Non-dispersive phloem-protein bodies (NPBs) ofPopulus trichocarpaconsist of a SEOR protein and do not respond to cell wounding and Ca²⁺

Abstract

Differentiating sieve elements in the phloem of angiosperms produce abundant phloem-specific proteins before their protein synthesis machinery is degraded. These P-proteins initially form dense bodies, which disperse into individual filaments when the sieve element matures. In some cases, however, the dense protein agglomerations remain intact and are visible in functional sieve tubes as non-dispersive P-protein bodies, or NPBs. Species exhibiting NPBs are distributed across the entire angiosperm clade. We found that NPBs in the model tree, Populus trichocarpa, resemble the protein bodies described from other species of the order Malpighiales as they all consist of coaligned tubular fibrils bundled in hexagonal symmetry. NPBs of all Malpighiales tested proved unresponsive to sieve tube wounding and Ca2+. The P. trichocarpa NPBs consisted of a protein encoded by a gene that in the genome database of this species had been annotated as a homolog of SEOR1 (sieve element occlusion-related 1) in Arabidopsis. Sequencing of the gene in our plants corroborated this interpretation, and we named the gene PtSEOR1. Previously characterized SEOR proteins form irregular masses of P-protein slime in functional sieve tubes. We conclude that a subgroup of these proteins is involved in the formation of NPBs at least in the Malpighiales, and that these protein bodies have no role in rapid wound responses of the sieve tube network.

Facile Labeling of Sieve Element Phloem-Protein Bodies Using the Reciprocal Oligosaccharide Probe OGA488

“…Higher magnification images of longitudinal sections probed with OGA 488 further confirmed the presence of the rounded bodies, but also revealed labeling of more disbursed bodies within the cell, sometimes associated with the basal ends of elongated cells (Figure 1G), while control sections lacked signal (Figure 1H). These results are reminiscent of the morphology of P-proteins previously presented for poplar (Mullendore et al, 2018).…”

“…However, while filaments failed to form or label, globular bodies did label within sieve elements for both complementation tests. SEOR-like proteins have also been described in poplar, and are associated with spherical P-proteins that do not respond to phloem wounding ( Mullendore et al, 2018 ).…”

Proteomics of isolated sieve tubes fromNicotiana tabacum: sieve element–specific proteins reveal differentiation of the endomembrane system

“…No traces of these proteins were detected in phloem exudates from two legumes, Lupinus albus ( 26 ) and Lupinus texensis ( 28 ), although forisomes have been documented in seven Lupinus spp., including L. albus ( 36 ). The similar but noncontractile SEOR (SEO-related) proteins are common and often abundant P-proteins in angiosperms including legumes, cucurbits, and model species such as Populus trichocarpa and Arabidopsis ( 12 , 35 , 37 , 38 ). Nonetheless, SEORs are conspicuously absent from phloem exudates; we are aware of only one detection of apparent SEOR proteins, namely several putative homologs of the Arabidopsis SEOR1 (At3G01680) in exudates from Cucurbita maxima ( supplemental table 1 in ref.…”

Species-Specific and Distance-Dependent Dispersive Behaviour of Forisomes in Different Legume Species

“…Structural P-proteins, some of which belong to the SEO (sieve-element occlusion) or the sieve-element occlusion-related (SEOR) family, come in a multitude of shapes [ 19 , 20 , 21 , 22 , 23 ]. Apart from the structural variants of loosely organized filaments [ 21 , 23 ], P-proteins also occur in densely packed, highly organized structures [ 24 , 25 , 26 , 27 ]. These spindle-shaped so-called forisomes [ 17 , 26 ] have only been found in the legume subfamily Papilionoideae [ 28 ].…”