An extracellular lipid transfer protein is relocalized intracellularly during seed germination

Abstract

Plant lipid transfer proteins (LTPs) constitute a family of small proteins recognized as being extracellular. In agreement with this notion, several lines of evidence have shown the apoplastic localization of HaAP10, a LTP from Helianthus annuus dry seeds. However, HaAP10 was recently detected intracellularly in imbibing seeds. To clarify its distribution, immunolocalization experiments were performed during the course of germination and confirmed its intracellular localization upon early seed imbibition. Further assays using a hydrophobic dye, FM4-64, inhibitors of vesicular traffic, and immunolocalization of the pectin rhamnogalacturonan-II, allowed the conclusion that endocytosis is activated as soon as seed imbibition starts. Furthermore, this study demonstrated that HaAP10 is endocytosed throughout imbibition. Biochemical and cellular approaches indicate that the intracellular fraction of this LTP appears associated with oil bodies and some evidence also suggest its presence in glyoxysomes. So, HaAP10 is apoplastic in dry seeds and upon imbibition is rapidly internalized and relocalized to organelles involved in lipid metabolism. The results suggest that HaAP10 may be acting as a fatty acid shuttle between the oil body and the glyoxysome during seed germination. This concept is consistent with the initial proposition that LTPs participate in the intracellular transfer of lipids which was further denied based on their apparent extracellular localization. This report reveals for the first time the relocalization of a lipid transfer protein and opens new perspectives on its role.

The Nonspecific Lipid Transfer Protein AtLtpI-4 Is Involved in Suberin Formation of Arabidopsis thaliana Crown Galls

“…GPI-anchored nsLTPs also have been postulated to be involved in suberin formation (Edstam et al, 2013). Most other nsLTPs are synthesized as precursor proteins, with an N-terminal signal peptide that targets them to the apoplastic space (Thoma et al, 1994;Pagnussat et al, 2012).…”

On the role of a Lipid-Transfer Protein. Arabidopsis ltp3 mutant is compromised in germination and seedling growth.

“…Although LTP3 is predicted to be extracellular, a putative ortholog of LTP3 from sunflower seeds has been shown to change its localization from the apoplast to intracellular oil bodies when germination is triggered. 40 Thus, the bulk of evidence prompt us to hypothesize that LTP3 could play a direct role in oil mobilization. A previous report by Tsuboi et al 29 supports this speculation since a LTP from castor bean cotyledons was detected in the glyoxysomal/peroxisomal fraction.…”

Germination Potential of Dormant and Nondormant Arabidopsis Seeds Is Driven by Distinct Recruitment of Messenger RNAs to Polysomes

“…For example, the transcripts for LTP6 and LTP5 (At3g51600 and At3g08770, respectively; Fig. 6), which ranged within this category, correspond to lipid transfer proteins, and they have been shown to participate in the intracellular transfer of lipids during seed germination (Pagnussat et al, 2012).…”