Submitted to: Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/24/2013
Publication Date: 4/1/2014
Citation: Arias, R.S., Sobolev, V.S., Orner, V.A., Dang, P.M., Lamb, M.C. 2014. Potential involvement of Aspergillus flavus laccases in peanut invasion at low water potential. Plant Pathology. 63(2):354-364.
Interpretive Summary: Aspergillus flavus (Link) accumulates carcinogenic aflatoxins in peanuts, mainly affecting immature kernels during drought. A. flavus can produce enzymes (laccases) able to degrade the mechanism of defense of peanut plants (phytoalexins). Though the genome of A. flavus is published, nothing is known about the possible role of A. flavus laccases on pathogenicity. In this work we used molecular tools (real-time PCR) to determine the regulation of A. flavus laccases in the presence of peanut kernels, peanut hulls and drought (low water potential). The fungal biomass was monitored as well as the phytoalexin content in peanut kernels. Some phytoalexins were identified as possible targets of A. flavus laccases. The significant up-regulation of expression of these enzymes under drought and in presence of sucrose could explain in part the higher incidence of carcinogenic aflatoxins in immature peanuts under drought conditions
Technical Abstract: Aspergillus flavus (Link) accumulates aflatoxins in peanuts, mainly affecting immature kernels during drought. Peanut invasion by A. flavus induces synthesis of phytoalexins, mostly stilbenoids, as a plant defense mechanism. Fungal laccases are often related to pathogenicity, and among other substrates they can also degrade stilbenoids. In this work we monitored, for the first time, the levels of expression of A. flavus (NRRL 3357) laccases by real-time PCR in relation to fungal biomass, presence of kernels and hulls, low water potential, and to the accumulation of phytoalexins in peanut kernels. Fungal biomass as packed-cell volume (PCV) of A. flavus was significantly higher (p = 0.01) in presence of mature kernels. Dead kernels, mature- and immature-peanut hulls also sustained significantly higher A. flavus biomass than regular liquid medium. Presence of kernels and hulls lowered the level of expression of three A. flavus laccases by 4-6 fold (p < 0.01), whereas 3% sucrose up-regulated them by 35-304 fold and low water potential (-1.1 MPa) up-regulated them by 85-248 fold (p < 0.01). Phytoalexins produced by peanut kernels in presence of A. flavus and quantified by HPLC-DAD-MS were primarily stilbenoids: 3’-isopentadienyl-3,5,4'-trihydroxystilbene (IPD), chiricanine A, arachidin-2, arachidin-3 and arahypin-1. The use of a priori induction of phytoalexins in seeds and laccases in A. flavus as well as potential substrates of the studied laccases are discussed. The up-regulation of laccase expression observed at -1.1 MPa and at high sucrose concentration could contribute to the higher incidence of aflatoxin observed in immature kernels under drought conditions.