Ordering of the cellular arrangement and xylogenesis in wounded shoots of willow

Abstract

Development of living organisms is characterized by self-organization, which results in ordered cell and tissue patterns. Xylem formation in callus tissue may serve as a model to study these phenomena. Applying auxin on the apical transverse cut surface of willow shoot segments stimulates the proliferation of callus with an unorganized cell arrangement. In some areas of callus, the cells form an ordered system and partly differentiate into tracheary elements. Below the cut surface a zone of initially unorganized parenchymatous cells is produced by the cambium. Later, some of the cells formed ordered arrangements giving rise to differentiation in xylem rays with a subsequent layer of normal wood. Digital image processing software based on a structure tensor revealed a more coherent orientation of the cellular pattern in the callus region close to the cambial zone in the cut shoot surface, compared with the areas at further distances near the outer parts of the callus ring. Differentiation of tracheary xylem elements occurs mostly in the regions where a higher degree of cellular ordering in parenchyma tissue is observed. Digital image analysis is a useful tool for the quantitative estimation of subtle changes of cellular ordering in various regions of regenerating tissue. Wider application of this tool may open new opportunities in studies of the complex mechanisms that control morphogenetic patterns in plants.