Biological control agents in plant disease control - Disease Control in Crops - page 30

a novel mode of action in control of plant pathogens by some BCAs which deserves
further study (Molina et al ., 2003; Uroz et al ., 2003; Dong et al ., 2004).
Gene clusters responsible for production of several antibiotics produced by bacteria
have now been cloned and manipulated to examine the potential to enhance biocontrol
activity in other strains (Hammer et al ., 1997; Nowak-Thompson et al ., 1999; Timms-
Wilson et al ., 2000; Chin-A-Woeng et al ., 2001; Delany et al ., 2001; Huang et al ., 2004).
The potential for detection of additional, and potentially novel, gene clusters responsible
for antibiotic production and other modes of action is likely to ensue as complete genomic
sequences such as that for the BCA strain Pseudomonas fl uorescens Pf-5 (Paulsen et al .,
2005; Loper et al ., 2007) become available.
The situation is less clear cut with fungi where molecular manipulation methods
are generally less advanced and more complex, especially for producing single insert
gene mutants. Nevertheless, fl occulosin produced by Pseudozyma fl occulosa (Cheng
et al ., 2003) and gliotoxin and peptaibols produced by Trichoderma spp. (Wilhite et al .,
1994; Wiest et al ., 2002) have been clearly shown to be involved in disease biocontrol.
Interestingly, UV mutants of Trichoderma virens defi cient in either antibiotic production,
mycoparasitism or both, were equally effective at controlling damping-off in cotton as the
parent strains, indicating that other mechanisms such as some form of induced resistance
in the plant were more important for biocontrol in this system (Howell & Stipanovic,
1995; Howell et al ., 2000; Howell, 2002). Production of volatile groups of inhibitory
compounds including alcohols, esters, ketones, acids and lipids by Muscador albus and
M. roseus may be responsible for the control of seedling diseases of sugar beet and eggplant
(Strobel et al ., 2001; Stinson et al ., 2003) and use of these BCAs as 'mycofumigants' may
be an interesting avenue to pursue.

3.2.3 Parasitism and production of extracellular lytic enzymes

Parasitism and associated production of extracellular lytic enzymes has been thoroughly
explored as a mode of action in biocontrol. This is a relatively simple phenom-
enon for bacteria where degradation of target cell walls is generally considered to
refl ect parasitism, and may range from simple attachment of bacterial cells to hyphae
with minimal degradation, through biofi lm formation to complete lysis and cell wall
breakdown (Mitchell & Hurwitz, 1965; Nelson et al ., 1986; Bolwerk et al. , 2003). Not
surprisingly, lists of extracellular enzymes produced by bacterial BCAs have been pro-
duced but only relatively rarely have unequivocal roles of enzymes produced by bacteria
been demonstrated. These include a number of chitinases (Chernin et al ., 1995, 1997;
Pleban et al ., 1997; Kamensky et al ., 2003), proteases (Dunne et al ., 1997) and

-1,3-
glucanases (Fridlender et al ., 1993; Palumbo et al ., 2005). Regulation of protease and
chitinase production in bacteria involves the two part regulatory systems GacA/GacS and
GrrA/GrrS (Sacherer et al. , 1994; Corbell & Loper, 1995; Ovadis et al ., 2004) similar to
the production of siderophores and antibiotics.
With fungal BCAs the process of parasitism of fungal plant pathogens, or mycoparasitism,
is more complex than that for bacteria and a series of interlinked phases of hyphal-
hyphal interactions have been recorded especially for Trichoderma spp. including:
sensing, directed growth, contact and binding, sometimes involving production of appres-
soria, coiling or alignment of hyphae of the mycoparasite around the host, penetration

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