No.42 - è¾²æ¥çç©è³æºç 究æ
No.42 - è¾²æ¥çç©è³æºç 究æ
No.42 - è¾²æ¥çç©è³æºç 究æ
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2<br />
Yuji KAMIYA<br />
Fig. 1. Gibberellin biosynthesis in Arabidopsis.<br />
It has been known that GA promotes seed germination in many plant species. In Arabidopsis,<br />
severe GA deficient-mutants, such as ga1-3 and ga2-1, are defective in seed germination<br />
(KOORNNEEF and VAN DER VEEN 1980), and chemical inhibitors of GA biosynthesis inhibit<br />
germination (NAMBARA et al. 1991). These observations indicate that de novo GA biosynthesis is<br />
necessary for seed germination in Arabidopsis (HEDDEN and KAMIYA 1997).<br />
Light is a critical environmental factor for seed germination in some small-seeded plants such<br />
as lettuce, tomato and Arabidopsis (SHINOMURA, 1997). The effect of light on seed germination is<br />
primarily mediated by phytochromes (BORTHWICK et al. 1952; BUTLER et al. 1959). Genes<br />
encoding GA 3-oxidases, which convert inactive precursor to active GAs, are regulated by<br />
phytochromes in germinating lettuce and Arabidopsis seeds (TOYOMASU et al. 1998; YAMAGUCHI<br />
et al. 1998).<br />
Temperature is another crucial external cue that controls seed germination (BEWLEY and<br />
BLACK 1982). In many plant species, exposure of seeds to low temperature (typically 2-5)<br />
immediately after imbibition is effective to promote germination. (SHROPSHIRE et al. 1961; CONE<br />
and SPRUIT 1983). This treatment is called “stratification”. Although this method is widely used to<br />
improve the frequency and synchronization of germination, the mechanism for the thermoregulation<br />
of seed germination has been unclear.<br />
The effect of cold treatment on GA content has been reported in the 1970s, based on semiquantitative<br />
analysis of endogenous GAs using -amylase bioassay and/or gas chromatographymass<br />
spectrometry (GC-MS) (ROSS and BRADBEER 1971; SINSKA et al. 1973; WILLIAMS et al.<br />
1974). DERKX et al. (1994) analyzed the effect of pre-chilling of Arabidopsis seeds and reported<br />
that bioactive GA4 was detectable only in pre-chilled seeds, but not in dark-imbibed seeds. It has<br />
not been clear whether the effect of pre-chilling is a direct response to low temperature because pre-
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