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P18<br />
Visualizing molecular changes associated with gravitropism and tension wood<br />
formation in Populus<br />
S. GERTTULA, H. BRUMER 2 , S. MANSFIELD 2 , M. ZINKGRAF 1 and A. GROOVER 1,3<br />
1 US Forest Service, Pacific Southwest Research Station, Davis CA, USA; 2 University of British Columbia,<br />
Vancouver BC, Canada; 3 Department of Plant Biology, University of California Davis, USA<br />
Woody species have evolved specialized mechanisms for responding to gravity. In angiosperms, the<br />
upper surface of a leaning stem develops tension wood in which a strong tensile force is generated<br />
and used to counteract the force of gravity. Here, confocal imaging was used to visualize molecular<br />
events occurring during graviperception and tension wood development in wild type and trees<br />
transformed with ARBORKNOX2 (ARK2), a Class I Homeobox transcription factor. When ARK2<br />
transcript levels are lowered, gravibending is compromised, while elevated ARK2 transcript levels<br />
enhance gravibending. Immunolocalization against the poplar ptPIN3 auxin transport protein shows<br />
that ptPIN3 undergoes dynamic polar relocalization events in response to gravity perception with<br />
differences among the genotypes. Arabinogalactan proteins (AGPs), key markers of tension wood,<br />
were visualized using an AGP specific antibody (JIM14) and showed strong labelling in developing<br />
tension wood fibers. JIM14 labelling appeared in fibers closer to or further from the cambium<br />
depending on the genotype. Xyloglucan endotransglycosylase (XET) activity has been proposed to<br />
play an important role in force generation of tension wood fibers. XET activity was visualized after<br />
incubation of wood tissue sections with a rhodamine-labelled XXXG oligomer. XET activity was strong<br />
in developing tension wood fibers. Similar to JIM14 labelling, XET labelling occurred in fibers closer<br />
to the cambium in ARK2 overexpressing plants. Together, these imaging techniques provide new<br />
insights and tools for the study of graviperception, gravitropism, and wood development in<br />
angiosperm trees.<br />
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