The role of hormones and gradients in the initiation of cortex proliferation and nodule formation in Pisum sativum L.

Abstract

The effect of exogenous phytohormones on proliferation of the root cortex, and their relation to the division factors from Rhizobium which participate in the initiation of root nodules, were studied using explants of root-cortex tissue from 7-day-old, sterile pea plants. The explants were cultured for 7 days on a synthetic nutrient medium supplemented with auxin, or auxin and cytokinin. With only auxin present in the medium, ca. 10% of the explants showed cell proliferation. With both auxin and cytokinin this percentage was much higher (ca. 80%). The active explants showed proliferation patterns which were similar to or could be derived from a pattern with three predominant meristematic areas in the inner cortex opposite the three xylem radii of the excised central cylinder. These proliferation patterns were similar to the initial proliferative stages in root-nodule formation in seedling intact roots. From this restricted division response of the explants to the hormones, a hypothesis of endogenous division factors is proposed. To test this hypothesis, extractions of root tissue were performed. The addition of a crude alcoholic extract from the central cylinder or the cortex to the medium resulted in cell divisions throughout the cortex. The results are interpreted as evidence for the presence of a transverse gradient system of (an) unknown cell-division factor(s) in the root cortex which may control the induction of cell divisions in nodule initiation brought about by the release of auxin and cytokinin from Rhizobium.

Nodulation Phenotypes of Gibberellin and Brassinosteroid Mutants of Pea

“…This might suggest the existence of a mechanism that compensates for changes in nodule numbers by regulating the size of individual nodules. Taken together, our findings support the theory proposed by Libbenga et al (1973) that a delicate balance in hormone levels is required to achieve optimum nodule development. This theory is further supported by our finding that GAs, in addition to cytokinins (Lorteau et al, 2001), are stimulatory to pea nodule formation at low concentrations but inhibitory when increased beyond a threshold level.…”

Infection and Invasion of Roots by Symbiotic, Nitrogen-Fixing Rhizobia during Nodulation of Temperate Legumes

“…These divide, and some of them (those that are not infected) will form the nodule meristem. The spatial localization of pericycle activation next to protoxylem poles may require an inducing factor, termed stele factor and thought to perhaps be uridine, coming from the protoxylem, or it may be caused by inhibition of the response of cells near the protophloem poles (100,142,151). mRNA encoding 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase, which catalyzes the final step of ethylene synthesis, has been shown by in situ hybridization to be enriched in cells adjacent to the protophloem poles (74).…”

Nod factors and cytokinins induce similar cortical cell division, amyloplast deposition and MsEnod12A expression patterns in alfalfa roots

“…The extent of nodulation is controlled at the level of cortical cell division sites by factors from the upper parts of the plant ('shoot factor') (for review see Caetano-Anoll~s and Gresshoff, 1991). The position of cortical cell division sites was proposed to be determined by gradients of further factors along the root, like the 'stele factor' (Libbenga et al, 1973;Smit et al, 1995). In addition, plant hormones appear to play a role in Nod factor action since treatment with auxin transport inhibitors and secretion of trans-zeatin by bacteria can result in pseudo-nodule formation (Cooper and Long, 1994;Hirsch et al, 1989; for review see .…”