Cytological Studies on Rhizobial Nodule Initiation inPisum

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Abstract
A cytological analysis has been made of the first cell divisions in the cortical cells of the seedling root of the garden pea, Pisum sativum L., variety Little Marvel, resulting from the penetration of the infection thread of the bacterial symbiont Rhizobium leguminosarum during rhizobial root nodule formation. In the study of a carefully timed sequence during nodule initiation it has been found that first mitoses occur six days after inoculation of the seedling root with the bacterium, that each such mitosis is typically polyploid, usually tetraploid but sometimes octaploid or higher and that in this first mitotic event typically one can observe diplochromosomes, indicating that the cortical cell responsive to the bacterial thread penetration had undergone endo-mitotic reduplication prior to the infection. Using Feulgen-stained squash preparations together with phase contrast microscopy it has been possible to establish clearly the association between these first polyploid mitoses and the presence in the cell of the terminal portion of the bacterial thread. In this system, morphologically visible nodule primordia are distinguishable by about the eighth day after inoculation; these primordia are comprised of dividing diploid and polyploid cell populations. Bacteroid-containing polyploid cells are evident by the tenth day. It is suggested that the response of polyploid cells to stimulation by the rhizobial infection thread is analogous to the stimulation of cortical cells of mature pea root segments by auxin-cytokinin treatment cultured in vitro. The possible relationship between the initiation of lateral root primordia and the stimulation of nodule initiation by the rhizobial infection thread is discussed.