Associational Effects of Desmodium Intercropping on Maize Resistance and Secondary Metabolism.

Intercropping is drawing increasing attention as a strategy to increase crop yields and manage pest pressure, however the mechanisms of associational resistance in diversified cropping systems remain controversial. We conducted a controlled experiment to assess the impact of co-planting with silverleaf Desmodium (Desmodium uncinatum) on maize secondary metabolism and resistance to herbivory by the spotted stemborer (Chilo partellus). Maize plants were grown either in the same pot with a Desmodium plant or adjacent to it in a separate pot. Our findings indicate that co-planting with Desmodium influences maize secondary metabolism and herbivore resistance through both above and below-ground mechanisms. Maize growing in the same pot with a Desmodium neighbor was less attractive for oviposition by spotted stemborer adults. However, maize exposed only to above-ground Desmodium cues generally showed increased susceptibility to spotted stemborer herbivory (through both increased oviposition and larval consumption). VOC emissions and tissue secondary metabolite titers were also altered in maize plants exposed to Desmodium cues, with stronger effects being observed when maize and Desmodium shared the same pot. Specifically, benzoxazinoids were strongly suppressed in maize roots by direct contact with a Desmodium neighbor while headspace emissions of short-chain aldehydes and alkylbenzenes were increased. These results imply that direct root contact or soil-borne cues play an important role in mediating associational effects on plant resistance in this system.

Interaction diversity explains the maintenance of phytochemical diversity.

The production of complex mixtures of secondary metabolites is a ubiquitous feature of plants. Several evolutionary hypotheses seek to explain how phytochemical diversity is maintained, including the synergy hypothesis, the interaction diversity hypothesis, and the screening hypothesis. We experimentally tested a set of predictions derived from these hypotheses by manipulating the richness and structural diversity of phenolic metabolites in the diets of eight plant consumers. Across 3940 total bioassays, there was clear support for the interaction diversity hypothesis over the synergy or screening hypotheses. The number of consumers affected by a particular phenolic composition increased with increasing richness and structural diversity of compounds. Furthermore, the bioactivity of phenolics was consumer-specific. All compounds tested reduced the performance of at least one consumer, but no compounds affected all consumers. These results show how phytochemical diversity may be maintained in nature by a complex selective landscape exerted by diverse communities of plant consumers.

Activity and structure of human acetyl-CoA carboxylase targeted by a specific inhibitor.

We have studied a series of human acetyl-CoA carboxylase (ACC) 1 and ACC2 proteins with deletions and/or Ser to Ala substitutions of the known phosphorylation sites. In vitro dephosphorylation/phosphorylation experiments reveal a substantial level of phosphorylation of human ACCs produced in insect cells. Our results are consistent with AMPK phosphorylation of Ser29 , Ser80 , Ser1,201 , and Ser1,216 . Phosphorylation of the N-terminal regulatory domain decreases ACC1 activity, while phosphorylation of residues in the ACC central domain has no effect. Inhibition of the activity by phosphorylation is significantly more profound at citrate concentrations below 2 mm. Furthermore, deletion of the N-terminal domain facilitates structural changes induced by citrate, including conversion of ACC dimers to linear polymers. We have also identified ACC2 amino acid mutations affecting specific inhibition of the isozyme by compound CD-017-0191. They form two clusters separated by 60-90 Å: one located in the vicinity of the BC active site and the other one in the vicinity of the ACC1 phosphorylation sites in the central domain, suggesting a contribution of the interface of two ACC dimers in the polymer to the inhibitor binding site.