The Chloroplast as Mediator of Phenolic Induction

Open Access
Walton, Anne Barber
Graduate Program:
Plant Physiology
Doctor of Philosophy
Document Type:
Date of Defense:
December 20, 2002
Committee Members:
  • Jack C Schultz, Committee Chair
  • Simon Gilroy, Committee Chair
  • Wayne Roger Curtis, Committee Member
  • Daniel J Cosgrove, Committee Member
  • John Edward Carlson, Committee Member
  • chloroplast
  • phenolics
  • chemical defenses
  • resource allocation
  • Arabidopsis
  • tobacco
  • photosynthesis
  • carbon allocation
  • HPLC
The degree to which insect herbivores and plant pathogens elicit accumulation of phenolic defenses in plants is variable in nature. Because light and leaf development, factors which can alter resource levels in chloroplasts, influence enemy responses to plants, I was interested in the degree to which chloroplast carbon resources constrain phenolic induction responses. To address the central role of the chloroplast in supplying carbon resources for induced phenolic accumulation, I devised the Chloroplast Control Hypothesis: constraints on phenolic induction occur when stromal substrates are limited. Levels of carbon resources were manipulated by shading plants and monitoring phenolic accumulation responses to Jasmonic acid (JA), a wound signal that elicits phenolic metabolism in plant leaves much as does wounding or herbivory. Phenolic accumulation was prevented in darkened tobacco plants, and low light constrained phenolic accumulation, but photosynthetic carbon assimilation was a poor predictor of resource investment in phenolic defenses. While young leaves which are carbon resource sinks were more responsive to JA, metabolite import was not enhanced in these leaves. In fact, activity of the chloroplast phosphoenolpyruvate/ phosphate transporter was not required for Arabidopsis plants to mount strong shikimate-based defense responses that deterred an insect herbivore. These results suggest that chloroplasts must possess alternate means of acquiring substrates for induced phenolic synthesis, possibly via starch breakdown. I was unable to disprove the chloroplast control hypothesis, as mechanisms of resource allocation to induced phenolics are much more complex than I originally envisioned. Chloroplasts may possess at least three avenues to acquire carbon resources for the shikimate pathway, allowing plants to compensate for deficiencies in one route when others are limiting. Such redundancy in resource supply may not be surprising since products of this pathway play central roles in plant physiology.