Slower phloem transport in gymnosperm trees can be attributed to higher sieve element resistance

J. Liesche; C. Windt; T. Bohr; A. Schulz; K. H. Jensen

doi:10.1093/treephys/tpv020

Aspartate Residues in a Forisome-Forming SEO Protein Are Critical for Protein Body Assembly and Ca2+ Responsiveness

Plant and Cell Physiology ◽

10.1093/pcp/pcaa093 ◽

2020 ◽

Vol 61 (10) ◽

pp. 1699-1710

Author(s):

Yan Liu ◽

Winfried S Peters ◽

Daniel R Froelich ◽

Alexander H Howell ◽

Sutton Mooney ◽

...

Keyword(s):

Molecular Mechanisms ◽

Protein Body ◽

Phloem Transport ◽

Sieve Element ◽

Amino Acid Sequences ◽

Yeast Saccharomyces Cerevisiae ◽

Binding Motifs ◽

Sieve Elements ◽

Protein Protein Interaction ◽

Transgenic Cells

Abstract Forisomes are protein bodies known exclusively from sieve elements of legumes. Forisomes contribute to the regulation of phloem transport due to their unique Ca2+-controlled, reversible swelling. The assembly of forisomes from sieve element occlusion (SEO) protein monomers in developing sieve elements and the mechanism(s) of Ca2+-dependent forisome contractility are poorly understood because the amino acid sequences of SEO proteins lack conventional protein–protein interaction and Ca2+-binding motifs. We selected amino acids potentially responsible for forisome-specific functions by analyzing SEO protein sequences in comparison to those of the widely distributed SEO-related (SEOR), or SEOR proteins. SEOR proteins resemble SEO proteins closely but lack any Ca2+ responsiveness. We exchanged identified candidate residues by directed mutagenesis of the Medicago truncatula SEO1 gene, expressed the mutated genes in yeast (Saccharomyces cerevisiae) and studied the structural and functional phenotypes of the forisome-like bodies that formed in the transgenic cells. We identified three aspartate residues critical for Ca2+ responsiveness and two more that were required for forisome-like bodies to assemble. The phenotypes observed further suggested that Ca2+-controlled and pH-inducible swelling effects in forisome-like bodies proceeded by different yet interacting mechanisms. Finally, we observed a previously unknown surface striation in native forisomes and in recombinant forisome-like bodies that could serve as an indicator of successful forisome assembly. To conclude, this study defines a promising path to the elucidation of the so-far elusive molecular mechanisms of forisome assembly and Ca2+-dependent contractility.

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Sieve element and companion cell: the story of the comatose patient and the hyperactive nurse

Functional Plant Biology ◽

10.1071/pp99172 ◽

2000 ◽

Vol 27 (6) ◽

pp. 477 ◽

Cited By ~ 21

Author(s):

Aart J. E. van Bel ◽

Michael Knoblauch

Keyword(s):

Phloem Transport ◽

Division Of Labour ◽

Sieve Element ◽

Companion Cell ◽

Concerted Action ◽

Sieve Elements ◽

Cell Complexes ◽

Photoassimilate Translocation ◽

Companion Cells ◽

Evolutionary Descent

Sieve elements and companion cells constitute the modules of the conducting elements in the phloem ofAngiosperms. Consequently, phloem transport basically relies on the concerted action of the sieve element/companion cell complexes. Sieve elements and companion cells are highly interactive units and show an extreme division of labour as exemplified by their state of life. Whereas the sieve element is almost ‘clinically’ dead, the companion cell is a paragon of bubbling activity. In the course of evolution, the sieve element has sacrificed all of its genetic and most of its metabolic equipment to serve photoassimilate translocation. A small part of the structural and metabolic outfit has been retained for a proper accomplishment of its function. In contrast, the cells bordering the sieve element have gained metabolic weight during evolution. With reference to their evolutionary descent, the peculiarities of sieve elements and companion cells are discussed in the light of recent cell-biological and molecular-biological findings. Emphasis is focused on their interaction, which is the secret of the success of the sieve element/companion cell complex.

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