Hormonal regulation of root growth under moderately elevated temperatures.

Background: Roots anchor plants in the ground, providing an interface for interactions with the environment and sensing potential stressors. At the same time, they contribute to the acclimatization to stressful conditions through their growth plasticity. Root growth is a combination of cell division and cell elongation, ultimately shaping root system architecture depending on environmental stimuli. Root thermomorphogenesis refers to the altered root growth response under moderately elevated ambient temperatures, characterized, for example, by an increase in primary root growth during early seedling development. While the molecular regulation of shoot thermomorphogenesis is comparatively well understood, the gene- and hormone-regulatory networks underlying root growth responses to warm temperature have only begun to be uncovered in recent years.

Scope: In this article, we review the latest findings of how root growth, comprised of cell division and elongation, is regulated by the phytohormones auxin, cytokinins and brassinosteroids at optimal temperatures. We then summarize our current understanding of root growth responses to warm temperatures during early seedling development and the key role of auxin in this process. Furthermore, we address the contributions of cell division versus cell elongation to root thermomorphogenesis, discuss whether the root is autonomous in sensing and reacting to increased temperatures, and provide an outlook of how root thermomorphogenesis research can be applied to crops.

Conclusions: Root growth is a complex process which is tightly regulated and strongly depends on environmental factors. During early seedling development, elevated ambient temperatures stimulate auxin signalling which leads to an increase in both cell division and elongation, resulting in elongated primary roots. It appears that the root can autonomously sense and react to temperature changes at this stage. Root thermomorphogenesis seems to be conserved among many plants including crops, but its ecophysiological relevance remains open to further research.