Adaptive differences in response to flooding in Populus alba and Populus tremula

Abstract

This study assesses the adaptation of Populus trees to flooding stress. Due to current and predicted changes in the climate, there is an increased need for research into tolerant species to be grown for both flood defence and as crops. Two species of Populus that naturally hybridise were used to assess the adaptation to flooding stress. Due to differences in their native habitat, Populus alba and Populus tremula, are predicted to be relatively flood tolerant and flood sensitive species respectively. Here, they were used to highlight key physiological mechanisms related to flood tolerance and to elucidate genes underlying these processes. This will aid the development of flood tolerant crops and improve understanding of adaptation to flooding in sensitive and tolerant tree species. This research employed physiological and morphological monitoring techniques, along with global gene expression microarrays to identify the key responses related to flooding stress acclimation and adaptation.

Many of the traits measured including stomatal conductance, transpiration, leaf size, leaf number, height and chlorophyll content index (CCI) revealed family differences in flooding response though these were not consistent within species therefore failing to reveal why the species differ in natural environments. Furthermore, these responses varied between flooding events suggesting epigenetic changes were in play. Adventitious roots (AR), able to grow and function in oxygen-deficient flooded soil, probably by virtue of their aerenchyma, were produced in response to both summer and winter flooding. During winter flooding, only two P. tremula individuals produced them whereas in P. alba they were produced by 16 individuals..

The final focus was on the transcriptomics of root flooding. A set of gene expression changes that occur in roots in response to flooding were identified. This was in addition to a set of gene expression differences between flooded root types. The flooding response revealed 3,687 genes were differentially expressed, of which 1,961 were up-regulated in flooded roots. Similarly the comparison of flooded root types revealed 7,589 genes were differentially expressed, of which 3,866 genes were up-regulated in adventitious roots (AR).

The results of this project may be due to epigenetic regulation and therefore future analysis of the DNA methylation may elucidate the effects of repeated flooding. I can conclude that the flooding stress response does differ within the population but there are no significant differences between the two Populus species. Therefore the two species are similarly flood tolerant but where present the flooding stress responses expressed were adventitious root formation (ARF), stomatal closure, and shoot and leaf growth reduction.

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