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optimum and maximum temperature for F. graminearum but a lower minimum temperature
(5°C). These difference could be related to the different growth substrate that was used.
Ramirez et al. (2006) also studied the effect of a w and reported an optimum level for F.
graminearum growth at the highest tested level: 0.995 a w . No growth was observed at 0.900
a w .
Once the spores have germinated, the propagation of the hyphae is more favoured in the
flower parts than in the other organs. The fungus can develop both inter- and intracellularly
and cause severe damages. In 2001, Bushnell provided a summary on of the way that head
blight Fusaria can infect the spikelets. The stomates can be entered by F. graminearum (Kang
and Buchenauer, 2000a; Pritsch et al., 2000). The glume, palea, lemma and the awn can all be
colonized by Fusarium mycelia. Hyphae can enter into the floret mouth opening where it can
colonize caught and retained anthers or the apical brush of the caryopsis. Another potential
pathway of entry is the crevice between the palea and lemma, especially near the floret base.
Whether entering from the mouth or crevice, the fungus can develop abundantly on and
within interior tissues of the lemma and palea. Surface tissues of the ovary and lodicules are
especially susceptible. However, the mode of penetration of F. graminearum into these
interior tissues has not been determined yet. F. graminearum can grow within tissues between
cells instead of entering them, establishing a biotrophic relationship with the host tissues.
How and where the intercellular fungus penetrates the cells for its subsequent growth within
the cells is still not known. Once established within the floret, the fungus colonizes and
follows vascular tissues in the floret stalk through the rachilla or rachis into other florets
(Kang and Buchenauer, 2000a; Pritsch et al., 2000; Bushnell, 2001; Wagacha and Muthomi,
2007).
Stress-related genes are known to be activated in both resistant and susceptible cultivars
in response to F. graminearum invasion. The genes activation is observed in both the invaded
and in the adjacent uncolonized spikelet, which demonstrates that this response is expressed
systemically. However, the molecular and physiological responses of heads to invasion by
Fusarium head blight have not been fully investigated (Pritsch et al., 2000; Pritsch et al.,
2001; Kong et al., 2005).
On the other hand, Fusarium head blight may produce cell wall degrading enzymes such
as cellulases, xylanases and pectinases during infection and colonization of wheat spike
tissues (Kang and Buchenauer, 2000b).
3.4. Deoxynivalenol Biosynthesis and Its Role in the Pathogenesis
Deoxynivalenol (DON, vomitoxin) is a type B trichothecene, an epoxy-sesquiter-penoid
(Figure 1). DON is probably the most frequently encountered mycotoxin in the FHB of wheat
throughout the world (Parry et al., 1995; FAO, 2003).
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