We present evidence that heterotrimeric G protein signaling is involved in cell death associated with the unfolded protein response (UPR) in Arabidopsis. Seedlings of homozygous agb1-2 (G␤-null mutation) mutant plants are markedly more resistant to growth inhibition by the protein glycosylation inhibitor tunicamycin (Tm) than either wild-type plants or gpa1-4 (G␣-null mutation) mutants. Leaves of older G␤ mutant plants show much less cell death when infiltrated with Tm than leaves of wild-type plants. The transcriptional response of G␤ mutant plants to Tm is less pronounced than that of wild-type plants, as is the accumulation of BiP chaperone proteins. A majority of the Arabidopsis G␤ protein is associated with the endoplasmic reticulum (ER) and cofractionates with membrane-associated ER luminal BiP. Consistent with its ER localization, G␤ protein is degraded during the UPR, whereas G␣ protein is not. Taken together, these observations imply that the G␤ protein, which forms a stable heterodimer with the G␥ subunit, is involved in the signaling events that trigger UPR-associated cell death. The different Tm sensitivities of G␣ and G␤ mutants, the ER localization of G␤, and the differential stabilities of G␣ and G␤ proteins during the UPR suggest that the G␤␥ complex serves a signaling function in the ER independent of its function in the G␣␤␥ heterotrimer.endoplasmic reticulum ͉ tunicamycin

Ascorbate peroxidase (APX) exists as several isoforms that are found in various compartments in plant cells. The cytosolic and chloroplast APXs appear to play important roles in antioxidation metabolism in plant cells, yet the function of peroxisomal APX is not well studied. In this study, the localization of a putative peroxisomal membrane-bound ascorbate peroxidase, APX3 from Arabidopsis, was confirmed by studying the green fluorescent protein (GFP)-APX3 fusion protein in transgenic plants. GFP-APX3 was found to co-localize with a reporter protein that was targeted to peroxisomes by the peroxisomal targeting signal 1. The function of APX3 in Arabidopsis was investigated by analysing an APX3 knockout mutant under normal and several stress conditions. It was found that loss of function in APX3 does not affect Arabidopsis growth and development, suggesting that APX3 may not be an important antioxidant enzyme in Arabidopsis, at least under the conditions that were tested, or the function of APX3 could be compensated by other antioxidant enzymes in plant cells.