The genus Alternaria includes several of fungi that are darkly pigmented by DHNmelanin.
These are pathogenic to plants but are also associated with human respiratory
allergic diseases and with serious infections in immunocompromised individuals. The
present work focuses on the alterations of the composition and structure of the
hyphal cell wall of Alternaria alternata occuring under the catabolism of L-tyrosine and
L-phenylalanine when cultured in minimal salt medium (MM). Under these growing
conditions, we observed the released of a brown pigment into the culture medium.
FTIR analysis demonstrates that the produced pigment is chemically identical to the
pigment released when the fungus is grown in MM with homogentisate acid (HGA),
the intermediate of pyomelanin, confirming that this pigment is pyomelanin. In contrast
to other fungi that also synthesize pyomelanin under tyrosine metabolism, A. alternata
inhibits DHN-melanin cell wall accumulation when pyomelanin is produced, and this
is associated with reduced chitin cell wall content. When A. alternata is grown in
MM containing L-phenylalanine, a L-tyrosine percursor, pyomelanin is synthesized but
only at trace concentrations and A. alternata mycelia display an albino-like phenotype
since DHN-melanin accumulation is inhibited. CmrA, the transcription regulator for
the genes coding for the DHN-melanin pathway, is involved in the down-regulation
of DHN-melanin synthesis when pyomelanin is being synthetized, since the CMRA
gene and genes of the enzymes involved in DHN-melanin synthesis pathway showed
a decreased expression. Other amino acids do not trigger pyomelanin synthesis and
DHN-melanin accumulation in the cell wall is not affected. Transmission and scanning
electron microscopy show that the cell wall structure and surface decorations are altered
in L-tyrosine- and L-phenylalanine-grown fungi, depending on the pigment produced. In
summary, growth in presence of L-tyrosine and L-phenylalanine leads to pigmentation
and cell wall changes, which could be relevant to infection conditions where these amino
acids are expected to be available.
