Following on from a previous blog on the interactions between soybean plants and soybean pests, new research on soybean (Glycine max) responses to the soybean aphid (Aphis glycines) published in Molecular Plant-Microbe Interactions has revealed some of the complex and fascinating interactions between pests and their plant hosts. This recent research led by Dr Gustavo Macintosh and Matthew Studham from Iowa State University has shown that soybean aphids can suppress the natural plant defense response of soybean plants to the aphids through the activation of what is known as an antagonistic decoy response. For example, the aphid will induce a plant defense that is not particularly effective against the pest (the ‘decoy’ defense) while suppressing the effective defense in order for it to continue feeding on the plant. It has further been found that aphids can actively suppress the effective defence responses of the plant while at the same time ‘hijacking’ the plant metabolism to improve the nutritional value of the plant for their own benefit. Soybean aphids do this by inducing asparagine synthase transcripts which improve the nutritional content of the phloem sap from which they feed.
Plants have evolved complex biochemical defense mechanisms that begin with the detection of elicitors, which are compounds that indicate a pest or pathogen attack. In the case of aphid attack, it is thought that elicitors could include aphid salivary proteins, which trigger an appropriate response in the plant to defend against the pest. The plant response is specific according to the type of pathogen or pest, for example when a plant is attacked by an insect pest which causes tissue damage it will produce toxins such as alkaloids. When attacked by a virus or bacteria plants may destroy cells to deprive the pathogen of nutrients required for growth. In addition, some of the plant volatiles emitted when the plant is under attack by insects pests such as aphids can be detected by the natural enemies of aphids, thereby ‘attracting’ the predators to a source of prey. These biochemical defense mechanisms in plants are controlled by plant hormones, which in soybeans include jasmonic acid (JA), ethylene (ET) and salicylic acid (SA). In addition to defence hormones, the abiotic stress hormone abscisic acid (ABA) has been shown to have various effects on pathogen resistance, and appears to be part of the plant’s response to aphid infestation. Interestingly, pests such as soybean aphids have evolved mechanisms to take advantage of the hormone signalling that controls plant defences, with some pests and pathogens producing hormones or hormone analogs, presumably to manipulate plant signalling to produce an ineffective decoy response that suppresses that effective defense response of the plant.
The soybean aphid is a phloem feeding insect pest that causes significant soybean yield loss worldwide. The aphid is native to Asia and has since spread throughout North America since it was first discovered there in 2000. This research has shown that salicyclic acid (SA) regulates the effective defence against the soybean aphid, while induction of the abiotic stress hormone abscisic acid (ABA) pathway may be a ‘decoy’ response that the aphids induce to counter the plant defences, since ABA suppresses SA response in soybeans. Furthermore, the aphids can ‘hijack’ the plant into producing a phloem sap with a higher nitrogen content, thereby making the sap more nutritious to the aphids. The changes induced by the soybean aphids have further implications for the plant since they appear to then make it easier for other pests, such as the soybean cyst nematode, to subsequently attack the plant. It is hoped further research in this area can help to identify soybean varieties that are more resistant to aphid and other insect pest attack and to predict how soybeans defences may react to new pests in the future.
‘ISU study: Aphid attacks weaken genetic defences in soybeans, may open door for other pests’, Iowa State University News Service, Oct 2012
‘Weakened genetic Mechanism of Soybeans is Susceptible for Other Insects’, Future Path, Oct 2012
Studham, M., & MacIntosh, G. (2012). Multiple phytohormone signals control the transcriptional response to soybean aphid infestation in susceptible and resistant soybean plants Molecular Plant-Microbe Interactions DOI: 10.1094/MPMI-05-12-0124-FI
Studham ME, & Macintosh GC (2012). Phytohormone signaling pathway analysis method for comparing hormone responses in plant-pest interactions. BMC research notes, 5 PMID: 22846705
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