Abundant type III lipid transfer proteins in Arabidopsis tapetum are secreted to the locule and become a constituent of the pollen exine

Plant Physiol. 2013 Nov;163(3):1218-29. doi: 10.1104/pp.113.225706. Epub 2013 Oct 4.

Abstract

Lipid transfer proteins (LTPs) are small secretory proteins in plants with defined lipid-binding structures for possible lipid exocytosis. Special groups of LTPs unique to the anther tapetum are abundant, but their functions are unclear. We studied a special group of LTPs, type III LTPs, in Arabidopsis (Arabidopsis thaliana). Their transcripts were restricted to the anther tapetum, with levels peaking at the developmental stage of maximal pollen-wall exine synthesis. We constructed an LTP-Green Fluorescent Protein (LTP-GFP) plasmid, transformed it into wild-type plants, and monitored LTP-GFP in developing anthers with confocal laser scanning microscopy. LTP-GFP appeared in the tapetum and was secreted via the endoplasmic reticulum-trans-Golgi network machinery into the locule. It then moved to the microspore surface and remained as a component of exine. Immuno-transmission electron microscopy of native LTP in anthers confirmed the LTP-GFP observations. The in vivo association of LTP-GFP and exine in anthers was not observed with non-type III or structurally modified type III LTPs or in transformed exine-defective mutant plants. RNA interference knockdown of individual type III LTPs produced no observable mutant phenotypes. RNA interference knockdown of two type III LTPs produced microscopy-observable morphologic changes in the intine underneath the exine (presumably as a consequence of changes in the exine not observed by transmission electron microscopy) and pollen susceptible to dehydration damage. Overall, we reveal a novel transfer pathway of LTPs in which LTPs bound or nonbound to exine precursors are secreted from the tapetum to become microspore exine constituents; this pathway explains the need for plentiful LTPs to incorporate into the abundant exine.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Arabidopsis / genetics
  • Arabidopsis / metabolism*
  • Arabidopsis Proteins / genetics
  • Arabidopsis Proteins / metabolism*
  • Carrier Proteins / classification
  • Carrier Proteins / genetics
  • Carrier Proteins / metabolism*
  • Endoplasmic Reticulum / metabolism
  • Flowers / genetics
  • Flowers / metabolism*
  • Flowers / ultrastructure
  • Gene Expression Regulation, Plant
  • Green Fluorescent Proteins / genetics
  • Green Fluorescent Proteins / metabolism
  • Microscopy, Confocal
  • Microscopy, Electron, Scanning
  • Microscopy, Electron, Transmission
  • Microscopy, Immunoelectron
  • Phylogeny
  • Plants, Genetically Modified
  • Pollen / genetics
  • Pollen / metabolism*
  • Pollen / ultrastructure
  • Protein Transport
  • RNA Interference
  • Reverse Transcriptase Polymerase Chain Reaction
  • trans-Golgi Network / metabolism

Substances

  • Arabidopsis Proteins
  • Carrier Proteins
  • lipid transfer protein
  • Green Fluorescent Proteins