- True potato (Solanum tuberosum) seed (TPS) is used for preservation of
variable genetic lines of wild and cultivated potatoes (Hawkes et al., 2000) and for
propagation of food crops in some developing countries. TPS has advantages over
seed potato tubers in storage and transportation and favors lower virus infection
levels in fields. However, TPS has thermodormancy and will not readily germinate
at 25°C and above (D'Antonio and McHale, 1988; Pallais, 1995a, b; Alvarado et
al., 2000). TPS can be extremely unreliable when planted directly in fields due to
poor emergence related to diseases and soil crusting.
Germination tests were conducted with two lots of TPS derived from cvs.
EB-8109 and All Blue, respectively, to study dormancy mechanisms. Seeds were
germinated under four temperature regimes (10°C, 15°C, 20°C and 25°C). The two
lots showed distinctly different germination characteristics. EB-8109 seeds showed
only thermodormancy whereas All Blue seeds showed very deep dormancy.
A carotenoid synthesis inhibitor, fluridone, which blocks abscisic acid
(ABA) synthesis, effectively broke thermodormancy in EB-8109 TPS but did not
break primary dormancy in All Blue seeds. Additional treatments, including pre-chilling
and hormonal regimes, also failed to break All Blue deep dormancy. When
the micropylar region of the endosperm (endosperm cap) was removed from seeds
of both seed lots, radicle elongation was observed, suggesting that mechanical
resistance from the endosperm cap restrains radicle protrusion, and that weakening
of the endosperm cap is requisite for TPS germination.
Endo-β-mannanase expression was measured to help characterize
mechanism underlying the weakening of endosperm cap tissues. This enzyme is
thought to permit radicle protrusion by degrading cell walls thereby weakening the
tissues of the endosperm cap (Groot et al., 1988). The coding region of
germination-specific mannanase was isolated from the potato genome by use of
polymerase chain reaction (PCR) with primers specifically designed for the tomato
germination-specific mannanase gene (LeMAN2, Nonogaki et al., 2000). The
cDNA of the TPS mannanase was identical to that of LeMAN2. The expression of
mannanase mRNA was detected in the endosperm cap of germinating TPS after 72
h of imbibition at 15°C, while no expression was detected at 25°C (thermodormant
condition). Fluridone induced mannanase expression in the micropylar region of
the endospem at 25°C. Thus, there was a correlation between induction of
mannanase and dormancy breakage.
A major increase in TPS post-germinative endo-β-mannanase activity was
detected by use of gel diffusion assay. Two isoforms of mannanases were detected
in the protein extracts of germinated TPS by activity staining of native
polyacrylamide gel electrophoresis. The post-germinative mannanase was detected
in the whole endosperm of germinated TPS by using tissue printing with the
LeMAN1 (Bewley et al., 1997) RNA probe. These results suggest that, as with
tomato, TPS also expresses post-germinative mannanase activity.
The promoter region of a new tomato mannanase was isolated during this
research. This promoter was shown to be involved in anther-specific expression of