The plant cell wall (CW) is a complex extracellular matrix mostly composed of polysaccharides, phenolics and glycoproteins. The CW surrounds every plant cell and provides mechanical support, countering turgor pressure and determining cell extensibility and thus controlling cell shape and size. Once germinated, seedlings form a protective structure, the apical hook, to prevent damage to the apical meristem. The formation of a proper hook requires hypocotyl bending via differential epidermal cell elongation among the inner and external sides of the hook. The phytohormone auxin orchestrates differential growth through its asymmetric accumulation at the concave side, with auxin maxima correlating with the inhibition of cell elongation. A tight interaction between CW and hormonal pathways mediates the differential cell elongation in hook formation. Xyloglucan deficiency in the CW compromises auxin distribution, impairing auxin maxima formation and disrupting hook development. Mutations or chemical treatments altering CW composition determine the activation of compensatory mechanisms aimed to reestablishing the correct CW functionality, such as CW stiffening and accumulation of jasmonic acid (JA) and salicylic acid (SA), which are known to antagonize apical hook formation. In this project I will investigate the actors downstream of CW alterations which impair proper apical hook development. In particular, throughout phenotypic analysis of CW mutants, quantification of the physiological response, localization and quantification of ROS levels I expect to gain new insight over the role of different CW components, of CW integrity maintenance responses and of ROS homeostasis in hook formation and, more generally, in differential cell expansion.
In dicotyledonous plants, the hook formation is a crucial step to begin photomorphogenic development and start the juvenile stage of life cycle. Previous studies focused their attention on the role of phytohormones in the control of this developmental process. Understanding how cell wall composition interacts with the molecular mechanisms underlying this process would provide novel insights in the regulation of plant growth and development. In particular, crosstalk between alteration of cell wall structure and phytohormonal regulation have been suggested by some recent studies; nevertheless the molecular mechanisms of this interactions still remain elusive. This project aims at the identification and characterization of these mechanisms, and in particular at the elucidation of the role of the role of apoplastic reactive oxygen species (ROS) in controlling hook formation and/or opening. An open question that this projects will address is whether cell wall ROS production controls hook development by altering cell wall structure or affecting phytohormone homeostasis. Throughout real time phenotypical analysis coupled to pharmacological and genetic approaches, I will gain new insights on the role of ROS during developmental programming. High resolution histochemical analyses using specific fluorescent dyes will also help gain information on ROS localization during apical hook development at a single cell resolution. This kind of information will be helpful to understand how cell wall changes influence ROS levels in specific cell types and how these in turn affect hook development. This knowledge might be also relevant to understand other plant developmental processes that involve asymmetric growth, like tropisms.