Polar auxin transport in the wood-forming tissues of hybrid aspen is under simultaneous control of developmental and environmental signals

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Abstract
Recent research has highlighted the importance of auxin concentration gradients during plant development. Establishment of these gradients is believed to involve polar auxin transport through specialized carrier proteins. We have used an experimental system, the wood-forming tissue of hybrid aspen, which allows tissue-specific expression analysis of auxin carrier genes and quantification of endogenous concentrations of the hormone. As part of this study, we isolated the putative polar auxin transport genes, PttLAX1–PttLAX3 and PttPIN1–PttPIN3, belonging to the AUX1-like family of influx and PIN1-like efflux carriers, respectively. Analysis of PttLAX and PttPIN expression suggests that specific positions in a concentration gradient of the hormone are associated with different stages of vascular cambium development and expression of specific members of the auxin transport gene families. We were also able demonstrate positive feedback of auxin on polar auxin transport genes. Entry into dormancy at the end of a growing season leads to a loss of auxin transport capacity, paralleled by reduced expression of PttLAX and PttPIN genes. Furthermore, data from field experiments show that production of the molecular components of the auxin transport machinery is governed by environmental controls. Our findings collectively demonstrate that trees have developed mechanisms to modulate auxin transport in the vascular meristem in response to developmental and environmental cues.