The importance of the phloem and CHO transport in shaping plant defense responses and herbivore feeding behaviors

Background/Question/Methods

“The budget of nature is fixed; but she is free to dispose of particular sums by an appropriation that may please her. In order to spend on one side, she is forced to economize on the other side”  (Darwin 1872)

Fixed resource budgets are a fact of life for plants facing attack by herbivores. Grazing threatens plants with the immediate loss of nutrient-rich tissues as well as the loss of future ‘revenue’ by suppressing photosynthesis and nutrient assimilation.  In response, plants may employ two basic strategies: (1) to resist herbivory plants mobilize their dwindling resources for the production of chemical defenses (2) to tolerate herbivory valuable resources are quickly withdrawn. Both strategies require long-distance resource transport among plant modules which, in turn, force trade-offs between plant defense and the processes of growth, reproduction, and storage. 

Results/Conclusions

In wounded plants the long-distance flow of resources can increase, stop, or reverse within hours.  For many wounded tissues, especially young, developing sink leaves, the import of resources (especially carbohydrates; CHOs) is crucial.  Rapid increases in sink strength, facilitated by the up-regulation of cell wall invertases and H⁺ coupled transporters, are common and may increase the CHO import by as much as 400%.  This supports and stimulates the synthesis of plant defenses, especially carbon-based phenolics.  Without induced CHO import polyphenol accumulations do not occur.  We’ve observed that sink strength can be cooperative.  Nearby wounds result in greater CHO import strength and galling insects, which induce invertase activities, may more effectively import CHOs (and “redden” galls by accumulating polyphenols) when grouped together.  On the other hand, CHO import may be reduced by sink-sink competition or insect feeding behaviors such as stem girdling.  We hypothesize that locally induced defenses are more likely to be “carbon-based”, in part, because we have found no evidence that nitrogen transport is similarly induced by wounding.  We also consider plants that adopt a strategy of tolerance instead of resistance by “bunkering” resources, e.g. transporting them away from wound sites to belowground tissues.  Recent studies suggest that both types of resource allocation occur, sometimes in the same wounded plant, at the same time.  Actively managing long-distance resource flow is a critical, often unappreciated component of plant defense responses.