Arabidopsis metacaspase 2d is a positive mediator of cell death induced during biotic and abiotic stresses

Abstract

SUMMARYCysteine proteases such as caspases play important roles in programmed cell death (PCD) of metazoans. Plant metacaspases (MCPs), a family of cysteine proteases structurally related to caspases, have been hypothesized to be ancestors of metazoan caspases, despite their different substrate specificity. Arabidopsis thaliana contains six type II MCP genes (AtMCP2a-f). Whether and how these individual members are involved in controlling PCD in plants remains largely unknown. Here we investigated the function and regulation of AtMCP2d, the predominant and constitutively expressed member of type II MCPs, in stress-inducible PCD. Two AtMCP2d mutants (mcp2d-1 and mcp2d-3) exhibited reduced sensitivity to PCD-inducing mycotoxin fumonisin B1 as well as oxidative stress inducers, whereas AtMCP2d over-expressors were more sensitive to these agents, and exhibited accelerated cell-death progression. We found that AtMCP2d exclusively localizes to the cytosol, and its accumulation and self-processing patterns were age-dependent in leaves. Importantly, active proteolytic processing of AtMCP2d proteins dependent on its catalytic activity was observed in mature leaves during mycotoxin-induced cell death. We also found that mcp2d-1 leaves exhibited reduced cell death in response to Pseudomonas syringae carrying avirulent gene avrRpt2, and that self-processing of AtMCP2d was also detected in wild-type leaves in response to this pathogen. Furthermore, increases in processed AtMCP2d proteins were found to correlate with conditional cell-death induction in two lesion-mimic mutants (cpr22 and ssi4) that exhibit spontaneous cell-death phenotypes. Taken together, our data strongly suggest that AtMCP2d plays a positive regulatory role in biotic and abiotic stress-induced PCD.

The Arabidopsis METACASPASE9 Degradome

“…Both MC9 gain-of-function or loss-of-function plants did not display any obvious aberrant phenotypes under normal vegetative or reproductive growth conditions. MC4 expression was the highest (Figure 1G), consistent with a previous report that it is the most expressed metacaspase at various growth stages (Watanabe and Lam, 2011a). In young seedlings, corresponding to a developmentally active stage with a dynamic proteome, the MC9 expression was the second highest; therefore, this stage was chosen for analysis of the MC9 degradome ( Figure 1G).…”

Metacaspases

“…Loss of the predominant type II metacaspase in Arabidopsis, AtMC4, has been found recently to compromise cell death induction by the fungal toxin fumonisin B-1 (FB1), an incompatible bacterial pathogen, and chemical inducers of oxidative stress in seedlings. 32 This work has further shown that AtMC4 processing from its zymogen is accelerated during activation of cell death by these agents, consistent with its deduced role in orchestrating PCD under these conditions. Lastly, a type I metacaspase AtMC1 was shown to be a positive regulator of the HR cell death in Arabidopsis, whereas another type I metacaspase, AtMC2, was found to counteract this pro-cell death effect of AtMC1 (see also review by Coll et al, in this issue).…”

Sheathing the swords of death: Post-translational modulation of plant metacaspases

“…2 During the past decade, experimental evidence in multiple species using reverse-genetic methods have supported type-II MCPs as positive mediators of PCD. [3][4][5] Among the 6 type-II MCPs in Arabidopsis, the nomenclature of which has recently been unified, 2 AtMC8 was found to be involved in UV-C and oxidative stress-induced PCD while the catalytic activity of AtMC4 is demonstrated to be essential for its in planta function during oxidative stresses as well as toxininduced PCD. 4,5 Among the three type-I MCPs in Arabidopsis, two of them were recently shown to antagonistically modulate PCD with the anti-death function of AtMC2 being independent of its catalytic activity.…”

“…[3][4][5] Among the 6 type-II MCPs in Arabidopsis, the nomenclature of which has recently been unified, 2 AtMC8 was found to be involved in UV-C and oxidative stress-induced PCD while the catalytic activity of AtMC4 is demonstrated to be essential for its in planta function during oxidative stresses as well as toxininduced PCD. 4,5 Among the three type-I MCPs in Arabidopsis, two of them were recently shown to antagonistically modulate PCD with the anti-death function of AtMC2 being independent of its catalytic activity. 6 Based on these new data, it is appropriate to state that MCPs are important cell death regulators in plants, especially under induced-stress scenarios.…”