The Arabidopsis Lipid Transfer Protein 2 (AtLTP2) Is Involved in Cuticle-Cell Wall Interface Integrity and in Etiolated Hypocotyl Permeability

Abstract

Plant non-specific lipid transfer proteins (nsLTPs) belong to a complex multigenic family implicated in diverse physiological processes. However, their function and mode of action remain unclear probably because of functional redundancy. Among the different roles proposed for nsLTPs, it has long been suggested that they could transport cuticular precursor across the cell wall during the formation of the cuticle, which constitutes the first physical barrier for plant interactions with their aerial environment. Here, we took advantage of the Arabidopsis thaliana etiolated hypocotyl model in which AtLTP2 was previously identified as the unique and abundant nsLTP member in the cell wall proteome, to investigate its function. AtLTP2 expression was restricted to epidermal cells of aerial organs, in agreement with the place of cuticle deposition. Furthermore, transient AtLTP2-TagRFP over-expression in Nicotiana benthamiana leaf epidermal cells resulted in its localization to the cell wall, as expected, but surprisingly also to the plastids, indicating an original dual trafficking for a nsLTP. Remarkably, in etiolated hypocotyls, the atltp2-1 mutant displayed modifications in cuticle permeability together with a disorganized ultra-structure at the cuticle-cell wall interface completely recovered in complemented lines, whereas only slight differences in cuticular composition were observed. Thus, AtLTP2 may not play the historical purported nsLTP shuttling role across the cell wall, but we rather hypothesize that AtLTP2 could play a major structural role by maintaining the integrity of the adhesion between the mainly hydrophobic cuticle and the hydrophilic underlying cell wall. Altogether, these results gave new insights into nsLTP functions.

Plant lipid transfer proteins: are we finally closing in on the roles of these enigmatic proteins?

“…An Atltp1.4-mutant has an increased cuticle permeability and structural defects at the cell wall-cuticle interphase. It was therefore proposed that AtLTP1.4 could play a major structural role by maintaining the integrity of the adhesion between the mainly hydrophobic cuticle and the underlying hydrophilic cell wall (32).An adhesive role for LTPs is also supported from the investigation of the Arabidopsis www.jlr.org Downloaded from 9 both functions in transfer and adhesion could be fulfilled by singular LTPs, as already been suggested for AtLTP1.4 (24,32). Further studies will hopefully reveal whether particular LTPs are involved in specific processes during lipid polymer deposition.…”

“…However, there are data accumulating suggesting that these proteins are required for the deposition and function of wax and lipidbased polymers, such as suberin, sporopollenin and the cuticle that form water-proof barriers on plant surfaces (24,(26)(27)(28)(29)(30)(31)(32). Additionally, some LTPs with cell wall loosening activities could be important for plant growth (33).…”

“…An Atltp1.4-mutant has an increased cuticle permeability and structural defects at the cell wall-cuticle interphase. It was therefore proposed that AtLTP1.4 could play a major structural role by maintaining the integrity of the adhesion between the mainly hydrophobic cuticle and the underlying hydrophilic cell wall (32).…”

Deciphering the Evolution and Development of the Cuticle by Studying Lipid Transfer Proteins in Mosses and Liverworts

“…An alternative hypothesis for the role of LTPs in cuticle assembly comes from investigations of AtLTP2 [ 31 ]. We previously renamed AtLTP2 to AtLTP1.4 to emphasize that it is of type LTP1 [ 7 ], and will use AtLTP1.4 onwards in this review.…”

“…However, in this mutant there is a 30% increase in 18:2 dicarboxylic acid, which is a major cutin component. The At ltp1.4- mutant also shows structural defects at the cell wall–cuticle interphase [ 31 ]. It was therefore proposed that AtLTP1.4 could play a major structural role by maintaining the integrity of the adhesion between the mainly hydrophobic cuticle and the underlying hydrophilic cell wall [ 31 ].…”