Agriculture Reference
In-Depth Information
Deleterious organisms are only a small proportion of the aerial and edaphic micro-
bial communities which surround crop plants. Many of these other micro-organisms
are potentially valuable adjuncts for sustainable production, especially soil borne
microbes. But currently there is relatively little understanding of soil properties,
ecology and the microbes living in and around plant roots, the rhizosphere. Since
the vast majority of soil microbes are benign, many must ultimately offer opportuni-
ties for aiding crop growth. There is a huge amount of nano-scale molecular signal-
ing within microbial communities, with plant roots, and within and between plants
themselves. Understanding these activities is the task of molecular microbiologists
who are unraveling opportunities for the creation of new generations of fertilisers,
crop stimulants, and protectant and eradicant treatments. These will be biologically-
based and provide increased yields linked firmly with environmental sustainability.
Already it is known that plants sense the approach of pests and pathogens and ac-
tivate defense mechanisms. These processes are aided by benign soil microbes es-
pecially, but not exclusively, mycorrhizal types which themselves mobilise supplies
of nutrients for their hosts. Free living microbes are being to be used in biological
control and this trend will accelerate. It will also be possible to exploit characteris-
tics of forms of self defense in plants. For example, some root exudates themselves
suppress the growth of competitors and it has been recognised that some soils are
naturally suppressive to disease-causing organisms and that this property stems from
the cumulative effects of benign microbes. Understanding natural plant signaling
and diverting it for the benefit of crop production is a possible sustainable pathway
to increased yield and profits. It is becoming apparent also that there is an immense
amount of signaling within individual plants that integrates responses to the envi-
ronment and that finely tunes the plant's responses appropriately. Additionally there
is signaling between plants which can detect and warn of approaching pests and
pathogens and develop appropriate responses. Furthermore, research demonstrates
that the plant's own defense compounds, like resveratrol in grapes, can form natural
and ecological alternatives to chemical pesticides (Jimenez Sanchez et al.
2008
).
Plants can, for instance, respond to feeding or egg deposition by herbivorous
arthropods by changing the volatile blend that they emit (Mumm and Dicke
2010
).
These herbivore-induced plant volatiles (HIPVs) can attract carnivorous natural en-
emies of the herbivores, such as parasitoids and predators, a phenomenon termed
indirect plant defense. The volatile blends of infested plants can be very complex,
sometimes consisting of hundreds of compounds. Most HIPVs can be classified as
terpenoids (e.g., (E)-beta-ocimene, (E, E)-alpha-farnesene, (E)-4,8-dimethyl-1,3,7-
nonatriene), green leaf volatiles (e.g., hexanal, (Z)-3-hexen-1-ol, (Z)-3-hexenyl ac-
etate), phenylpropanoids (e.g., methyl salicylate, indole), and sulphur- or nitrogen-
containing compounds (e.g.,
iso
thiocyanates or nitriles, respectively). One highly
intriguing question has been which volatiles out of the complex blend are the most
important ones for the carnivorous natural enemies to locate “suitable” host plants.
Electrophysiological methods such as electroantennography have been used with
parasitoids to elucidate which compounds can be perceived by the antennae. Dif-
ferent types of elicitors and inhibitors have widely been applied to manipulate plant
volatile blends. Transgenic plants that were genetically modified in specific steps
