Cross-talk interactions of sucrose and Fusarium oxysporum in the phenylpropanoid pathway and the accumulation and localization of flavonoids in embryo axes of yellow lupine

This study investigated the effects of cross-talk interactions of sucrose and infection caused by a pathogenic fungus Fusarium oxysporum f.sp. lupini on the regulation of the phenylpropanoid pathway, i.e. the level of expression of genes encoding enzymes participating in flavonoid biosynthesis, as well as cell location and accumulation of these compounds in embryo axes of Lupinus luteus L. cv. Polo. Embryo axes, both non-inoculated and inoculated, were cultured for 96 h on Heller medium with 60 mM sucrose (+Sn and +Si) or without it (−Sn and −Si). Real-time RT-PCR to assess expression levels of the flavonoid biosynthetic genes, phenylalanine ammonialyase (PAL), chalcone synthase (CHS), chalcone isomerase (CHI) and isoflavone synthase (IFS) were used. Sucrose alone strongly stimulated the expression of these genes. There was a very high expression level of these genes in +Si embryo axes in the early phase of infection. Signal amplification by sucrose and the infection was most intense in the 48-h +Si axes, resulting in the highest level of expression of flavonoid biosynthetic genes. In −Si tissues, the expression level of these genes increased at 48 and 72 h after inoculation relative to 24 h; however, the relative level of expression was much lower than in +Si axes, except at 72 h for PAL and CHS.Moreover, at 48 h of culture, considerably higher activity of CHI (EC 5.5.1.6) was observed in axes with a high level of sucrose than in those with a sucrose deficit. CHI activity in +Si axes at 48 and 96 h post-inoculation was over 1.5 and 2 times higher than that in +Sn axes, as well as higher than in −Si axes.Observations of yellow lupine embryo axes under a confocal microscope showed an increased post-infection accumulation of flavonoids, particularly in cells of embryo axes infected with F. oxysporum and cultured on a medium containing sucrose (+Si). Up to 48 h post-infection in +Si axes, a very intensive emission of green fluorescence was observed, indicating high accumulation of these compounds in whole cells. Moreover, a nuclear location of flavonoids was recorded in cells. Strong staining of flavonoid end products in +Si embryo axes was consistent with the expression of PAL, CHS, CHI and IFS.These results indicate that, in the early phase of infection, the flavonoid biosynthesis pathway is considerably enhanced in yellow lupine embryo axes as a strong signal amplification effect of sucrose and the pathogenic fungus F. oxysporum.