Role of Gene and Hormone Regulation in Adventitious Root Formation in Trees

Dear Colleagues,

De novo root regeneration, a crucial step for clonal propagation of plant species by cuttings, is a complex genotype-dependent trait affected by several inner and outer cues, including the physiological status and age of the plant, phytohormone homeostasis, nutrient availability, carbohydrate dynamics, light conditions, biotic factors, etc. The recalcitrance to root regeneration of many trees is the main hurdle hindering the vegetive propagation of elite genotypes and forest industry development. In cuttings of woody species, current models suggest that both initial wounding and auxin are essential to trigger the process, where auxin plays a master-regulator role and integrates into its plastic signaling pathways the inputs from several other gene and phytohormone routes.

Initial wounding causes a major shift in the balance of several phytohormones, including a transient burst of jasmonic acid and ethylene, followed by the local accumulation of auxin in the wounding zone through auxin biosynthesis and transport. This spatial-constraint buildup of auxin derives in the formation of a tissue gradient of the hormone that results in the priming of specific cells in the vasculature or neighboring zones, and those cells change their previous fate and become the so-called root founder cells.

In general terms, ethylene and jasmonic acid have an early positive role with auxin in the formation of adventitious roots, whereas gibberellins, abscisic acid, and cytokinins are expected to negatively influence the development of the new roots. However, given the complex nature of the induction and growth of adventitious roots, no unambiguous model has been established that could be valid for every species, and the identity of many genes involved in this morphogenetic route remains elusive. Furthermore, several other molecular cues exert an influence in the process, such as microtubule and cell wall remodelling, epigenetics modifications, and microRNAs activity, probably through a complex interaction with phytohormone homeostasis and derived regulation of specific gene expression.

The release of draft genomes of many woody species, the affordable access to transcriptomics analysis, and the popularization of mass spectrometry techniques to quantify phytohormones in plant tissues has enabled the expansion of studies concerning the characterization of the different steps of adventitious rooting in many non-model species. The integration of results obtained through these techniques facilitates the characterization of specific signaling networks and the development of conceptual models. With this Special Issue, our aim is to provide a platform for the communication of research related to the gene and phytohormone control of adventitious root formation in trees and woody species, allowing for a precise description of the molecular pathways involved in every phase of the process, which may pave the way for an optimized exploitation of the available resources.

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

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• adventitious rooting
• cuttings
• phytohormones
• signaling networks
• translational research
• transcriptomics
• recalcitrance
• woody species