Poster: Hormones

Abs # 614: The interaction between biosynthesis and response of ABA during seed germination in Arabidopsis thaliana

Presenter:	Okamoto, Masanori , okamo@postman.riken.go.jp
Authors	Okamoto, Masanori  (A) (B)  Kamiya, Yuji  (A)   Koshiba, Tomokazu  (B)   Nambara, Eiji  (A)
Affiliations:	(A): Plant Science Center, RIKEN ( The Insititute of Physical and Chemical Research) (B): Department of Biologigal Sciences, Tokyo Metropolitan University

Abscisic acid (ABA) plays an important role in seed dormancy, stomata closure, and plant adaptation to environment stresses. ABA action is accomplished by regulation of both ABA content and its sensitivity. Although numerous studies have reported the identification of genes involved in ABA biosynthesis and its response, the mechanism underlying the interaction between ABA biosynthesis and its sensitivity remains unclear. To reveal this molecular interaction during seed germination, we have analyzed ABA content in dry seeds using two ABA-insensitive mutants (abi4, abi5). ABI4 encodes an APETALA2 (AP2) domain protein whereas the ABI5 gene encodes a basic Leucine Zipper (bZIP) transcription factor. ABI5 expression is most abundant in dry seeds and decreases during imbibition, while ABI4 expression is enhanced during imbibition. The abi5 mutant contained higher levels of ABA in dry seeds compared to that of wild type. Moreover, when we monitered transcript levels of the NCED genes, which encode key enzymes involved in ABA biosynthesis, only NCED6 showed higher transcript levels in the abi5 mutant than wild type. These results indicate that ABI5 is involved in the feedback regulation of ABA biosynthesis in the seed. By contrast, the ABA content in the abi4 mutant dry seeds was comparable to that in wild type. However, the reduction of NCED6 mRNA levels in the abi4 mutant was significantly more than that observed in wild type during imbibition. This result suggests that ABI4 is the positive regulator of NCED6 during germination. Now, we are investigating ABA content during imbibition in abi4 and abi5 mutants.