Sieve elements rapidly develop 'nacreous walls' following injury − a common wounding response?. (28th January 2020)

• Record Type: Journal Article
• Authors: Knoblauch, Jan; Knoblauch, Michael; Vasina, Viktoriya V.; Peters, Winfried S.
• Is Part Of: Plant journal. Volume 102:Number 4(2020)
• Page Start: 797
• Page End: 808
• Publication Date: 2020-01-28
• Subjects: cell wall swelling -- Gerrardanthus macrorhizus -- in situ microscopy -- nacreous wall -- phloem transport -- pressure–flow theory -- sieve tube occlusion -- wounding response; Plant molecular biology -- Periodicals; Plant cells and tissues -- Periodicals; Botany -- Periodicals; 580
• DOI: 10.1111/tpj.14665
• Languages: English
• ISSNs: 0960-7412

Abstract

Summary: Thick glistening cell walls occur in sieve tubes of all major land plant taxa. Historically, these 'nacreous walls' have been considered a diagnostic feature of sieve elements; they represent a conundrum, though, in the context of the widely accepted pressure–flow theory as they severely constrict sieve tubes. We employed the cucurbit Gerrardanthus macrorhizus as a model to study nacreous walls in sieve elements by standard and in situ confocal microscopy and electron microscopy, focusing on changes in functional sieve tubes that occur when prepared for microscopic observation. Over 90% of sieve elements in tissue sections processed for microscopy by standard methods exhibit nacreous walls. Sieve elements in whole, live plants that were actively transporting as shown by phloem‐mobile tracers, lacked nacreous walls and exhibited open lumina of circular cross‐sections instead, an appropriate structure for Münch‐type mass flow of the cell contents. Puncturing of transporting sieve elements with micropipettes triggered the rapid (<1 min) development of nacreous walls that occluded the cell lumen almost completely. We conclude that nacreous walls are preparation artefacts rather than structural features of transporting sieve elements. Nacreous walls in land plants resemble the reversibly swellable walls found in various algae, suggesting that they may function in turgor buffering, the amelioration of osmotic stress, wounding‐induced sieve tube occlusion, and possibly local defence responses of the phloem. Significance Statement: The occurrence of 'nacreous walls' in sieve tubes of all major land plant taxa questions the validity of the pressure–flow theory, because these drastically thickened walls severely constrict the sieve tube lumen. We show that 'nacreous walls' do not exist in transporting sieve tubes, and rather represent a wounding response based on rapid cell wall swelling, an unusual phenomenon in angiosperms.