Understanding the role of starch sheath layer in graviception of Alternanthera philoxeroides: a biophysical and microscopical study.

Plants' ability to sense and respond to gravity is a unique and fundamental process. When a plant organ is tilted, it adjusts its growth orientation relative to gravity direction, which is achieved by a curvature of the organ. In higher, multicellular plants, it is thought that the relative directional change of gravity is detected by starch-filled organelles that occur inside specialized cells called statocytes, and this is followed by signal conversion from physical information to physiological information within the statocytes. The classic starch statolith hypothesis, i.e., the starch accumulating amyloplasts movement along the gravity vector within gravity-sensing cells (statocytes) is the probable trigger of subsequent intracellular signaling, is widely accepted. Acharya Jagadish Chandra Bose through his pioneering research had investigated whether the fundamental reaction of geocurvature is contractile or expansive and whether the geo-sensing cells are diffusedly distributed in the organ or are present in the form of a definite layer. In this backdrop, a microscopy based experimental study was undertaken to understand the distribution pattern of the gravisensing layer, along the length (node-node) of the model plant Alternanthera philoxeroides and to study the microrheological property of the mobile starch-filled statocytes following inclination-induced graviception in the stem of the model plant. The study indicated a prominent difference in the pattern of distribution of the gravisensing layer along the length of the model plant. The study also indicated that upon changing the orientation of the plant from vertical position to horizontal position there was a characteristic change in orientation of the mobile starch granules within the statocytes. In the present study for the analysis of the microscopic images of the stem tissue cross sections, a specialized and modified microscopic illumination setup was developed in the laboratory in order to enhance the resolution and contrast of the starch granules.

A microscopic study on scattering in tissue section of Alternanthera philoxeroides under polarized light.

Like any other biological tissue, plant tissue also exhibits optical properties like refraction, transmission, absorption, coloration, scattering and so on. Several studies have been conducted using different parts of plants such as leaves, seedlings, roots, stems and so on, and their optical properties have been analyzed to study plant physiology, influence of environmental cues on plant metabolism, light propagation through plant parts and the like. Thus, it is essential to study in detail the optical properties of several plant parts to determine their structural relationship. In this backdrop, an experimental study was conducted to observe and analyze the optical properties of node and inter-nodal tissue cross-sections of the plant Alternanthera philoxeroides under a polarizing microscope constructed and standardized in the laboratory. The observed optical properties of the microscopic tissue sections have been then studied to determine a significant structural relationship between nodal and inter-nodal tissue arrangement patterns as a whole. Tissue sections that have undergone a sort of biological perturbation like loss of water (dried in air for 15 min) have also been studied to study the change in the pattern of tissue optical property when compared with that of normal plant-tissue cross-sections under a polarizing microscope. This type of biological perturbation was chosen for the study because water plays an important role in maintenance of the normal physiological processes in plants and most other forms of life.