Semagenesis in Nonparasitic Plants Público

Abstract

Abstract

Semagenesis in Nonparasitic Plants

The parasitic plant Striga asiatica detects monocot host roots by semagenesis: Striga's root tip exudes reactive oxygen species which oxidize phenols on nearby hosts' cell walls, releasing quinones, which trigger parasitism in Striga. In another plant-plant detection phenomenon, many nonparasitic plants adjust their root architecture so that their roots avoid or grow toward competitor roots. This density-dependent phenotype is not fully explained by the detection of limited resources around another root or by the toxicity of allelochemicals. Semagenesis may be used by nonparasitic plants as a third mechanism of detecting nearby competitors. This study considers the two unconfirmed steps of semagenesis in nonparasitic dicots: 1) do reactive
oxygen species (e.g. H2O2) result in the release of quinones from nonparasites' roots? and 2) is the response to semagenesis signals the same as the response to high population densities? When ten day old Arabidopsis thaliana seedlings were treated with 0, 10, 25, or 50 µM H2O2, dimethoxy-p- benzoquinone (DMBQ) and methoxy-quinone were not detected in an ethyl acetate extraction of the growth medium, possibly because this dicot plant does not have enough phenols in its cell walls to produce high levels of quinones. The same extraction of the growth media showed that 48 h treatment with 25 or 50 µM H2O2 causes an increase in exudation of both camalexin and indole-3-carboxylic acid, indicating that such treatments elicit a stress response. In the second step of semagenesis, elicited quinones should trigger a morphological and physiological change consistent with density-dependent morphology. Ten day old Arabidopsis seedlings treated with DMBQ had shorter roots and more lateral roots than untreated seedlings. With Arabidopsis seedlings grown at varying densities, no consistent density-dependent morphology was observed. Semagenesis is more likely to occur in 20 day old Arabidopsis, as these older plants have more phenols and a known root exudate-dependent root architecture. Semagenesis may serve as a detection mechanism for stressed dicots, which release phenols in quantity, or as a mechanism for dicots to detect monocots, which have more phenols in their walls than do dicots.

Table of Contents

Table of Contents

Introduction ..........................................................................................................................1
Chemical signaling between organisms ..........................................................................................1
Competition in plants ................................................................................................................6
Semagenesis in nonparasitic plants-a method of detecting competition? .............................................9
Testing the semagenesis model in nonparasites..............................................................................13

Methods ...............................................................................................................................14
Germination of seeds ................................................................................................................14
Treatment with H2O2 ...............................................................................................................14
Sample extraction of DMBQ .......................................................................................................15
Aqueous extraction of root exudate ............................................................................................15
Organic extraction of root exudate .............................................................................................16
Effect of population on root morphology and on ·O2^- accumulation in the root tip ................................17
Effect of DMBQ on morphology and .............................................................................................17
Effect of population density on morphology ...................................................................................18
Data analysis ..........................................................................................................................19

Results .................................................................................................................................20
DMBQ extraction.......................................................................................................................20
Treatment with H2O2................................................................................................................20
Organic extraction of root exudate ..............................................................................................32
Effect of population on root morphology and on ·O2^- accumulation in the root tip ................................33
Effect of DMBQ on morphology ...................................................................................................36
Effect of population density on morphology ...................................................................................37

Discussion.............................................................................................................................42

References ............................................................................................................................48

Figures
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About this Honors Thesis

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School	Emory College
Department	Chemistry
Degree	BS
Submission	Honors Thesis
Language	English
Research Field	Chemistry, Biochemistry Biology, Ecology
Palavra-chave	biology chemistry ecology plants
Committee Chair / Thesis Advisor	Lynn, David, Emory University
Committee Members	Gerardo, Nicole Marie, Emory University De Roode, Jacobus, Emory University

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