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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., (© 2022 The Authors. New Phytologist © 2022 New Phytologist Foundation.)

Angiosperms transport their photoassimilates through sieve tubes, which comprise longitudinally-connected sieve elements. In dicots and also some monocots, the sieve elements contain parietal structural proteins known as phloem proteins or P-proteins. Following injury, P proteins disperse and accumulate as viscous plugs at the sieve plates to prevent the loss of valuable transport sugars. Tobacco (Nicotiana tabacum) P-proteins are multimeric complexes comprising subunits encoded by members of the SEO (sieve element occlusion) gene family. The existence of multiple subunits suggests that P-protein assembly involves interactions between SEO proteins, but this process is largely uncharacterized and it is unclear whether the different subunits perform unique roles or are redundant. We therefore extended our analysis of the tobacco P-proteins NtSEO1 and NtSEO2 to investigate potential interactions between them, and found that both proteins can form homomeric and heteromeric complexes in planta.

The sieve element occlusion (SEO) gene family includes several members that are expressed specifically in immature sieve elements (SEs) in the developing phloem of dicotyledonous plants. To determine how this restricted expression profile is achieved, we analysed the SE-specific Medicago truncatula SEO-F1 promoter (PMtSEO-F1) by constructing deletion, substitution and hybrid constructs and testing them in transgenic tobacco plants using green fluorescent protein as a reporter. This revealed four promoter regions, each containing cis-regulatory elements that activate transcription in SEs. One of these segments also contained sufficient information to suppress PMtSEO-F1 transcription in the phloem companion cells (CCs). Subsequent in silico analysis revealed several candidate cis-regulatory elements that PMtSEO-F1 shares with other SEO promoters. These putative sieve element boxes (PSE boxes) are promising candidates for cis-regulatory elements controlling the SE-specific expression of PMtSEO-F1., (© 2012 German Botanical Society and The Royal Botanical Society of the Netherlands.)

The sieve element occlusion (SEO) gene family originally was delimited to genes encoding structural components of forisomes, which are specialized crystalloid phloem proteins found solely in the Fabaceae. More recently, SEO genes discovered in various non-Fabaceae plants were proposed to encode the common phloem proteins (P-proteins) that plug sieve plates after wounding. We carried out a comprehensive characterization of two tobacco (Nicotiana tabacum) SEO genes (NtSEO). Reporter genes controlled by the NtSEO promoters were expressed specifically in immature sieve elements, and GFP-SEO fusion proteins formed parietal agglomerates in intact sieve elements as well as sieve plate plugs after wounding. NtSEO proteins with and without fluorescent protein tags formed agglomerates similar in structure to native P-protein bodies when transiently coexpressed in Nicotiana benthamiana, and the analysis of these protein complexes by electron microscopy revealed ultrastructural features resembling those of native P-proteins. NtSEO-RNA interference lines were essentially devoid of P-protein structures and lost photoassimilates more rapidly after injury than control plants, thus confirming the role of P-proteins in sieve tube sealing. We therefore provide direct evidence that SEO genes in tobacco encode P-protein subunits that affect translocation. We also found that peptides recently identified in fascicular phloem P-protein plugs from squash (Cucurbita maxima) represent cucurbit members of the SEO family. Our results therefore suggest a common evolutionary origin for P-proteins found in the sieve elements of all dicotyledonous plants and demonstrate the exceptional status of extrafascicular P-proteins in cucurbits.

Forisomes are remarkable protein bodies found exclusively in the phloem of the Fabaceae. When the phloem is wounded, forisomes are converted from a condensed to a dispersed state in an ATP-independent reaction triggered by Ca(2+), thereby plugging the sieve tubes and preventing the loss of photoassimilates. Potentially, forisomes are ideal biomaterials for technical devices because the conformational changes can be replicated in vitro and are fully reversible over a large number of cycles. However, the development of technical devices based on forisomes has been hampered by the laborious and time-consuming process of purifying native forisomes from plants. More recently, the problem has been overcome by the production of recombinant artificial forisomes. This is a milestone in the development of forisome-based devices, not only because large quantities of homogeneous forisomes can be produced on demand, but also because their properties can be tailored for particular applications. In this review, we discuss the physical and molecular properties of native and artificial forisomes, focusing on their current applications in technical devices and potential developments in the future.

Forisomes are protein aggregates found uniquely in the sieve elements of Fabaceaen plants. Upon wounding they undergo a reversible, calcium-dependent conformational switch which enables them to act as cellular stopcocks. Forisomes begin to form in young sieve elements at an early stage of metaphloem differentiation. Genes encoding forisome components could therefore be useful as markers of early sieve element development. Here we present a comprehensive analysis of the developmental expression profile of for1, which encodes such a forisome component. The for1 gene is highly conserved among Fabaceaen species and appears to be unique to this phylogenetic lineage since no orthologous genes have been found in other plants, including Arabidopsis and rice. Even so, transgenic tobacco plants expressing reporter genes under the control of the for1 promoter display reporter activity exclusively in immature sieve elements. This suggests that the regulation of sieve element development is highly conserved even in plants where mature forisomes have not been detected. The promoter system could therefore provide a powerful tool for the detailed analysis of differentiation in metaphloem sieve elements in an unexpectedly broad range of plant species.