What happened to plant caspases?

Abstract

The extent of conservation in the programmed cell death pathways that are activated in species belonging to different kingdoms is not clear. Caspases are key components of animal apoptosis; caspase activities are detected in both animal and plant cells. Yet, while animals have caspase genes, plants do not have orthologous sequences in their genomes. It is 10 years since the first caspase activity was reported in plants, and there are now at least eight caspase activities that have been measured in plant extracts using caspase substrates. Various caspase inhibitors can block many forms of plant programmed cell death, suggesting that caspase-like activities are required for completion of the process. Since plant metacaspases do not have caspase activities, a major challenge is to identify the plant proteases that are responsible for the caspase-like activities and to understand how they relate, if at all, to animal caspases. The protease vacuolar processing enzyme, a legumain, is responsible for the cleavage of caspase-1 synthetic substrate in plant extracts. Saspase, a serine protease, cleaves caspase-8 and some caspase-6 synthetic substrates. Possible scenarios that could explain why plants have caspase activities without caspases are discussed.

The role of vacuole in plant cell death

“…Many studies have shown that activities similar to those of caspases are required for various types of cell death of plants. [34][35][36] As plants lack genes homologous to caspases, the question is, what proteinases are responsible for caspase-like activity?…”

Arabidopsis root K+-efflux conductance activated by hydroxyl radicals: single-channel properties, genetic basis and involvement in stress-induced cell death

“…Fluorescently labelled protease inhibitor zVAD-fmk (FITC-VAD-fmk) is a widely used tool for the observation of PCD protease activation in intact tissues, because, at micromolar concentrations, this substance does not significantly inhibit proteases, although it binds to and fluorescently labels them (Elbaz et al, 2002;Bonneau et al, 2008). Increased intracellular fluorescence indicates a higher protease activity and tests on Arabidopsis roots showed only a little background staining under control conditions (Fig.…”

The role of vacuolar processing enzymes in plant immunity

“…18 However, no caspase homolog has been found in plants. 9 Therefore, the molecular identification of protein kinases with caspase activity will help us elucidate elicitor signal transduction. As VPE was identified to have YVADase activity and to be involved in various cell death responses, 12,19 we postulated that elicitor signals are transduced via VPEs in plants.…”

“…At least eight caspase activities have now been measured in plant extracts, which were found using caspase substrates, and various caspase inhibitors can block many forms of plant programmed cell death. 9 In the past several years, vacuolar-processing enzyme (VPE) has been determined to play important roles in plant immunity responses. In this review paper, I describe the current view on the role of VPE in plant immunity, based on our own research and recent developments in this field.…”