EXPRESSION OF DESICCATION-INDUCED AND LIPOXYGENASE GENES DURING THE TRANSITION FROM THE MATURATION TO THE GERMINATION PHASES IN SOYBEAN SOMATIC EMBRYOS

Transitions between developmental programs during plant embryogenesis are usually characterized by differential gene expression. Soybean (Gl ycine max (L.) Merr.) somatic embryos from embryogenic suspension cult ures germinate poorly on a germination medium even after being grown o n medium containing abscisic acid until developmental changes are indu ced by desiccation. The objectives of this study were to investigate: (i) the relationship between water loss and germination of somatic emb ryos, (ii) changes in desiccation-induced polypeptides, and (iii) the expression of maturation- and germination-associated genes during the transition from maturation to germination of soybean somatic embryos. The results revealed that partial drying of somatic embryos at 76% rel ative humidity for 96 h resulted in more than 90% germination. Germina tion frequency was correlated with the percent fresh weight loss of th e somatic embryos. Polypeptides induced by desiccation were classified into three groups based on the tissues in which they were expressed a nd the inducing agents to which they responded. Desiccation-induced po lypeptides decreased or became undetectable upon imbibition. The expre ssion of a maturation-associated gene, Mat1, was induced in both cotyl edons and hypocotyl/radicle tissues of somatic embryos after 72 h desi ccation. The transcript for this gene increased in concert with water loss from the embryos. During maturation of somatic embryos, the expre ssion of embryonic lipoxygenase genes was turned on. Desiccation alone did not directly induce expression of germination-associated lipoxyge nase genes SC514 and LOXB2 in somatic embryos. However, upon rehydrati on for 24 h and prior to any visible developmental change in embryos o n the germination medium, the lipoxygenase transcripts and proteins be came detectable in germinable somatic embryos but not in non-germinabl e somatic embryos. These data suggest that for somatic embryos which d o not complete their normal developmental processes, desiccation is re quired for the transition from a maturation to a germination program.