Poster: Seed Biology

Abs # 558: A novel vacuolar processing enzyme functions in seed coat formation at the early stage of seed development

Presenter:	Nakaune, Satoru , nakaune@gr.bot.kyoto-u.ac.jp
Authors	Nakaune, Satoru  (A)   Yamada, Kenji  (A)   Hara-Nishimura, Ikuko  (A)
Affiliations:	(A): Kyoto University

Vacuolar processing enzyme (VPE) is a cysteine endopeptidase with specificity towards asparaginyl bonds. Arabidopsis has three VPE homologs (aVPE, bVPE, and gVPE); bVPE is specific to seeds, and both aVPE and gVPE are specific to vegetative organs. This suggests that each VPE has its own function at different organs in plants. Recently, the 4th VPE homolog (dVPE) was found in the genome database of Arabidopsis. A phylogenetic tree of VPE homologs showed that dVPE is different from seed-type VPEs and vegetative-type VPEs. This suggests that dVPE is a novel type VPE. An immunoblot of Arabidopsis plants revealed that dVPE was specifically expressed in testa of developing seeds at the early stage, but not in embryo. We found that dVPE was localized at the electron-dense small structures in the outer-cell layers of the inner integuments, which consists of mostly three cell layers. It has been shown that programmed cell death (PCD) causes the reduction of the number of cell layers of inner and outer integuments of testa during seed development of B. napus. Arabidopsis seed coats at the late stage consist of one cell-layered outer integument and one cell-layered inner integument derived from innermost inner integument. Immunofluorescence analysis showed that dVPE was specifically expressed in outer cell layers of inner integument during PCD. We also found that PCD of outer layers of inner integument was delayed in the developing seeds of the dVPE-deficient mutant. Our results suggest that dVPE plays an important role in the PCD for seed coat formation at the early stage of seed development.