Figure 6.

Model of FvZBD1- and FvABC3-facilitated pyrrocidine tolerance in F. verticillioides WT. Pyrrocidine enters the F. verticillioides WT cell by an unknown mechanism, inducing FvZBD1 expression. FvZBD1 is translated into FvZBD1p, a putative zinc-binding dehydrogenase with an enoyl reductase domain that presumably functionally requires zinc. Fumonisin biosynthesis also requires zinc (e.g., for FUM21p function—the Zn(II)2Cys6 transcription factor of the fumonisin biosynthetic gene cluster). Under pyrrocidine challenge, abundant FvZBD1p sequesters zinc, decreasing zinc availability in the cell, and thereby inhibiting fumonisin biosynthesis via competitive inhibition of FUM21p. FvZBD1p enzyme then becomes active and abundantly present in the cytoplasm. Via its enoyl reductase domain, FvZBD1p may detoxify the more toxic pyrrocidine A by reducing its double-bonded lactam ring—detoxifying pyrrocidine A into pyrrocidine B, at which FvABC3p (with specificity to pyrrocidine B) can effectively pump it out of the cell, conferring tolerance to F. verticillioides WT. PM, plasma membrane; PA, pyrrocidine A; PB, pyrrocidine B. Figure created with Biorender.com.