SEORious business: structural proteins in sieve tubes and their involvement in sieve element occlusion

Abstract

The phloem provides a network of sieve tubes for long-distance translocation of photosynthates. For over a century, structural proteins in sieve tubes have presented a conundrum since they presumably increase the hydraulic resistance of the tubes while no potential function other than sieve tube or wound sealing in the case of injury has been suggested. Here we summarize and critically evaluate current speculations regarding the roles of these proteins. Our understanding suffers from the suggestive power of images; what looks like a sieve tube plug on micrographs may not actually impede translocation very much. Recent reports of an involvement of SEOR (sieve element occlusion-related) proteins, a class of P-proteins, in the sealing of injured sieve tubes are inconclusive; various lines of evidence suggest that, in neither intact nor injured plants, are SEORs determinative of translocation stoppage. Similarly, the popular notion that P-proteins serve in the defence against phloem sap-feeding insects is unsupported by empirical facts; it is conceivable that in functional sieve tubes, aphids actually could benefit from inducing a plug. The idea that rising cytosolic Ca(2+) generally triggers sieve tube blockage by P-proteins appears widely accepted, despite lacking experimental support. Even in forisomes, P-protein assemblages restricted to one single plant family and the only Ca(2+)-responsive P-proteins known, the available evidence does not unequivocally suggest that plug formation is the cause rather than a consequence of translocation stoppage. We conclude that the physiological roles of structural P-proteins remain elusive, and that in vivo studies of their dynamics in continuous sieve tube networks combined with flow velocity measurements will be required to (hopefully) resolve this scientific roadblock.

Redox Control of Aphid Resistance through Altered Cell Wall Composition and Nutritional Quality

“…4; Supplemental Table S3). These proteins are believed to have a role in sieve element occlusion that might impede aphid feeding, although this widely accepted view has been challenged more recently (Knoblauch et al, 2014). Several transcripts associated with cell wall metabolism and remodeling were greatly increased in TAO lines but were not changed significantly Microarray probe identification numbers are indicated to the left of each row, and a brief description of the gene product is provided to the right.…”

Faba bean forisomes can function in defence against generalist aphids

“…However, the universality of P‐proteins effectively sealing sieve plates has recently been questioned (Knoblauch et al . ). While there is strong experimental evidence that P‐proteins of legumes can form effective plugs that stop the flow of sap (Knoblauch et al .…”

“…While there is strong experimental evidence that P‐proteins of legumes can form effective plugs that stop the flow of sap (Knoblauch et al . ) and there is a correlation between formation of P‐protein plugs and stoppage of sap flow in cucurbits (Furch et al . ), P‐proteins of Arabidopsis readily pass through the sieve plates without stopping the flow of sap (Froelich et al .…”

Combined microscopy and molecular analyses show phloem occlusions and cell wall modifications in tomato leaves in response to ‘Candidatus Phytoplasma solani’

“…Recently, Knoblauch et al . () highlighted the difficulties concerning investigation on P‐proteins stressing that their physiological role is still a puzzle.…”

“…They appear involved in maintaining turgor pressure after injury (Read & Northcote, 1983a), facilitating rapid wound sealing (Knoblauch & van Bel, 1998), and in defence, blocking pathogen spread and acting as signalling molecules (Read & Northcote, 1983a,b). Recently, Knoblauch et al (2014) highlighted the difficulties concerning investigation on P-proteins stressing that their physiological role is still a puzzle. P-proteins have been widely studied in cucurbits.…”