Plant cystatins are naturally occurring protease inhibitors that prevent proteolysis by papain-like cysteine
proteases. Their protective action against environmental stresses has been relatively well characterised.
Still, there is a need to greatly improve both potency and specificity based on the current rather poor
performance of cystatins in biotechnological applications. Research in creating more potent and specific
cystatins, including amino acid substitutions in either conserved cystatin motifs and/or at variable amino
acid sites, is reviewed. Existing gaps for better understanding of cystatin-protease interactions are further
explored. Current knowledge on multi-cystatins or hybrid protease inhibitors involving cystatins as an
additional option for cystatin engineering is further outlined along with the nuances of how cystatins
with rather unusual amino acid sequences might actually help in cystatin engineering. Finally, future
opportunities for application of cystatins are highlighted which include applications in genetically modified
transgenic plants for environmental stress protection and also as nutraceuticals, as part of more
nutritious food. Further opportunities might also include the possible management of diseases and disorders,
often associated with lifestyle changes, and the most immediate and promising application which
is inclusion into plant-based recombinant protein production platforms.