Liquid Culture Systems for in vitro Plant Propagation

Picea abies somatic embryo development 311
4. Discussion
The mechanism for the stimulation of SIV SEs by all concentrations of
AgNO3 remains unclear. The microbial suppressant properties of the silver
salt may have had an effect, but the ‘master cultures’ had been tested and no
microbial colonies had been recovered. Silver nitrate is generally regarded
as an inhibitor of ethylene synthesis (e.g. Kawano et al., 2002), but recent
work by Lim et al. (2002) reported that S-adenosylmethionine synthetase,
which catalyses the conversion of methionine to S-adenosylmethionine, a
precursor for ethylene synthesis, was upregulated by AgNO3. Nevertheless,
Mandal et al. (2001) reported increased somatic embryo production in
AgNO3–treated safflower cultures. Similarly, Achar (2002) reported
increased somatic embryo yields from anther cultures of cabbage. Kvaalen
(1994) reported that ethylene, supplied via ethephon to P. abies embryogenic
cultures did not inhibit growth. In relation to exogenous application of
AgNO3, it could be that the additional nitrate content is stimulatory or that
the silver ion is itself a stressor which induces abscisic acid synthesis in the
cultures and thereby promotes SE maturation.
The differences relating to the origin of the cultures (suspensions vs agar)
may reflect the high numbers of 'early-stage' pro-embryos in the suspension
culture compared with the later stage pro-embryos which exhibit clear signs
of polar development, thus whilst the SA vs AA cultures were notionally of
the same age, because of the developmental differences (Plates 3 and 4), the
AA cultures were at a more advanced early stage embryo and these
differences were reflected throughout the subsequent period of observation.
References
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