Identification and functional characterisation of an allene oxide synthase from grapevine (Vitis vinifera L. Sauvignon blanc)

Abstract

Jasmonic acid (JA) is known to be an important phytohormone that orchestrates plant defence mechanisms against a range of herbivores and pathogens. Studies have suggested allene oxide synthase (AOS; E.C 4.2.1.92), the first committed step in JA biosynthesis, is essential for JA biosynthesis, yet clear evidence of its role as a biosynthetic regulatory point is lacking, in the main due to conflicting results derived from transgenic studies. However other studies lend support to a biosynthetic regulatory role for AOS. These studies have suggested that certain amino acid substitutions can increase the biosynthetic capacity of the enzyme and consequently improve pathogen tolerance in plants. To explore the role of AOS in Grapevine we isolated and functionally characterised this enzyme for the first time from Vitis vinifera L. Sauvignon blanc. The cloned AOS consisted of a single 1563 bp open reading frame. Comparative sequence analysis showed that the cloned gene (VvAOS) was highly conserved compared to those from other species. Complementation of an Arabidopsis AOS null mutant (aos) with VvAOS recovered the male sterile mutant phenotype and confirmed its function. Transcript analysis showed that VvAOS was wound responsive in leaves and was detectable in most tissues, with the highest levels of transcript in the mesocarp (pulp) of mature berries. Sub-cellular localisation of the VvAOS protein indicated that VvAOS is associated with the chloroplast membrane. Unexpectedly high levels of VvAOS transcript in complemented aos lines did not lead to predicted increases in JA. We have functionally characterised the sole AOS from Grapevine. Patterns of transcript accumulation in grapevine suggest roles in growth, development as well as an important role for JA in fruit ripening. Expression of VvAOS in Arabidopsis suggest complex epigenetic interactions between transgenic and endogenous AOS alleles, providing a possible explanation for why transgenic studies of AOS have delivered conflicting data pointing to a questionable role of AOS as a key regulatory point in JA biosynthesis.