Abstract
Aphids are insects that cause direct damage to crops by the removal of phloem sap, but more importantly they spread devastating viruses. Aphids use their sophisticated mouthpart (i.e. stylet) to feed from the phloem sieve elements of the host plant. To identify genes that affect host plant resistance to aphids, we previously screened an Arabidopsis thaliana activation tag mutant collection. In such mutants, tagged genes are overexpressed by a strong 35S enhancer adjacent to the natural promoter, resulting in a dominant gain-of-function phenotype. We previously identified several of these mutants on which the aphid Myzus persicae showed a reduced population development compared with wild type. In the present study we show that the gene responsible for the phenotype of one of the mutants is At5g65040 and named this gene Increased Resistance to Myzus persicae 1 (IRM1). Overexpression of the cloned IRM1 gene conferred a phenotype identical to that of the original mutant. Conversely, an IRM1 knockout mutant promoted aphid population development compared to the wild type. We performed Electrical Penetration Graph analysis to investigate how probing and feeding behaviour of aphids was affected on plants that either overexpressed IRM1 or contained a knockout mutation in this gene. The EPG results indicated that the aphids encounter resistance factors while reaching for the phloem on the overexpressing line. This resistance mechanism also affected other aphid species and is suggested to be of mechanical nature. Interestingly, genetic variation for IRM1 expression in response to aphid attack was observed. Upon aphid attack the expression of IRM1 was initially (after 6 hours) induced in ecotype Wassilewskija followed by suppression. In Columbia-0, IRM1 expression was already suppressed six hours after the start of the infestation. The resistance conferred by the overexpression of IRM1 in A. thaliana trades off with plant growth.
Highlights
Phloem-feeding insects display a variety of activities during settlement and feeding on a host plant
Mutant 3646 was previously identified as an A. thaliana activation tag mutant with a significantly smaller number of M. persicae than the wild type WS [19]
To determine the cause of the phenotype of mutant 3646, we performed inverse PCR with primers designed on transposon and T-DNA sequences (Table 1)
Summary
Phloem-feeding insects display a variety of activities during settlement and feeding on a host plant. After landing on a host plant, they intercellularly probe plant tissue and frequently puncture epidermis, mesophyll, and parenchyma cells to determine the suitability of the host [2]. Once they established a feeding site, aphids can continue feeding from a single phloem sieve element for hours or even days [3]. These probing and feeding activities of aphids can be monitored using the Electrical Penetration Graph (EPG) technique in which the aphid and the plant are wired in a low-voltage circuit connected to a recording system [4,5]. Information on the aphid activities can be extracted from the recorded signal waveforms and provides insight into the location of plant resistance factors [5]
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