Analysis and modelling of gravi and phototropic reactions in trees – TROPIC

- Construction of an original experimental device that enables to tilt plant in a isotropic light environment in order to disentangle gravitropic and phototropic reactions.
- Study of tropic reactions to study the movements of trunks and the molecular responses by recent methods (microRNA, phosphoproteomics)
-An original method to retrieve by computations gravitropic movements from the quantification of reaction wodd and measurements of geometry and mass of logs of felled adult trees.

Experiments in controlled conditions enabled :
- to conclude to an additivity of gravitropic and phototropic responses (movements) for poplars
- poplars and oaks exhibit different sensitivity to gravitropic and phototropic signals
- to isolate genes specifically implicated in gravitropism
A model has been built to simulate the perception of gravitropic and phototropic signals and revealed that a perception of inclination but also a perception of curvature is required to simulate gravitropic and phototropic responses.

Perspectives of the project will be defined at the end of year 2014.

One part of the first results leaded to the participation to the writing of two book chapters about tension wood, the other part of the first results are being written for peer reviewed scientific journals.

Scenarios of climatic changes predict an increase of frequency and amplitude of storms, which destroy forest stands. In this context and the actual awareness of environment preservation, management of tree stands tends to promote mixed and heterogeneous forests with using natural regeneration instead of planting monospecific even-aged stands. In even-aged and monospecific stands the light environment is regular and determined by neighbouring trees. Trees from these stands are straight. In irregular and mixed stands, the light environment of individual trees is more irregular from a spatio-temporal point of view: trees are not compulsorily the same size, nor from the same species, the light environment which is more or less isotropic during the first tree stages becomes more and more anisotropic during tree spatial development within the canopy. Anisotropy is reinforced when a canopy opening occurs (harvesting of an adult tree for wood production). This anisotropy can lead to phototropic movements which lead to gravitropic movements. Both processes have bad consequences on timber shape and lead to the formation of reaction wood which affects wood quality. It is thus important to evaluate consequences of tropic reactions in mixed and irregular forests. The TROPIC project aims at studying photo and gravitropic processes and their interactions by an interdisciplinary approach in order to characterise, quantify and model, their contributions in the acquisition of tree shape in the context of irregular and mixed forests.
The majority of tropic studies have been performed on young organs exhibiting primary growth where the motor process of tropic movements is differential growth. Among these studies only a few have established quantitative relationships between the stimuli like gravity and light and tropic responses. There are only a few studies on trees where the formation of reaction wood is a fundamental process of tropism. To our knowledge, there is no study that has established dose-response curves between stimuli and tropic responses for trees so that the prediction of tropic movements remains difficult. The TROPIC project, in a first time, will perform a study of tropic reactions and of the signalling pathway in controlled conditions. In order to disentangle gravi- and phototropic responses and to quantify their relative contributions, anisotropic light devices and an innovating isotropic light design will be used and combined to tree tilting. These studies encompass an analysis of rapid responses linked to the perception-transduction phases of stimuli at the molecular scale on poplar, the plant model for woody plants. This analysis of rapid responses aims at identifying the signalling pathway implicated in photo- and gravitropism and at studying the regulation of molecular actors by light and gravity. An analysis on more long term will also be performed at the plant scale in controlled conditions in order i) to quantify the level of genericity of tropic movements between poplar and two forest species: oak and beech ii) to establish doses responses curves between tropic reactions and stimuli (light and gravity). In a second time, the TROPIC project aims at utilising the results obtained on controlled conditions in a modelling step: models of gravitropic reactions in trees will be enriched with the phototropic process. The simulations issued of these models will be compared to tropic reactions that will be measured in natural conditions for oak and beech in understorey of mixed and irregular forests.