TRANSCRIPT PROFILING OF NODULE SENESCENCE IN MEDICAGO TRUNCATULA DELINEATES DISTINCT PHASES AND MECHANISMS

摘要

Leguminous plants can grow under nitrogen-lmiiting conditions because of their ability to establish endosymbiosis with rhizobia. To fix atmospheric nitrogen, the bacteria internalize into cells of the central tissue, get surrounded by a plant-derivedmembrane, and differentiate into bacteroids. As such, they exist as intracellular organelles, called symbio-somes, and exchange fixed nitrogen for carbon sources. The symbiotic relationship is lost after some tune and nodule senescence is visible as a color change in the N2-fixing zone from pink (associated with functional leghemoglobin) to green (associated with heme degradation; Roponen, 1970). Changes indicating degradation of bacterial and plant cells have also been observed. A typical hallmark forsenescence is the triggering of a wide range of proteolytic activities that cause large-scale protein degradation (Pladys and Vance, 1993) and finally, death of bacteroids and nodule cells. The signal-transduction cascades and regulatory functions that control nodule senescence are unknown. To unravel the molecular processes that govern nodule senescence, developmental nodule senescence in the model legume Medicago truncatula has been described (Van De Velde et al., 2006). In-depth light and electron microscopy analyses show two stages; an early stage in which symbiosomes degrade but the plant cells stay rigid, followed by a later stage wherein the plant cells disintegrate. As described for leaf senescence, nodule senescence is slow and consists of different developmental stages. In addition to this slow disintegration, some cells also collapse directly and die, and their number increases as the senescence process proceeds.