Section 4 - Root physiology and plant structure (original) (raw)

Journal of Experimental Botany Ultrastructural and physiological changes in root cells of

1996

The xerophytic, but salt-sensitive Sorghum cultivar 'Sweet Sioux' is known as an ion excluder with a high K/Na selectivity at the plasmalemma and tonoplast of epidermal root cells. The aim of this study is the correlation of salt-effected changes in physiological parameters with structural and ultrastructural changes in root cells. The investigation was carried out with root cells because these cells are most directly exposed to the growth medium. Sorghum bicolor xS. sudanensis cv. Sweet Sioux plants were grown under steady-state conditions on nutrient solutions with or without 40 mol m" 3 NaCI. Sorghum sustained this treatment but showed several salt-induced structural and physiological changes which were studied in various cell types of the root tip. (1) NaCI salinity led to a shorter growth region and to salt-induced alterations in the chemical and physical properties of the cell walls in the root tips. (2) Salt treatment also increased the membrane surface in root cells: root cells showed an increase in the quantity of vesicles in the epidermis and in the middle cortex cells. Additionally, some of the epidermis cells of salt-treated plants revealed a characteristic build-up of transfer cells, suggesting an increase in membrane surfaces to increase the uptake and storage of substances. (3) The number of mitochondria increased in the epidermal and in the cortex cells after salt stress thus indicating an additional supply of energy for osmotic adaptation and for selective uptake and transport processes. (4) In the epidermal cytoplasm NaCI stress led to a significant decrease of the P, K, Ca, and S concentrations accompanied by an increase of Na concentration. Electron micrographs show an increase in electron optical contrast within the cytosol and in the matrix of the mitochondria. These results are discussed with regard to the possibility of influence on the part of metabolic functions. (5) The NaCI concentrations were seen to increase and the K concentrations to decrease during salt stress in the vacuoles of the epidermis and cortex cells. The salt-induced increase in vacuolar NaCI concentrations of epidermis and cortex cells are in the region 2 cm behind the root tip, which is sufficient for an osmotic balance towards the growth medium. Additional solutes are necessary 0.5 mm behind the root tip to facilitate osmotic adaptation. The results show ultrastructural changes caused by an Na-avoiding mechanism characterized by a high level of energy consumption. The exclusion of Na from the symplast seems to lead additionally to a decrease in cytoplasmic concentrations of such essential elements as Mg, P, S, and Ca and is thus responsible directly (via energy supply in mitochondria, homeostasis, selectivity of K over Na) or indirectly (via enzyme conformation, cytoplasmic hydration) for the ultrastructural degradation indicated. The salinity-induced multiplicity of structural and functional changes within cell compartments constitutes a group of indicators for the limited NaCI tolerance of Sorghum.

PAPER II-PLANT PHYSIOLOGY AND BIOCHEMISTRY

Osmosis, plasmolysis, deplasmolysis Adsorption. Absorption of water. Ascent of sap. Concept of water potential. Transpiration (mechanism of opening &closing of stomata) factors affecting transpiration and its importance Role of macro and micro elements. Unit-II Photosynthesis: Photosynthetic pigments (Chlorophylls, caratenoids and phycobilins)-structure and function. Light reactions mechanism of carbon fixation in C3 and C4 plants. Brief description of C.A.M. plants. , compensation point. Factors affecting phosynthesis.

Review on the Root, Stem and Leaf Initiations in Plants

Asian Plant Research Journal

The root and shoot apical meristem serve as sources of pluripotent cells and provide new cells for repetitive organ initiation, they are the major meristematic regions on which plant development take place. New meristems are incessantly formed as plants produce new branches or lateral roots thus making the understanding of meristem function central to how plants can establish different growth types, ranging from tiny herbs to huge trees. The sizes and numbers of meristems that are initiated during advanced development control the size and number of fruits and the generation of seeds. The development of a lateral root from a limited number of cells requires compactly coordinated asymmetric cell divisions to generate cell diversity and tissue patterns which characteristically involves the specification of founder cells, followed by a number of cellular changes until the cells divide and give rise to unequally sized daughter cells. Leaf development exemplifies the dynamic nature and fl...

The organization of roots of dicotyledonous plants and the positions of control points

Annals of botany, 2011

The structure of roots has been studied for many years, but despite their importance to the growth and well-being of plants, most researchers tend to ignore them. This is unfortunate, because their simple body plan makes it possible to study complex developmental pathways without the complications sometimes found in the shoot. In this illustrated essay, my objective is to describe the body plan of the root and the root apical meristem (RAM) and point out the control points where differentiation and cell cycle decisions are made. Hopefully this outline will assist plant biologists in identifying the structural context for their observations. This short paper outlines the types of RAM, i.e. basic-open, intermediate-open and closed, shows how they are similar and different, and makes the point that the structure and shape of the RAM are not static, but changes in shape, size and organization occur depending on root growth rate and development stage. RAMs with a closed organization lose...

INVITED REVIEW Form matters: morphological aspects of lateral root development

2016

† Background The crucial role of roots in plant nutrition, and consequently in plant productivity, is a strong motivation to study the growth and functioning of various aspects of the root system. Numerous studies on lateral roots, as a major determinant of the root system architecture, mostly focus on the physiological and molecular bases of developmental processes. Unfortunately, little attention is paid either to the morphological changes accompanying the formation of a lateral root or to morphological defects occurring in lateral root primordia. The latter are observed in some mutants and occasionally in wild-type plants, but may also result from application of external factors. † Scope and Conclusions In this review various morphological aspects of lateral branching in roots are analysed. Morphological events occurring during the formation of a typical lateral root are described. This process involves dramatic changes in the geometry of the developing organ that at early stages are associated with oblique cell divisions, leading to breaking of the symmetry of the cell pattern. Several types of defects in the morphology of primordia are indicated and described. Computer simulations show that some of these defects may result from an unstable field of growth rates. Significant changes in both primary and lateral root morphology may also be a consequence of various mutations, some of which are auxin-related. Examples reported in the literature are considered. Finally, lateral root formation is discussed in terms of mechanics. In this approach the primordium is considered as a physical object undergoing deformation and is characterized by specific mechanical properties.

Plant Physiol.-2014-Kumpf-632-43

2014

The stem cell niche of the Arabidopsis (Arabidopsis thaliana) primary root apical meristem is composed of the quiescent (or organizing) center surrounded by stem (initial) cells for the different tissues. Initial cells generate a population of transit-amplifying cells that undergo a limited number of cell divisions before elongating and differentiating. It is unclear whether these divisions occur stochastically or in an orderly manner. Using the thymidine analog 5-ethynyl-29-deoxyuridine to monitor DNA replication of cells of Arabidopsis root meristems, we identified a pattern of two, four, and eight neighboring cells with synchronized replication along the cortical, epidermal, and endodermal cell files, suggested to be daughters, granddaughters, and great-granddaughters of the direct progeny of each stem cell. Markers of mitosis and cytokinesis were not present in the region closest to the transition zone where the cells start to elongate, suggesting that great-granddaughter cells switch synchronously from the mitotic cell cycle to endoreduplication. Mutations in the stem cell niche-expressed ASH1-RELATED3 (ASHR3) gene, encoding a SET-domain protein conferring histone H3 lysine-36 methylation, disrupted this pattern of coordinated DNA replication and cell division and increased the cell division rate in the quiescent center. E2Fa/E2Fb transcription factors controlling the G1-to-S-phase transition regulate ASHR3 expression and bind to the ASHR3 promoter, substantiating a role for ASHR3 in cell division control. The reduced length of the root apical meristem and primary root of the mutant ashr3-1 indicate that synchronization of replication and cell divisions is required for normal root growth and development.