Solute and matric potential stress on Penicillium verrucosum: impact on growth, gene expression and ochratoxin A production

Penicillium verrucosum survives in soil and on cereal debris. It colonises grain during harvesting, drying and storage. There is no information on the relative tolerance of P. verrucosum to solute and matric stress in terms of colonisation, or on the biosynthetic toxin gene clusters or ochratoxin A (OTA) production. The objectives were to examine the effect of ionic and non-ionic solute and matric potential stress on (a) growth, (b) expression of two toxin biosynthetic genes otapksPV and otanrpsPV, and (c) OTA production by a strain of P verrucosum. Optimum growth and OTA production were at -7.0 MPa (= 0.95 water activity, a(w)) and -1.4 MPa (= 0.99 a(w)), respectively, regardless of whether solute (Psi(s)) or matric (Psi(m)) stress was imposed. P. verrucosum was more sensitive to ionic solute stress (NaCl) with no growth at -19.6 MPa (=0.86 a(w)) while growth still occurred in the non-ionic solute (glycerol) and matric stress treatments. Relative gene expression of the biosynthetic genes using PCR (RT-qPCR) showed that the otapksPV gene was expressed over a wide range of ionic/non-ionic solute stress conditions (-1.4 to -14.0 MPa; = 0.99-0.90 a(w)). The highest expression was in the non-ionic Psi(s) stress treatments at -7.0 MPa (= 0.95 a(w)). However, the otanrpsPV gene was significantly up regulated under Psi(m) stress, especially with freely available water (-1.4 MPa = 0.99 a(w)). OTA production was significantly decreased as Psi(s) or Psi(m) stress were imposed. Limited OTA production occurred in the driest treatments under Psi(s) and Psi(m) stress respectively. The impact of these two types of stresses on the growth of P verrucosum was quite different from that for OTA production. The results are discussed in the context of the life cycle and ecological characteristics of this species in contaminating cereals with OTA in the post-harvest phase of the cereal chain.