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Title: The Populus transcriptome in elevated CO2 reveals increased anthocyanin biosynthesis during delayed autumnal senescence      
Transcriptome or Gene expression
Over recent decades the progression of natural autumnal senescence has been increasingly delayed across wide areas of the Northern Hemisphere and is thought to be caused by rising temperature. Recently this delay has been shown to be partly attributable to rising atmospheric carbon dioxide concentrations [CO2]. Here, for the first time, we present a possible mechanistic explanation for this phenomenon. Using a plantation of Populus x euramericana grown at elevated [CO2] (e[CO2]) using Free-Air CO2 Enrichment (FACE) technology, we investigated the molecular and biochemical basis underlying this response. Using a poplar cDNA microarray where late growing season and senescing leaves from ambient [CO2] (a[CO2]) and e[CO2] were compared, revealed unique increases and decreases in transcript abundance in the e[CO2] treatment. Growth at e[CO2]caused the greatest increase in the abundance of transcripts catalysing steps in the anthocyanin biosynthetic pathway and an increase in leaf anthocyanin content. Leaf sucrose and starch content were also increased during senescence in e[CO2] and associated with a higher abundance of transcripts in the sucrose and starch biosynthetic pathways. We propose that in e[CO2], autumnal photosynthesis and sugar accumulation results in changes in genes expression that include further investment in secondary carbon metabolism leading to anthocyanin production. This enables prolonged leaf longevity during natural autumnal senescence through increased availability of carbon and improved stress tolerance while in contrast this may also have a negative effect on the control of dormancy. Overall design: Samples GSM398292..GSM398314 (late senescence samples only): Six replicate trees were labelled in the unfertilised sub-plots of plots 1-4 in the POP/EUROFACE site ( and the 10th leaf down from the closed apical bud of the dominant branch were labelled. Leaves were sampled to liquid nitrogen (18th October 2004) and stored at -80 oC. Extracted RNA was tested for quality and quantity and of that passing the test equal quantities of RNA from each replicate were taken and each sample from the control plots (plots 2 and 3) were randomly paired with a sample from and elevated CO2 plot (plots 1 and 4). From the resulting 9 dual channel hybridisations 6 had the control sample in the Cy3 channel and 3 had the elevated CO2 sample in the Cy3 channel. The 6 samples with Cydye orientation as Cy3=control (ambient CO2) and Cy5 = treated (elevated CO2) are as follows: GSM398296, GSM398301, GSM398303, GSM398304, GSM398305 and GSM398306. The 3 samples with dye swap i.e Cy3 = treated (elevated CO2) are as follows and Cy5 = control (ambient CO2) are as follows: GSM398292, GSM398313 and GSM398314. Samples GSM398426..GSM398474: Four replicate leaves per sub-plot were sampled and flash frozen into liquid nitrogen, on 31 August 2004, and prior to visible senescence but around the time of bud-set as given by Calfapietra et al., (2003) [1]. The tenth leaf down the dominant stem (which represented the first fully expanded mature leaf of maximum size) from a reference leaf of 30 mm in length (leaf one) were selected. On the 14th September twenty trees from within each experimental plot were taken. The tenth leaf down the dominant stem (which represented the first fully expanded mature leaf of maximum size) from a reference leaf of 30 mm in length (leaf one) were labelled on the main-stem above the petiole. All labelled leaves were a uniform height (approx 0.3 m) from the canopy top across all treatments and not shaded by the canopy. Four replicate leaves from within each sub-plot were sampled during each harvest (except on 18th October when six replicate leaves were sampled). Each sampling time was restricted to two hours within the same day and across sampling day’s uniform cloud free skies were chosen. September samplings took place between 15.00 and 17.00 when climatic variables were generally consistent; later in the season time of sampling was adjusted to an earlier two hour period after mid-day when climatic variables were generally consistent. A chlorotic canopy was evident in plots five and six, as described by Liberloo et al. (2007) therefore any subsequent analysis was for plots one to four only. Six replicate labelled leaves were sampled (as above) from each sub-plot on 18th October 2004
Populus x canadensis
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