Biology Reference
In-Depth Information
germ tube. Maturation of the appressorial lobe results in a functional unit
capable of generating turgor and producing those enzymes necessary for
degradation of host cuticle/cell wall complex, allowing the fungal penetration
peg to enter the cell lumen and produce the haustorium (or feeding structure).
Although the fungus lacks a discernable globin, and globins are generally
regarded as the main defence against NO,
B. graminis
f.sp.
hordei
does generate
NO, which is a pathogenesis determinant on barley (
Prats et al., 2008
).
Transient NO generation occurs in the appressorium during maturation.
The mammalian NO synthase inhibitor L-NAME or the NO scavenger
c-PTIO affected the number of appressorial lobes produced by the fungus,
indicating that NO plays a key role in formation of
B. graminis
appressoria.
Might then the absence of a globin circumvent the problem of uncontrolled
NO quenching in this organism? Might the absence of globins in most/all true
biotrophs represent a necessary condition for communication via NO with
the host plant?
Globins are also absent in certain Pucciniomycotina (
Table 9.4
), a group
that contains a number of important obligately biotrophic plant pathogens.
Those lacking globins appear to be
Microbotryum violaceum
(formerly
Ustilago
violacea
),
Melampsora laricis-populina
(responsible for poplar leaf rust disease)
and
Phakopsora pachyrhizi
(a basidiomycete fungus that causes rust disease
in soybean plants). There is no evidence in this group for an FHb, the class
of globin most clearly correlated with NO removal. In contrast, in the same
group,
Puccinia graminis
—an obligate biotroph that infects wheat—there is
evidence from the sequence of two SSDgbs, each containing an unidentified
C-terminal extension of 140-150 amino acids. In two members of the
Ustilagomycyes, globins also seem to be absent; these are
Sporisorium reilanum
(a biotrophic pathogen of maize) and
Ustilago maydis
(a biotrophic pathogen
of maize causing smut disease). There are too few examples and too little
published information on NO tolerance in these fungi to draw conclusions
but the hypothesis that NO-consuming globins will be absent from fungi
that produce NO during infection is worthy of investigation.
An apparent exception is
Cladosporium fulvum
(Supplementary Table S2
troph that is a member of the Dothideomycetes fungi—one of the largest
and most diverse groups, which includes many plant pathogens. The group
makes a major contribution to local ecosystems by degrading biomass and
contributing to regulating the carbon cycle (
Ohm et al., 2012
). However,
C. fulvum
is atypical in that it can be cultivated
in vitro
. It causes grey leaf
mold of tomato and does contain an FHb. In the tomato, it has been shown
