Characterization of the transcriptional and metabolic changes induced in soybean by the soybean aphid, Aphis glycines.
Soybean is the second most important crop in the U.S. The soybean aphid is one of the pests that can reduce soybean production significantly, with damages adding up to 40% of crop loss. Limited information is available about plant-aphid interactions. Even less is known about the specific soybean aphid-soybean interaction because this aphid is a newly introduced species that until a few years ago had received little attention.
Plants use a diversity of defense strategies to fight pests. They change physical properties of the leaf surface (cuticle) to affect recognition and colonization, produce toxins as direct deterrents, and induce indirect defenses aimed at attracting predators of the pests. We used a combination of transcriptomics and metabolomics to dissect the changes that occur in soybean after soybean aphid colonization, both in susceptible and resistant varieties. Our results showed that in susceptible plants aphids trigger the induction of defense responses that do not seem to be effective. We also found that amino acid accumulation and lipid metabolism are important for this interaction.
Role of general stress responses in the interaction between plants and aphids.
Microarray experiments in several plant species have shown that in addition to defense pathways, many stress responses are induced during aphid infestations. In soybean, we found that ABA synthesis and response are highly induced during susceptible interactions but not in resistant plants. We are using VIGS to study the role of ABA in soybean, and we are also using Arabidopsis to investigate how this hormone affects the response to aphid attacks. We hypothesize that aphids use this pathway to suppress effective plant defenses.
Identification of novel signaling pathways that control defense responses to insects and mechanical wounding in Arabidopsis and soybean.
Plants injured by herbivores induce a wide range of genes whose products help to defend against the herbivore, and also repair the plant or to defend against opportunistic pathogens that might infect the wounded plant. In Arabidopsis thaliana, oligogalacturonides (OGAs) and jasmonic acid (JA) are the main regulators of the signaling pathways that control the local and systemic wound response, respectively. The secreted ribonuclease RNS1 is induced by wounding in Arabidopsis independent of these two signals, however, indicating that another wound-response signal exists. We showed that abscisic acid (ABA) also induces RNS1. In the absence of ABA signaling, wounding induces only approximately 45% of the levels of RNS1 mRNA that are induced when ABA pathways are operational. However, significant levels of RNS1 and other wound-induced nuclease activities still accumulate in the absence of ABA signaling. Our results suggest that wound-responsive increases in ABA production may amplify induction of RNS1 by a novel ABA-independent pathway. The wound induction of RNS1 is due in part to transcriptional regulation by wounding and ABA. We have also found evidence of post-transcriptional regulation which might contribute to the high levels of RNS1 transcript accumulation in response to wounding.
We are also investigating the role of ABA in plant-aphid interactions, both in Arabidopsis and soybean.
Biological roles of secreted ribonucleases in defense responses and also in other cellular processes.
While secreted ribonucleases (RNases) have been well studied at the enzymatic and structural levels, little is known regarding their biological functions. One family of secreted RNases, the RNase T2 family, is particularly widespread, with members throughout various kingdoms. In recent years, many plant members of this superfamily have been identified. Gametophytic self-incompatibility in several plant groups involves the activity of S-RNases, one subfamily of plant T2 RNases. Another subfamily, the S-like RNases, is found in self-compatible as well as self-incompatible plant species. RNS1, an RNase induced by insects and mechanical damage, is a member of the S-like RNase family. While expression analyses suggest that S-like RNases are part of the plant defense response, their biological role is not undestood.
To understand the role of S-like RNases we are working on the five S-like RNase genes from Arabidopsis. We found that RNS2 is necessary for normal rRNA decay, and mutant plants lacking RNS2 present constitutive autophagy. We are currently studying the link between rRNA decay and autophagy.
We have also extended our functional analysis of the RNase T2 family to other systems. We are currently studying the defense role of RNases in flower nectar. In addition, we have done evolutionary analyses of T2 RNases in several species, including plants (rice, soybean, and others) and animals (Caenorhabditis elegans and zebrafish) to understand why T2 RNases are conserved in almost all organisms.
Aphis glycines on soybean
Myzus persicae on Arabidopsis
Tobacco and Petunia nectaries
Expression of RNase Dre2 in zebrafish embryo