Identification and localization of a caleosin in olive (Olea europaea L.) pollen during in vitro germination

Abstract

In plant organs and tissues, the neutral storage lipids are confined to discrete spherical organelles called oil bodies. Oil bodies from plant seeds contain 0.6–3% proteins, including oleosins, steroleosins, and caleosins. In this study, a caleosin isoform of ∼30 kDa was identified in the olive pollen grain. The protein was mainly located at the boundaries of the oil bodies in the cytoplasm of the pollen grain and the pollen tube. In addition, caleosins were also visualized in the cytoplasm at the subapical zone, as well as in the tonoplast of vacuoles present in the pollen tube cytoplasm. The cellular behaviour of lipid bodies in the olive pollen was also monitored during in vitro germination. The number of oil bodies decreased 20-fold in the pollen grain during germination, whereas the opposite tendency occurred in the pollen tube, suggesting that oil bodies moved from one to the other. The data suggest that this pollen caleosin might have a role in the mobilization of oil bodies as well as in the reorganization of membrane compartments during pollen in vitro germination.

Morphological, histological and ultrastructural changes in the olive pistil during flowering

“…The depletion of starch in stigma cells at anthesis coincided with an increasing presence of carbohydrates in the stigmatic secretion, strongly suggesting the incorporation of products of starch hydrolysis into the exudate. It seems unlikely, though, that these carbohydrates serve as an energy source for early pollen tube growth, since during this phase they grow autotrophically at the expense of their own reserve substances (Rodríguez-García et al 2003;Zienkiewicz et al 2010). In the style, the passage of the pollen tubes across the transmitting tract was accompanied by the disappearance of starch, which likely serves as a nutritional source as described in other species (Herrero and Dickinson 1979;Herrero and Arbeloa 1989).…”

Integral Proteins in Plant Oil Bodies

“…According to the characterization of two independent insertion mutants lacking caleosin, it was proposed that caleosin might play a role in the degradation of storage lipids in oil bodies by inducing the interaction of oil bodies with vacuoles during germination [96]. Putative interaction between oil bodies and vacuoles was also observed in pollen cells after germination under electron microscopy; and the pollen oil bodies were presumably surrounded by tubular membrane structures and encapsulated in the vacuoles after germination [97][98][99]. The detailed molecular interaction between caleosin on the surface of oil bodies and its specific partner protein on the membrane of vacuoles remains to be studied.…”

“…In contrast with oleosin isoforms that are unique to oil bodies, caleosin isoforms or caleosin-like proteins are possibly present in other cellular locations, for example, ER membrane [140]. Moreover, the same caleosin isoform is possibly present in both seed oil bodies and membrane-bound fractions of other tissues [99].…”

Plant lipid bodies and cell-cell signaling: A new role for an old organelle?

“…19,42,44 In Arabidopsis, seven caleosin-like genes are identified, only one of which is strongly expressed in seeds. Once thought to be specific of seed LBs, caleosins are also found associated with LBs in post-embryonic tissues including root tips, 44 pollen 47,74 and shoots. 52 In addition, and in contrast to oleosin, caleosin is not restricted to plants as it is also found in fungi.…”

“…For example, in hydrating pollen, LBs gather near the germinative aperture, where the pollen tube emerges, and subsequently migrate into the forming pollen tube assisted by ER strands and cytoplasmic streaming. 73,74 Another example is tapetum, a tissue of just one cell layer thick which encloses the microspores in the anther locule and controls their maturation. Tapetum cells express nine oleosin genes, the proteins of which accumulate in tapetosomes, a novel organelle type containing LBs.…”