Dear Colleagues,

Trees are long-living organisms with complex life cycles, which must contend with a variable environment over their lifetimes. Therefore, trees are highly adaptable, displaying a wide range of phenotypes as a function of their environments. The complex architecture of the root system is the result of the adaptive growth below ground to the availability of both nutrients and water and to the rapidly changing climate conditions, highlighting the fascinating plasticity of plants. In addition, the root system is fundamentally important because it anchors a plant to its substrate, provides support for extensive growth, serves as a storage organ, perceives stress signals, and can form symbioses with microorganisms and fungi. The importance of the phytohormones in modulating root growth is well established. Acting in minute concentrations, these chemical substances enable the transduction of environmental and internal cues into plastic responses, for instance by regulating the meristematic activity, ranging from cell division, differentiation, and fate acquisition to the root outgrowth in response to physiological and stress conditions. Root development is the result of the interactions of phytohormones with one another and with other endogenous and exogenous factors in complex networks, in which auxin is considered the master regulator. 

Substantial progress has been made in the understanding of the role played by phytohormones in tree rooting in the last decades. Nonetheless, the information is still scattered and difficult to process, and important questions remain open for future investigations. These issues concern the functions of other unexplored plant growth regulators, the phase-specific function of phytohormones during root formation, the relationships between environmental cues, epigenetic modifications, and plant hormone action in trees, among others. Until recently, this information was inaccessible, mainly due to the unavailability of genomic resources and the lack of suitable tools for the quantification of metabolites. The release of draft genomes from conifers and improved genome assembly of several woody species, combined with the development of sensitive methods for the identification and quantification of metabolites and advances in imaging technologies for root phenomic studies, have opened new possibilities for functional studies regarding homeostatic mechanisms in tree rooting. This Special Issue aims to help researchers keep abreast of the most recent developments in elucidating the central role played by plant growth regulators during the formation of adventitious, lateral, and primary roots in tree species. We invite authors to submit original research, review, and opinion papers for this Special Issue, covering this topic for wild and cultivated trees, as well as for other woody and arborescent species.

Dr. Conchi Sánchez
Dr. Jesús M. Vielba
Dr. Federica Brunoni
Guest Editors

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• forest trees
• woody species
• phytohormones
• root system
• vegetative propagation
• plant hormone profiling
• genetic diversity
• epigenetics
• next-generation sequencing
• translational research