reactive oxygen species.

Funding: Our work was funded by AvH Research Fellowship from the Alexander von Humboldt Foundation [06/2009 to K.O.]; the Deutsche Forschungsgemeinschaft Grant [DFG LE720/6 to G.L.-M.]; the Deutscher Akademischer Austauschdienst Grant [DAAD D/07/01311 to G.L.-M.]; the Ministry of Education, Youth and Sports of the Czech Republic [MSM 6198959216 to M.S.]; Czech Science Foundation [GD522/08/H003 to M.S.]; the Grant Agency of the Academy of Sciences CR [KAN200380801 to M.S.] and EU funding Operational Program Research and Development for Innovations [CZ.1.05/2.1.00/01.0007 to M.S.].