**1. Introduction**

Every proteinaceous enzyme that performs hydrolysis of peptide bonds is correctly designed as protease (peptidase) [1, 2]. This term was first used by Vines [3] based on direct and indirect evidences from studies with algae, some fungi and Phanerogams. Long before this work, the word "proteolytic" was applied by Roberts [4] to describe the digestive process on human stomach, and in the first years of the twentieth century, the digestive proteins trypsin and pepsin as well as other autocatalytic enzymes were known as proteases, being pepsin credited to do "proteolysis" since 1877 [5]. In 1928, Grassmann and Dyckerhoff [6] established important definitions concerning the nature of the catalytic activities of proteases. In resume, the peptide cleavage pattern by proteases can be internal, for endopeptidases; on the N‐terminal portion, for aminopeptidases; or C‐terminal, for carboxypeptidases [2].

Despite this difference, all proteases share the same catalytic strategy, as they polarize the carbonyl group on the peptide bond of the substrate by the stabilization of the oxygen atom on an oxyanionic hole, what makes the carbon atom more vulnerable to the attack of an activated nucleophile. The nature of the nucleophile presents some variation and is determinant to the employed mechanism for enzymatic catalysis. As a result of these variations, the proteases are grouped into six different catalytic families: serine, threonine, cysteine, aspartic, glutamic and metallopeptidases (http://merops.sanger.ac.uk/). The larger families (cysteine, serine, aspartic and metallopeptidases) have a wide range of distribution on living organisms, and are also present in the "controversial" group of viruses [7].

About their function, these enzymes are well known for the promotion of protein degradation on amino acid unities. Besides this function, they regulate the destination of other proteins based on their cleavage specificity and participate on important cellular pathways, being key regulators in different response processes to environmental factors and developmental signals [2].

One of these cellular pathways is the event of programmed cell death (PCD) that, since the 2000s, is being studied with increased interest on non‐metazoan organisms, in particular on plant models. On metazoan, one of the main constituent of the pathway are cysteine proteases known as caspases. They are not found in plants, but an increasing number of reports shows strong evidences that caspase‐related proteases that belong to the metacaspase group, as well as proteases with caspase‐like activities are eminent on plant PCD [8].
