Agriculture Reference
In-Depth Information
3.6
Future research directions and conclusions
Great strides have been made over the last few years in understanding and utilising BCAs.
Molecular biology has played a big part in mode of action studies and, as these techniques
advance further, additional novel information that may help the practical application of
BCAs is likely to be generated. For example, analysis of expressed sequence tags (ESTs)
has provided novel information on genes expressed by
Trichoderma harzianum
(Liu &
Yang, 2005; Vizcaino
et al
., 2006), and genes differentially expressed during growth in
the presence of cell walls of host fungi or during mycoparasitism itself have been identi-
fi ed in several systems (Carpenter
et al
., 2005; Massart & Jijakli, 2006; Muthumeenakshi
et al
., 2007). Differential gene expression has also been used to identify novel genetic
markers associated with biocontrol activities in
Bacillus subtilis
(Joshi & Gardener,
2006). This can be scaled up further with gene chip technology or transcriptomics,
already applied to monitor broad changes in gene expression due to ISR in
Arabidopsis
(Verhagen
et al
., 2004). The increasing availability of whole genome sequences of BCAs,
such as that for
Pseudomonas fl uorescens
PF-5 (Loper
et al
., 2007) and
Trichoderma har-
zianum
(underway) is also likely to shed new insights into biocontrol. Proteomic studies
are also revealing new information on proteins expressed during growth of
Trichoderma
atroviride
on cell walls of
Rhizoctonia solani
(Grinyer
et al
., 2005) and during three-
way interactions between
T. atroviride
, bean plants and
Botrytis cinerea
and
Rhizoctonia
solani
(Marra
et al
., 2006) and are also likely to provide new concepts to improve bio-
control activity. Understanding more about signalling between microorganisms during
biocontrol may also allow a new way of controlling disease (Molina
et al
., 2003; Uroz
et al
., 2003; Dong
et al
., 2004; Lutz
et al
., 2004; Pierson & Pierson, 2007).
If genes active in biocontrol are identifi ed and sequenced it is also possible to constitutively
over express these genes and potentially enhance biocontrol activity. Recently, this con-
cept has been tested by introducing two genes, a
-1,6-glucanase
into
Trichoderma virens
, and despite a decrease in growth and sporulation, the double
over-expression transformants still provided enhanced bioprotection of cotton seedlings
against
Pythium ultimum, Rhizoctonia solani
and
Rhizopus oryzae
(Djonovic
et al
.,
2007). An endochitinase gene from
Trichoderma harzianum
has also been transformed
into potato and tobacco and these transgenic plants exhibited increased disease resistance
(Lorito
et al
., 1998). Nevertheless, all genetic modifi cation procedures are subject to strict
regulation relating to use in the environment. Consequently, even though they may show
great promise they are unlikely to become commercialised as they would face a double leg-
islation hurdle associated with use both as a pesticide and genetically modifi ed organism.
More practical areas for future research relate to the continuum of production, formula-
tion, application and ecology of BCAs. Any advances that can improve quantity, quality
and shelf life of inocula would be welcome, particularly bacteria such as
Pseudomonas
that do not form spores. Recently, for the fi rst time, both
Pseudomonas
and
Trichoderma
isolates have been simultaneously applied to seed via drum priming and found to sur-
vive and proliferate on roots similarly to when applied individually (Bennett & Whipps,
2007). This procedure may be a way to apply and maintain multiple BCAs in a com-
mercially relevant process. Combining different strains of BCAs has frequently been
suggested as a way ahead for biocontrol as they may express different biocontrol traits,
give higher levels of protection, reduce variability, and increase the range of pathogens
suppressed (Dunne
et al
., 1998; Guetsky
et al
., 2002; Jetiyanon & Kloepper, 2002;
β
-1,3-glucanase and a
β
