Programmed cell death in the root cortex of soybean root necrosis mutants

Abstract

The soybean root necrosis (rn) mutation causes a progressive browning of the root soon after germination that is associated with accumulation of phytoalexins and pathogenesis-related proteins and an increased tolerance to root-borne infection by the fungal pathogen, Phytophthora sojae. Grafting and decapitation experiments indicate that the rn phenotype is root-autonomous at the macroscopic level. However, the onset and severity of browning was modulated in intact plants by exposure to light, as was the extent of lateral root formation, suggesting that both lateral roots and the rn phenotype could be directly or indirectly controlled by similar shoot-derived factors. Browning first occurs in differentiated inner cortical cells adjacent to the stele and is preceded by a wave of autofluorescence that emanates from cortical cells opposite the xylem poles and spreads across the cortex. Before any visible changes in autofluorescence or browning, fragmented DNA was detected by TUNEL (Terminal deoxynucleotidyl transferase-mediated dUTP-digoxigenin nick end labeling) in small clusters of inner cortical cells that subsequently could be distinguished cytologically from neighboring cells throughout rn root development. Inner cortical cells overlying lateral root primordia in either Rn or rn plants also were stained by TUNEL. Features commonly observed in animal cell apoptosis were confirmed by electron microscopy but, surprisingly, cells with a necrotic morphology were detected alongside apoptotic cells in the cortex of rn roots when TUNEL-positive cells were first observed. The two morphologies may represent different stages of a common pathway for programmed cell death (pcd) in plant roots, or two separate pathways of pcd could be involved. The phenotype of rn plants suggests that the Rn gene could either negatively regulate cortical cell death or be required for cortical cell survival. The possibility of a mechanistic link between cortical cell death in rn plants and during lateral root emergence is discussed.

Lovastatin Triggers an Apoptosis-like Cell Death Process in the FungusMucor racemosus

“…These data support the hypothesis that a single ''core death pathway'' for PCD evolved in unicellular organisms and, through the process of evolution, gained complexicity (Ameisen, 1996;Cornillon et al, 1994;Kosslak et al, 1997;Markiewicz et al, 1997;Naito et al, 1995;Yarmolinsky, 1995). The existence of Bcl-2 cross-reactive and Bax cross-reactive proteins in M. racemosus provide additional evidence in support of this hypothesis.…”

Reactive oxygen species and ethylene play a positive role in lateral root base nodulation of a semiaquatic legume

“…Sections were analyzed under a stereomicroscope (Leica, Wetzlar, Germany). The differential staining was as described (30). Nonstained sections were observed with bright-field optics or epifluorescence (G-excitation, BP545 exciter filter) and an Olympus BH2 microscope (Olympus, Tokyo).…”

Soybean

“…Necrotic root genes, rn1, rn2, and rn3, were reported to be associated with the accumulation of isoflavonoid phytoalexins and group 2 peroxidases under axenic conditions (Kosslak et al 1996(Kosslak et al , 1997. The necrosis genes soon after germination also seemed to be associated with resistance to reniform nematode (Kosslak et al 1997), but those genes were not located.…”