Dissecting growth responses to gravity and light in poplar trees

In plants, the perception of Earth’s gravity leads to a gravity-driven growth process called gravitropism. In higher plants, this results in shoots growing upwards and roots downwards. Gravitropism includes the perception of gravity, signal transmission within the plant and the resulting growth response. A similar process, phototropism, is responsible for directional growth in response to light signals.

In a heterogeneous stand of trees, an individual tree may be subjected to an anisotropic light environment (in other words, light may be strongly directional in nature) causing phototropic reactions that will lead to gravitropic movements. The reorientation of the tree is due to the production of asymmetric reaction wood that affects the shape of the trunk and the quality of the wood. The molecular events leading to the formation of reaction wood are complex and little understood. One of the key reasons for this is that phototropic and gravitropic responses as well as the autotropic response, which is essentially the straightening of a curved organ, are processes in constant interaction.

In their new study published in AoBP, Lopez et al. characterise the early molecular responses occurring in the stem of poplar (Populus tremula × Populus alba) after gravistimulation in an isotropic environment. By tilting young poplars 30 min at 35° in an innovative isotropic light device, they are able to dissociate phototropic, gravitropic and autotrophic growth responses of the trees.

In their study, Lopez et al. highlight a list of 668 xylem-regulated genes that respond specifically to the gravitropic stimulus. Gene ontology analyses indicate that molecular reprogramming of processes such as ‘wood cell expansion’, ‘cell wall reorganization’ and ‘programmed cell death’ occur as early as 30 min after gravistimulation. Their work and their new experimental tool opens the door towards improved understanding of events triggering ‘reaction wood’ formation.