Publisher Summary Vesicular-arbuscular mycorrhizal fungi are obligate biotrophs that have so far resisted all attempts to be cultivated axenically (in pure culture). This lack of independent growth has not prevented vesicular-arbuscular mycorrhizal fungi from becoming distributed world-wide as a symbiotic partner of most vascular plants, under a wide variety of pedologic and climatic conditions. Cultivation of vesicular-arbuscular mycorrhizal fungi under axenic conditions continues to be a preoccupation and represents one of the most challenging goals of modern plant biology. The purpose of this chapter is to provide a detailed description of the procedures of the method and to suggest new avenues, to stimulate research on the biology of these symbiotic fungi. In this chapter, the use of transformed carrot roots inoculated with a single spore of G. margarita is proposed as a simple experimental system which allows reproducible observations of all stages of vesicular-arbuscular mycorrhizal development, including the extraradical phases. The control of colonization of selected root parts by G. margarita germ tubes is the key to making quantitative and consistent measurements of the initiation and development of a vesicular-arbuscular mycorrhizal symbiosis. The system has been useful for determining symbiotic factors provided by the root which govern fungal growth.

Establishment of vesicular-arbuscular mycorrhiza in root organ culture : review and proposed methodology

The effects of temperature on primary root growth dynamics and lateral root distribution in garden pea (Pisum sativum L., cv. Alaska)

Lactuca sativa cv. Baijianye seedlings do not normally produce lateral roots, but removal of the root tip or application of auxin, especially indole-butyric acid, triggered the formation of lateral roots. Primordia initiated within 9 h and were fully developed after 24 h by activating the pericycle cells opposite the xylem pole. The pericycle cells divided asymmetrically into short and long cells. The short cells divided further to form primordia. The effect of root tip removal and auxin application was reversed by 6-benzylaminopurine at concentrations >10(-8) M. The cytokinin oxidase inhibitor N1-(2chloro4pyridyl)-N2-phenylurea also suppressed auxin-induced lateral rooting. The elongation of primary roots was promoted by L-alpha-(2-aminoethoxyvinyl) glycine and silver ions, but only the latter enhanced elongation of lateral roots. The data indicate that the induction of lateral roots is controlled by basipetally moving cytokinin and acropetally moving auxin. Lateral roots appear to not produce ethylene.

https://www.jstor.org/stable/10.1086/314147

Initiation and elongation of lateral roots in Lactuca sativa

Auxin is essential for the regulation of root system architecture by controlling primary root elongation and lateral root (LR) formation. Exogenous auxin has been reported to inhibit primary root elongation and promote the formation of LRs. In this study, LR formation in the Zea mays primary root was quantitatively evaluated after exogenous auxin treatment by comparing the effects of auxin on two selected zones elongated either before or after auxin application. We determined two main variables in both zones: the LR density per unit of root length (LRD), and the mean phloem pericycle cell length. The total number of phloem pericycle cells (PPCs) per unit of root length was then calculated. Considering that each LR primordium is initiated from four founder cells (FCs), the percentage of PPCs (%PPC) that behave as FCs in a specific root zone was estimated by dividing the number of pericycle cells by four times the LRD. This index was utilized to describe LR initiation. Root zones elongated in the presence of a synthetic auxin (1-naphthalene acetic acid, NAA) at low concentrations (0.01 μM) showed reduced cell length and increased LRD. However, a high concentration of NAA (0.1 μM) strongly reduced both cell length and LRD. In contrast, both low and high levels of NAA stimulated LRD in zones elongated before auxin application. Analysis of the percentage of FCs in the phloem pericycle in zones elongated in the presence or absence of NAA showed that low concentrations of NAA increased the %PFC, indicating that LR initiation is promoted at new sites; however, high concentrations of NAA elicited a considerable reduction in this variable in zones developed in the presence of auxin. As these zones are composed of short pericycle cells, we propose that short pericycle cells are incapable to participate in LR primordium initiation and that auxin modulated initiation of LRs is linked to pericycle cell length.

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Auxin Modulated Initiation of Lateral Roots Is Linked to Pericycle Cell Length in Maize.

Pueraria lobata hairy roots have faster elongationand more branches than normal roots. The responses of hairy roots and normalroots to treatment with three auxins, indole-3-acetic acid (IAA),indole-3-butyric acid (IBA), and naphthalene acetic acid (NAA) were different.In normal roots, all three auxins strongly stimulated lateral root formation atall tested concentrations. Responses to IAA and IBA in primary root growth andlateral root elongation were similar and depended on concentration; promotionat0.1 μM, no effect at 1.0 μM, and inhibition at2.5 μM. In hairy roots, lateral root formation varied inresponseto the different auxins, i.e. depressed by NAA, unaffected by IAA, and promotedby IBA. Primary root growth was slightly inhibited by IBA and was unaffected byIAA. However, mean lateral root length was reduced in response to IAA and IBA.Only NAA exerted strong inhibition on primary and lateral root elongation inboth root types. The similar free IAA and conjugated IAA content but quitedifferent basal ethylene production and biosynthesis in hairy and normal rootssuggested different mechanisms of response to exogenous auxins in the two roottypes.

S. M. Zeadan

Papers

Apical Meristem Activity and Lateral Root Anlage Development in Excised Roots of Pisum sativum L.

Some Effects of Indol-3-yl Acetic Acid on Lateral Root Development in Attached and Excised Roots of Pisum sativum L.

Lateral Root Development in Attached and Excised Roots of Zea mays L. Cultivated in the Presence or Absence of Indol-3-yl Acetic Acid