The functionality of plant mechanoproteins (forisomes) is dependent on the dual role of conserved cysteine residues

Guardians of the phloem - forisomes and beyond

The phloem is a highly specialized vascular tissue that forms a fundamentally important transport and signaling pathway in plants. It is therefore a system worth protecting. The main function of the phloem is to transport the products of photosynthesis throughout the whole plant, but it also transports soluble signaling molecules and propagates electrophysiological signals. The phloem is constantly threatened by mechanical injuries, phloem-sucking pests and parasites, and the spread of pathogens, which has led to the evolution of efficient defense mechanisms. One such mechanism involves structural phloem proteins, which are thought to facilitate sieve element occlusion following injury and to defend the plant against pathogens. In leguminous plants, specialized structural phloem proteins known as forisomes form unique mechanoproteins via sophisticated molecular interaction and assembly mechanisms, thus enabling reversible sieve element occlusion. By understanding the structure and function of forisomes and other structural phloem proteins, we can develop a toolbox for biotechnological applications in material science and medicine. Furthermore, understanding the involvement of structural phloem proteins in plant defense mechanisms will allow phloem engineering as a new strategy for the development of crop varieties that are resistant to pests, pathogens and parasites.

The Ca2+ response of a smart forisome protein is dependent on polymerization

Forisomes are giant self-assembling mechanoproteins that undergo reversible structural changes in response to Ca²⁺ and various other stimuli. Artificial forisomes assembled from the monomer MtSEO-F1 can be used as smart biomaterials, but the molecular basis of their functionality is not understood. To determine the role of protein polymerization in forisome activity, we tested the Ca²⁺ association of MtSEO-F1 dimers (the basic polymerization unit) by circular dichroism spectroscopy and microscale thermophoresis. We found that soluble MtSEO-F1 dimers neither associate with Ca²⁺ nor undergo structural changes. However, polarization modulation infrared reflection absorption spectroscopy revealed that aggregated MtSEO-F1 dimers and fully-assembled forisomes associate with Ca²⁺ , allowing the hydration of poorly-hydrated protein areas. A change in the signal profile of complete forisomes indicated that Ca²⁺ interacts with negatively-charged regions in the protein complexes that only become available during aggregation. We conclude that aggregation is required to establish the Ca²⁺ response of forisome polymers.