Are root exudates more important than other sources of rhizodeposits in structuring rhizosphere bacterial communities

Microorganisms in soils and waters convert many synthetic organic chemicals to inorganic products. Other compounds are transformed only by cometabolism. These microbial processes may lead to environmental detoxication, the formation of new toxicants, or the biosynthesis of persistent products. Type reactions are proposed for major categories of enzymatic transformation of synthetic chemicals in soils, natural waters, and sewage. Some organic molecules are resistant to microbial attack, and explanations for the persistence of such compounds are suggested.

https://science.sciencemag.org/content/sci/211/4478/132.full.pdf

Biodegradation of Chemicals of Environmental Concern

The primary symptom of aluminium (Al) toxicity in higher plants is inhibition of root growth. In this study, we investigated the spatial sensitivity of maize (Zea mays L.) roots to Al. A divided-chamber technique indicated that only exposure of the terminal 10 to 15 mm of the root to Al resulted in inhibition of growth. Application of Al to all but this apical region of the root had little or no effect on growth for 24 h and caused minimal damage to the root tissue. Small agar blocks infused with Al were then applied to discrete areas of the apex of maize roots to determine which section (root cap, meristem or elongation zone) was more important to Al-induced inhibition of growth

Aluminium Toxicity in Roots: An Investigation of Spatial Sensitivity and the Role of the Root Cap

This review of plant gravity sensing examines sensing in organ gravitropism, sensing in single-cell gravitropism, and nongravitropic sensing. Topics related to sensing in organ gravitropism are (1) identification of the gravitropic susceptors, including intracellular asymmetry in equilibrium position and after reorientation, susceptor signal-to-noise ratio, signal integration over threshold stimulation periods, intracellular asymmetry and gravitropic competence, and starch deficiency and gravitropic competence; (2) possible root statocytes and receptors, including identification of presumptive statocytes, cytology, and possible receptors and models of sensing; and (3) negatively gravitropic organs, including identification and distribution of presumptive statocytes and cytology and possible receptors. Topics related to nongravitropic sensing include gravitaxis, reaction wood, gravimorphogenesis, other gravity-influenced organ movements, and cytoplasmic streaming.

Plant Gravity Sensing

The cap is widely accepted to be the site of gravity sensing in roots because removal of the cap abolishes root curvature. Circumstantial evidence favors the columella cells as the gravisensory cells because amyloplasts (and often other cellular components) are polarized with respect to the gravity vector. However, there has been no functional confirmation of their role. To address this problem, we used laser ablation to remove defined cells in the cap of Arabidopsis primary roots and quantified the response of the roots to gravity using three parameters: time course of curvature, presentation time, and deviation from vertical growth. Ablation of the peripheral cap cells and tip cells did not alter root curvature. Ablation of the innermost columella cells caused the strongest inhibitory effect on root curvature without affecting growth rates. Many of these roots deviated significantly from vertical growth and had a presentation time 6-fold longer than the controls. Among the two inner columella stories, the central cells of story 2 contributed the most to root gravitropism. These cells also exhibited the largest amyloplast sedimentation velocities. Therefore, these results are consistent with the starch-statolith sedimentation hypothesis for gravity sensing.

http://www.plantphysiol.org/content/plantphysiol/116/1/213.full.pdf

Mapping the Functional Roles of Cap Cells in the Response of Arabidopsis Primary Roots to Gravity

DURING investigations into the fine structure of apical meristems under the electron microscope, one of us (S. G.) discovered that it was possible, in maize and barley, to detach the intact root cap cleanly from the rest of the root tip. This is shown in Fig. 1. The roots of grasses are characterized by having a discrete cap meristem and, as can be seen in Fig. 2, a very well marked boundary between the cap and the root proper. The fracture takes place through the unstrengthened walls of the meristematic region of the cap as indicated in Fig. 2. We have not so far succeeded in removing the cap intact from other species such as broad bean or pea.

Root Cap and the Perception of Gravity

IN a recent paper1, evidence was presented to support the view that the detoxication of the selective herbicide, 2,4-dichlorophenoxyacetic acid in soil was due almost entirely to the action of microorganisms. The evidence from this and other similar confirmatory work is entirely indirect, and attempts were therefore made to isolate the organism or organisms concerned.

/pdf/biological-detoxication-of-2-4-dichlorophenoxyacetic-acid-in-43bcge2uh9.pdf

Biological detoxication of 2,4-dichlorophenoxyacetic acid in soils. Isolation of an effective organism.

Auxin Transport in Roots: Vicia faba

Limits to growth

SODIUM 2,4-dichlorophenoxy-ethyl-sulphate, a chemical of low intrinsic phytotoxicity, can be sprayed in relatively concentrated aqueous solutions on to plants normally sensitive to the hormone herbicides, without serious damage to them; but when it comes into contact with the soil, a high phytotoxicity is generated, sufficient to give satisfactory control of weed-seed germination in the surface layers of the soil1. The toxic compound arising from it has been assumed to be 2,4-dichlorophenoxyacetic acid (2,4-D).

/pdf/fate-of-sodium-2-4-dichloro-phenoxy-ethyl-sulphate-in-the-4xmfdqc9ss.pdf

L. J. Audus

Papers

Root Cap and the Perception of Gravity

Biological detoxication of 2,4-dichlorophenoxyacetic acid in soils. Isolation of an effective organism.

Auxin Transport in Roots: Vicia faba

Limits to growth

Fate of Sodium 2,4-Dichloro-phenoxy-ethyl-sulphate in the Soil