Aphid responses to drought: a combined physiological and transcriptomic approach
Vickers, Laura Helen (2012)
Ph.D. thesis, University of Birmingham.
Drought is arguably one the greatest future challenges for agriculture. The response of phloem feeders, such as aphids, to increased drought expected under climate change is still relatively undefined. The effect on aphid feeding of drought stress in plants can be viewed as potentially positive and negative. It is currently accepted that under drought conditions, host plant sieve elements will become more concentrated; hence increases in available amino acid concentrations will potentially benefit aphids. However, the increased need for aphid osmoregulation under drought, to deal with the osmotically challenging diet, may be detrimental to aphid performance. Using the electrical penetration graph technique (EPG), the probing behaviour of two clones of Myzus persicae (O Clone and B Clone), Brevicoryne brassicae, Rhopalosiphum padi and Sitobion avenae on the host plants Brassica nigra and Lolium perenne (respectively), were analysed under a defined drought regime. Drought treatment had a significant effect on the behaviours of all aphid species and clones, except S. avenae. In B. brassicae and B clones of M. persicae, xylem feeding was significantly increased on droughted host plants. Furthermore, both clones of M. persicae and R. padi spent significantly less time ingesting sieve element sap, and more time not probing or in plant pathway activities whilst on droughted host plants. These results suggest that drought stress may cause a reduced palatability of host plants.
In addition, fecundity measurements showed that drought resulted in a reduction in aphid reproductive performance in M. persciae (O and B clone), B. brassicae, R. padi
and S. avenae. However, fecundity was only significantly reduced in the M. persicae (O clone only), B. brassicae and R. padi. To understand the aphid response to droughted B. nigra at the genetic level, the gene expression of M. persicae exposed to different drought regimes was analysed using microarrays. Gene expression analysis showed up regulation of the osmoregulation associated enzyme, sucrase, as well as the up regulation of other enzymes such as amylases, cytochrome P450s, Heat Shock Proteins, and an aquaporin when exposed to droughted hosts. Furthermore, it was found that the level of drought had a noticeable effect on gene expression in M. persicae, showing aphids have a very adaptable response to drought stress. The combined physiological and transcriptomic approach of this study gives a complementary whole organism assessment of aphid responses to drought. This study has helped to confirm xylem feeding and sugar polymerisation as important mechanisms of aphid osmoregulation, as well as providing support for the hypothesis of water cycling within the body of the aphid. This study has highlighted that aphids respond heterogeneously to water stress, and although it has been possible to identify some general trends, this study has emphasised that the adaptability of aphids to stress is species and even ecotype specific.
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