A New Branch of Endoplasmic Reticulum Stress Signaling and the Osmotic Signal Converge on Plant-specific Asparagine-rich Proteins to Promote Cell Death

Abstract

NRPs (N-rich proteins) were identified as targets of a novel adaptive pathway that integrates endoplasmic reticulum (ER)and osmotic stress signals based on coordinate regulation and synergistic up-regulation by tunicamycin and polyethylene glycol treatments. This integrated pathway diverges from the molecular chaperone-inducing branch of the unfolded protein response (UPR) in several ways. While UPR-specific targets were inversely regulated by ER and osmotic stresses, NRPs required both signals for full activation. Furthermore, BiP (binding protein) overexpression in soybean prevented activation of the UPR by ER stress inducers, but did not affect activation of NRPs. We also found that this integrated pathway transduces a PCD signal generated by ER and osmotic stresses that result in the appearance of markers associated with leaf senescence. Overexpression of NRPs in soybean protoplasts induced caspase-3-like activity and promoted extensive DNA fragmentation. Furthermore, transient expression of NRPs in planta caused leaf yellowing, chlorophyll loss, malondialdehyde production, ethylene evolution, and induction of the senescence marker gene CP1. This phenotype was alleviated by the cytokinin zeatin, a potent senescence inhibitor. Collectively, these results indicate that ER stress induces leaf senescence through activation of plant-specific NRPs via a novel branch of the ER stress response.

Endoplasmic Reticulum Protein Quality Control and Its Relationship to Environmental Stress Responses in Plants

“…Recently, N-rich proteins (NRPs) were identified as targets of a novel adaptive pathway that integrates ER and osmotic stress signals in soybean based on coordinate regulation and synergistic upregulation by tunicamycin (UPR inducer) and polyethylene glycol (osmotic stress inducer) treatments. The cell death domains containing NRPs were demonstrated to induce caspase activity and to be the critical mediators of osmotic-and ER stress-induced cell death in plants (Costa et al, 2008). Metacaspases are believed to be the functional homologs of animal caspases in plants, although metacaspases ER stress activates the expression of NRPs, which induces the caspase-3-like activity to trigger cell death.…”

Endoplasmic Reticulum Stress Response

“…These genomic analyses suggested that soybeans, like Arabidopsis and mammals, have evolved at least three different mechanisms that mediate UPR: (1) transcriptional induction of genes encoding chaperones and vesicle trafficking proteins, (2) attenuation of genes that encode secretory proteins, and (3) upregulation of the ER-associated protein degradation (ERAD) system for rapid disposal of unfolded proteins in the ER. As further evidence that the UPR operates in soybean, the promoters of soybean BiP genes contain functional ER stress cis-acting elements (ERSEs), and soybean BiP functions as a regulator of the UPR as it does in mammals and yeast (Buzeli et al, 2002;Costa et al, 2008). Nevertheless, the roles of plant BiP in stress perception and in the mechanism of signal propagation remain to be elucidated.…”

“…Nevertheless, the roles of plant BiP in stress perception and in the mechanism of signal propagation remain to be elucidated. Overexpression of soybean BiP inhibits activation of the UPR and attenuates ER stress in response to tunicamycin, a potent inducer of ER stress (Alvim et al, 2001;Costa et al, 2008). Soybean BiP has also been demonstrated to confer tolerance to drought in transgenic soybean and tobacco plants (Alvim et al, 2001;Valente et al, 2009).…”

“…The most well-characterized ERbased integrative pathway in plants is the ER stress and osmotic stress integrated response that is mediated by N-rich proteins and transduces a cell death signal (Figure 1; Costa et al, 2008). This integrative pathway was first identified by genome-wide approaches and expression profiling studies, which revealed the existence of a modest overlap of the ER stress-and osmotic stress-induced transcriptomes in soybean seedlings treated with PEG (an inducer of osmotic stress) or tunicamycin and AZC, potent inducers of ER stress (Irsigler et al, 2007).…”

EARLY RESPONSIVE to DEHYDRATION 15, a new transcription factor that integrates stress signaling pathways

“…13,14 This pathway integrates the ER-stress and osmotic-stress signals to increase the synergistic expression of N-rich proteins (NRP-A and NRP-B) and an NAC domaincontaining protein, GmNAC6, which are critical mediators of stress-induced cell death in plants. [13][14][15][16] This cell death integrated pathway has emerged as a relevant adaptive response of plant cells to multiple environmental stimuli. 16 The soybean GmERD15 homolog has been described as a new ER stress-and osmotic stress-induced transcription factor that binds to the promoter and induces the expression of the NRP-B gene.…”