Spatial and temporal regulation of the forisome gene for1 in the phloem during plant development

Abstract

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.

Sieve element occlusion(SEO) genes encode structural phloem proteins involved in wound sealing of the phloem

“…In P NtSEO1 GUS transgenic plants, GUS activity appeared to be restricted to the vascular bundle (Fig. 1B), confirming the anticipated spatiotemporal expression pattern already observed for other SEO genes in sieve elements (24,28,31,32).…”

“…The sequence similarity indicates that forisomes and conventional P-proteins have a common evolutionary origin (28,35). This notion is supported by several shared structural characteristics, such as their fibrillar composition (9,56), their ultrastructure in the condensed and dispersed states (5,57), their spatiotemporal expression profiles (24,58), and their cellular function, although forisomes are unique in that the sealing of sieve plates is reversible (29). In this context, the Ca 2+ -mediated dispersion of forisomes has been investigated in detail (30,56), and it remains to be seen whether conventional Pproteins display any sensitivity to calcium or have any further functional similarities to forisomes.…”

“…Thus far, four sieve element occlusion (SEO) gene family members have been identified in Medicago truncatula (MtSEO-F1-F4) and Glycine max (GmSEO-F1-F4) and characterized as encoding forisome subunits, which assemble into a special type of crystalloid P-protein found uniquely in Fabaceae (23)(24)(25)(26)(27)(28). Forisomes differ from conventional P-proteins because they can undergo a reversible switch between condensed spindle-shaped and dispersed plug-forming states, triggered by a calcium influx (29,30).…”

“…Forisomes differ from conventional P-proteins because they can undergo a reversible switch between condensed spindle-shaped and dispersed plug-forming states, triggered by a calcium influx (29,30). Genes encoding forisomes are expressed in immature sieve elements (24,28,31,32), and the corresponding protein assemblies share several structural and functional characteristics with conventional P-proteins (33,34). SEO proteins contain three conserved domains: the SEO N-terminal domain (SEO-NTD), a potential thioredoxin fold, and the SEO C-terminal domain (SEO-CTD).…”

Binding Properties of the N-Acetylglucosamine and High-Mannose N-Glycan PP2-A1 Phloem Lectin in Arabidopsis

“…Light and confocal laser scanning microscopy studies have demonstrated that forisomes, crystalloid P-proteins found in Vicia faba and other faboid legumes, control the conductivity of sieve tubes (Knoblauch et al, 2001(Knoblauch et al, , 2003. The components of the forisomes have been identified and appear to be proteins unique to faboid legumes, although some of the motifs they contain have been identified in proteins from other plant taxa (Noll et al, 2007;Pelissier et al, 2008). These findings demonstrate a role of forisome-type P-proteins in the occlusion of sieve tubes in the Fabaceae, but the function of the P-proteins present in other species remains unclear.…”