No.42 - è¾²æ¥­ç”Ÿç‰©è³‡æºç ”ç©¶æ‰€

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Takeshi NAKANO, Shigeo YOSHIDA and Tadao ASAMI
Fig. 5 Screened bil (Brz-insensitive-long hypocotyl) mutants of Arabidopsis.
bil1 mutants with 3 M Brz had hypocotyls as long as those of wild-type plants
grown on unsupplemented medium.
and the bes1 mutant has the same nucleotide substitution (Yin et al., 2002). The plant-specific
gene family encompassing BZR1, BES1 and BIL1 encodes novel phosphoproteins containing a
putative nuclear localization signal. The BIL1/BZR1:CFP fusion protein localizes mainly to the
cytoplasm and also to the nucleus at low levels, but treatment with brassinosteroids results in a
significant increase of BIL1/BZR1:CFP levels in the nucleus within thirty minutes. In contrast, a
BZR1:CFP protein containing the mutation localizes continuously to the nucleus (Wang et al.,
2002). Then, the mutants felt that brassinosteroid signaling is on in every time, and these can
showed wild-type like phenotype in brassinosteroid deficient condition. These results suggest that
BIL1/BZR1 and BES1 is a key component in brassinosteroid signaling from the cell surface to the
nucleus (Fig. 6).
The phenotypes of the bzr1/bil1 and bes1 Brz-resistance and bri1-suppressor mutants,
respectively, are very strong, with the resistant mutant just like wild-type plants in appearance,
even though it is severely deficient in brassinosteroids. These mutants, the mutant alleles of which
are both dominants, resulted from the substitution of just one amino acid as compared to the wild
type. Overexpression of the BZR1 or BES1 wild-type genes via the CaMV 35S promoter resulted
in only weak resistance against brassinosteroid deficiency (Wang et al., 2002, Yin et al., 2002).
These results suggest that bzr1/bil1 and bes1 mutants would be difficult to identify from activationtagged
pools of plants in the background of a mutant deficient in brassinosteroids, such as det2 or
dwf4. An alternative method is to induce point mutations by chemical treatment of brassinosteroiddeficient
mutants. The most widely-used method to identify point mutations is genomic walking,
performed using backcrosses with another ecotype. The screening is best performed on
recombinant F2 plants, to allow identification of the brassinosteroid-deficiency mutation as a