Nodule morphogenesis: the early infection of Alfalfa (Medicago sativa) root hairs by Rhizobium meliloti

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Abstract
The growth and development of alfalfa (Medicago sativa) cv. Saranac root hairs and their infection by Rhizobium meliloti strain 102F51 was studied with Smith's interference contrast optics. Uninoculated root hairs grew and matured over a 10-h growth period. The nucleus migrated from a position opposite that of root-hair protrusion at initiation to the base of the root-hair protrusion, then into the growing root hair during the most active phase. When growth was nearly complete, the nucleus assumed a position near the base of the vacuolate root hair. If root hairs were inoculated during the first 2 h of growth after initiation, either "Shepherd's crooks" or root hairs deformed into a tight curl as the tip developed. Some of these Shepherd's rooks later demonstrated typical infection-thread formation. Root hairs that were inoculated between 4 and 6 h after root-hair initiation demonstrated branched growth, with the branch forming opposite the position of the nucleus at the time of infection. Infection threads occasionally formed in either the side branches or tip branches. Root hairs that were older than 6 h at the time of inoculation formed a variety of growth deformations, including ballooning, and elongate, spatulate, spiralling, or intertwined growth. Infections in this population of root hairs were rare.
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