Characterization of five subgroups of the sieve element occlusion gene family in Glycine max reveals genes encoding non-forisome P-proteins, forisomes and forisome tails

Abstract

P-proteins are structural phloem proteins discussed to be involved in the rapid sealing of injured sieve elements. P-proteins are found in all dicotyledonous and some monocotyledonous plants, but additional crystalloid P-proteins, known as forisomes, have evolved solely in the Fabaceae. Both types are encoded by members of the sieve element occlusion (SEO) gene family, which comprises seven phylogenetic subgroups. The Fabaceae-specific subgroup 1 contains genes encoding forisome subunits in e.g. Medicago truncatula, Vicia faba, Dipteryx panamensis and Canavalia gladiata whereas basal subgroup 5 encodes P-proteins in Nicotiana tabacum (tobacco) and Arabidopsis thaliana. The function of remaining subgroups is still unknown. We chose Glycine max (soybean) as a model to investigate SEO proteins representing different subgroups in one species. We isolated native P-proteins to determine the SEO protein composition and analyzed the expression pattern, localization and structure of the G. max SEO proteins representing five of the subgroups. We found that subgroup 1 GmSEO genes encode forisome subunits, a member of subgroup 5 encodes a non-forisome P-protein and subgroup 2 GmSEO genes encode the components of forisome tails, which are present in a restricted selection of Fabaceaen species. We therefore present the first molecular characterization of a Fabaceae non-forisome P-protein and the first evidence that forisome tails are encoded by a phylogenetically-distinct branch of the SEO gene family.

Similar Intracellular Location and Stimulus Reactivity, but Differential Mobility of Tailless (Vicia faba) and Tailed Forisomes (Phaseolus vulgaris) in Intact Sieve Tubes

“…The same applies to Phaseolus forisomes, the return of which to the original position must be more complex than in Vicia given the extensive longitudinal movement ( Fig 5F–5O ). Phaseolus forisomes may be anchored to a motive apparatus, perhaps via the tails, which have a SEO composition distinct from the rest of the forisome body [ 8 , 13 ]. Forisome tails do not disperse in response to Ca 2+ supply ( Fig 1 ) [ 19 ] and are only weakly fluorescent in forisomes in which certain SEO-proteins are GFP-tagged [ 8 ].…”

“…[ 1 , 5 , 7 ] which are largely of a proteinaceous nature. These proteins belong to the SEO family [ 8 ] which is presumably involved in sieve-element occlusion (SEO) and seems widespread among dicotyledons [ 9 – 13 ].…”

“…After withdrawal of Ca 2+ -ions, forisomes resume the original, condensed conformation [ 1 , 14 , 16 , 18 ]. The tails do not react to Ca 2+ supply and, therefore, may be of a different composition or architecture [ 8 , 13 , 19 ]. The reversible conformation changes reflect forisome behaviour in response to sieve-element injury [ 20 , 21 ].…”

Protein Structural Biology Using Cell-Free Platform from Wheat Germ

“…Using enterokinase, the 3XFLAG tag was then removed with >93% efficiency ( Figure 4B, Lane 8). The same purification procedure has been tested on many other proteins which some include GS, PPDK, Sieve Element Occlusion protein from M. truncatula (SEO1, 75 kDa) [38,39] and Pyridoxal 5'-Phosphate Synthase-Like subunit (PDX1.2, 34 kDa) from A. thalianaI [40].…”

“…This swelling/deformation is reversible, and it is a critical mechanism in sieve tube flow control [45]. Natural forisomes consist of several members of SEO protein family [38,39]. However, SEO proteins were never analyzed individually at the structural level.…”