Root cells

The hormonal regulation of root elongation in the intact plant is poorly understood. This is partly because all the major plant hormones can inhibit root cell elongation at physiological concentrations. Since roots produce or at least contain each of these inhibitors , it seems clear that root elongation must 

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The possible involvement of polygalacturonic acid-containing molecules in the defence reactions of tomato root cells against Fusarium oxysporum was suggested about 20 years ago by their accumulation at penetration sites. Since papillae are held to serve as a resistance mechanism to fungal penetration, it was assumed that the interrelation between pectin and other polymers - such as lignin - may contribute to enhancing the hardness of these newly formed structures (Benhamou et al., 1990). 

Benhamou N. Chamberland H. Pauze F.J. (1990) Implication of pectic components in cell surface interactions between tomato root cells and Fusarium oxysporum f. sp. radicis-lycopersici / / Plant Physiol. V. 92. P. 995-1003. 

N. Nishizawa and S. Mori, The particular vesicles appearing in barely root cells and its relation to mugineic acid secretion. J. Plant Nntr. 10 1013 (1987). 

R. Welch, W. Norvell, S. Schaefer, J. Shaff, and L. Kochian, Induction of iron (111) and copper (11) reduction in pea Pisiim. sativum L.) roots by Fe and Cu status. Does the root-cell plasma Fe(lll) reductase perform a general role in regulating cation uptake Planta 190 555 (1993). 

Other than a nutritional role linked to mineralization processes, humic compounds have been hypothesized to directly affect plant nutrition, since it has been suggested that roots may take up low-molecular-weight humic molecules (21). Interestingly, plants have been ob.served to express carriers for amino acids (22) and small peptides (23) at the root level. Certain components of the humic fraction have been found inside root cells and were, moreover, translocated to the shoots (24,25). Recent experiments performed on rice cells in suspension culture seem to suggest that they may use carbon skeletons from humic molecules to synthesize proteins and DNA (26). 

Further progress may derive from a more accurate definition of the chemical and physical properties of the humic substances present at the rhizosphere and how they interact with the root-cell apoplast and the plasma membrane. An interaction with the plasma membrane H -ATPase has already been observed however this master enzyme may not be the sole molecular target of humic compounds. Both lipids and proteins (e.g., carriers) could be involved in the regulation of ion uptake. It therefore seems necessary to investigate the action of humic compounds with molecular approaches in order to understand the regulatory aspects of the process and therefore estimate the importance of these molecules as modulators of the root-soil interaction. 

All soil metabolic proce.sses are driven by enzymes. The main sources of enzymes in soil are roots, animals, and microorganisms the last are considered to be the most important (49). Once enzymes are produced and excreted from microbial cells or from root cells, they face harsh conditions most may be rapidly decomposed by organisms (50), part may be adsorbed onto soil organomineral colloids and possibly protected against microbial degradation (51), and a minor portion may stand active in soil solution (52). The fraction of extracellular enzyme activity of soil, which is not denaturated and/or inactivated through interactions with soil fabric (51), is called naturally stabilized or immobilized. Moreover, it has been hypothesized that immobilized enzymes have a peculiar behavior, for they might not require cofactors for their catalysis. 

ATPase and specific modification of root cell membrane permeability directly mediated by low-molecular-weight (<5000 Da) fulvic acid-like compounds deriving from native soil organic matter (54-56) (see also Chap. 5). 

H. F. Bienfait, Regulated redox proces.ses at the plasmalemma of plant root cells and their function in iron uptake. J. Bioenerg. Biomemhr. 17 13 (1985). 

Endomycorrhizal hyphae adopt a variety of colonization patterns in their penetration of the host root cells. Glomalean fungi are highly dependent on their ho.st and cannot survive for long in its absence. Their hyphae form appressoria on the epidermal cells, penetrate the cortical tissue, and eventually form highly branched structures called arbuscules (Figs. 3-6) (10). 

Figures 7 and 8 (7) Hair root of Calhma vulgaris colonized by an ericoid mycorrhizal strain. The asconiycetous fungus is a dark sterile mycelium and produces an intercellular coil, which is surrounded by the host membrane. X 15,000. (8) Detail of an orchid root cell colonized by an orchid symbiont. The basidiomycetous fungus (F) has a thick wall and is surrounded by the host membrane (H). X21.000.

The influence of these phenolic acids on electrical potentials may reflect effects on either the diffusion potential or the electrogenic potential of plant root cells. Influence on the electrogenic component could result from inhibition of ATPases which generate the electrogenic component or from reductions in the substrate (ATP) for the ATPases. 

Phytostabilization on the root membranes. Proteins and enzymes directly associated with the root cell walls can bind and stabilize the contaminant on the exterior surfaces of the root membranes. This prevents the contaminant from entering the plant. 

Phytostabilization in the root cells. Proteins and enzymes present on the root cell walls can facilitate the transport of contaminants across the root membranes. Upon uptake, these contaminants can be sequestered into the vacuole of the root cells, preventing further translocation to the shoots. 

The process of infection of lupine nodule cells by Rhizobia was examined by the thin-section electron microscopic technique, as well as the freeze-fracture technique. Different membranes such as infection thread membranes, peribacterioid membranes, plasma membranes, membranes of cytoplasmic vesicles, and membranes of the Golgi bodies and ER were stained with uranium-lead, silver, phosphotungstic acid, and ZIO (31). ZIO stained the membranes of the proximal face of the Golgi bodies and endoplasmic reticulum. ZIO staining has given good contrast in thick sections such as a cotyledon cell, a root cell, and an aleurone layer for ER, dictyosomes cisternae, mitochondria, and nuclear envelopes (17,32-37). 

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