Unravelling the response of poplar (Populus nigra) roots to mechanical stress imposed by bending GABRIELLA S. SCIPPA 1 , DALILA TRUPIANO 1 , MARIAPINA ROCCO 2 , ANTONINO DI IORIO 3 , & DONATO CHIATANTE 3 1 Department of Scienze e Tecnologie per l’Ambiente e il Territorio, University of Molise, Pesche, Italy, 2 Department of Biologia e Scienze Ambientali, University of Sannio, Benevento, Italy, and 3 Department of Scienze Chimiche e Ambientali, University of Insubria, Como, Italy Abstract Mechanical stress is a widespread environmental condition that can be caused by several factors (i.e. gravity, touch, wind, soil density, soil compaction and grazing, slope) and that can severely affect plant stability. In response to mechanical stress and to improve their anchorage, plants have developed complex mechanisms to detect mechanical perturbation and to induce a suite of modifications at anatomical, physiological, biochemical, biophysical and molecular level. Although it is well recognized that one of the primary functions of root systems is to anchor the plant to the soil, root response to mechanical stresses have been investigated mainly at morphological and biomechanical level, whereas investigations about the molecular mechanisms underlying these important alterations are still in an initial stage. We have used an experimental system in which the taproot poplar seedlings are bent to simulate mechanical perturbation to begin investigate the mechanisms involved in root response to mechanical stress. The results reported herein show that, in response to bending, the poplar root changes its morphology by emitting new lateral roots, and its biomechanical properties by increasing the root biomass and lignin synthesis. In addition, using a proteomic approach, we found that several proteins involved in the signal transduction pathway, detoxification and metabolism are up-regulated and/or down-regulated in the bent root. These results provide new insight into the obscure field of woody root response to mechanical stress, and can serve as a basis for future investigations aimed at unravelling the complex mechanism involved in the reaction of root biology to environmental stress. Key words: Root system, mechanical stress, poplar, Populus nigra, woody root Introduction Mechanical stress is a common condition caused by several factors (i.e. gravity, touch, wind, soil density, soil compaction and grazing, slope) that can severely affect plant stability. Plants have developed sensory mechanisms to detect mechanical perturbations and to induce a suite of responses (anatomical, physio- logical, biochemical, biophysical and molecular) collectively termed ‘‘thigmomorphogenesis’’ (Jaffe & Forbes 1993; Braam 2005). Thigmomorphogen- esis is believed to improve plant anchorage (Mitchell et al. 1975; Jaffe & Biro 1979) and has been mainly investigated in the stem of herbaceous and woody plants. In the case of woody plants, particular attention has focused on the role of the hormones ethylene and auxin, and factors involved in reaction wood formation (Timell 1986; Sundberg et al. 1994; Little & Eklund 1999). Reaction wood represents a corrective growth that results from coordinately enhanced development of strength-contributing cells and wall metabolites (Wardrop & Davies 1964; Scurfield 1973; Barnett 1981; Timell 1986), and is one of the most critical defense systems in the long- term growth of woody species. Furthermore, in the stem of poplar mechanically stressed by bending, the formation of reaction wood is often accompanied by a stimulation of cambial cell division, whereas the cell division at the opposite site is more or less inhibited (Hellgren et al. 2004). It is well recognized that anchorage is one of the primary functions of root systems, but although root Correspondence: Gabriella S. Scippa, Department of Scienze e Tecnologie per l’Ambiente e il Territorio, University of Molise, 86090 Pesche, Italy. E-mail: scippa@unimol.it Plant Biosystems, Vol. 142, No. 2, July 2008, pp. 401–413 ISSN 1126-3504 print/ISSN 1724-5575 online ª 2008 Societa ` Botanica Italiana DOI: 10.1080/11263500802151058