Tissue-specific hormone profiles from woody poplar roots under bending stress

Mechanical forces induced by bending are able to trigger an asymmetrical response in Populus nigra L. woody taproots. This response includes the recruitment of new lateral roots on the convex side and the deposition of reaction wood (RW) on the opposite concave side. Since these responses seem to be induced by asymmetric activity and differentiation of cambium cells, we investigated, in the present work, how mechanical forces could influence the activation of specific phytohormone signaling pathways on the two sides of the vascular cambium. Thus, distinctive tissues were isolated from convex and concave sides of bent poplar root using cryosectioning. Successively, the isolated tissues, represented by the cambial zone, and the developing phloem and xylem, were analyzed using liquid chromatography coupled to tandem mass spectrometry to profile auxins, abscisic acid (ABA), cytokinins (CKs) and their metabolites. The auxin gradient on the concave side, with the IAA maximum localized in the cambium and decreasing level toward the developing phloem and xylem, suggests a pivotal role of IAA in the control of cambial growth rate, xylem differentiation and RW production. The IAA differences between the two bent root sides could be at the basis of the strictly unidirectional RW production. The higher levels of ABA and all CKs metabolites on the concave side support their involvement in RW production, whereby ABA could mediate the adaptation to the deforming conditions generated by bending, while CKs could act in synergy with IAA in controlling cell differentiation and meristem size.