Bis(guanylhydrazones) negatively affect in vitro germination of kiwifruit pollen and alter the endogenous polyamine pool

Bis(guanylhydrazones) are a class of compounds known to interfere with the metabolism of polyamines (PAs). Among them, the methylglyoxal derivative (MGBG) has been studied most thoroughly. Because PAs and their biosynthetic enzymes are strongly involved in pollen tube organization, emergence and elongation, a number of these inhibitors have been studied in the present work for their effects on the in vitro performance of kiwifruit (Actinidia deliciosa) pollen. Increasing concentrations of several bis(guanylhydrazones) in the range 0.05-1 mM were checked for their effect on pollen germination. Most of the compounds tested showed a dose-dependent inhibitory effect on tube emergence, which was established very early during incubation. At 0.5 mM, the methylpropylglyoxal derivative (MPGBG) had a stronger inhibitory effect than MGBG. To verify whether the inhibitors reached their metabolic target, PA levels and S-adenosylmethionine decarboxylase (SAMDC) activity were determined in pollen germinated in the presence or absence (controls) of 0.5 mM bis(guanylhydrazones). Spermidine (Spd) content was significantly reduced in the treated pollen, and this effect was more pronounced after treatment with MGBG than with MPGBG. An early and strong reduction in SAMDC activity was observed after exposure to either inhibitor. Inhibition of pollen germination by MGBG or MPGBG could not be reversed by the addition of exogenous Spd, which per se was inhibitory. Taken together, our results suggest that bis(guanylhydrazones) alter PA metabolism and negatively affect kiwifruit pollen germination, even though a strict cause-effect relationship could not be established, and other mechanisms, unrelated to PA activity, must be involved.