New insights into the early steps of oil body mobilization during pollen germination

Abstract

In some plants, pollen grains accumulate storage lipids that serve as energy supply during germination. Here, three enzymes involved in early steps of oil body mobilization in the male gametophyte were functionally characterized for the first time. The effect of extracellular sugars on pollen performance and oil body dynamics was also analysed. Olive pollen oil bodies showed phospholipase A, lipase, and lipoxygenase activities on their surface. Enzyme activity levels increased during germination with a maximum after 3h. Removal of extracellular sugars from the germination medium did not affect pollen performance but increased enzyme activity rates and sped up oil body mobilization. Inhibitors seriously hampered pollen germination and pollen tube growth, leading to a characteristic accumulation of oil bodies in the germinative aperture. It can be concluded that storage lipids are sufficient for proper olive pollen germination. A lipase and a lipoxygenase are likely involved in oil body mobilization. Extracellular sugars may modulate their function, while a phospholipase A may promote their access to the storage lipids.

PUX10 Is a Lipid Droplet-Localized Scaffold Protein That Interacts with CELL DIVISION CYCLE48 and Is Involved in the Degradation of Lipid Droplet Proteins

“…While seeds degrade their neutral lipids following germination, tobacco pollen tubes synthesize TAG (Mellema et al, 2002) and sterol esters (Müller and Ischebeck, 2018), and pollen tubes cultivated for 14 h still contain large amounts of these neutral lipids (Müller and Ischebeck, 2018). By contrast, pollen tubes from olive (Olea europaea) degrade their LDs during pollen tube growth (Zienkiewicz et al, 2013). It remains unclear, though, if LDs are synthesized de novo in tobacco pollen tubes.…”

“…For instance, caleosin and steroleosin (also referred to as hydroxysteroid dehydrogenases [HSDs]) are anchored to LDs by a hydrophobic hairpin sequence that contains conserved proline residues that form a so-called proline "knot" and "knob," respectively (Shimada and Hara-Nishimura, 2010). Caleosins are considered to have a structural role in maintaining LD integrity, but some isoforms have also been reported to have peroxygenase activity (Hanano et al, 2006;Blée et al, 2014) and are thought to act after the oxidation of fatty acids by LD-localized lipoxygenases (Rudolph et al, 2011;Zienkiewicz et al, 2013). Based on genetic evidence, steroleosins are associated with brassinosteroid metabolism (Li et al, 2007;Baud et al, 2009) and have been shown to be able to act on steroid substrates as dehydrogenases, interconverting hydroxyl-groups of the steroid backbone to keto-groups (d'Andréa et al, 2007).…”

Characterization of the enzymatic activity and physiological function of the lipid droplet‐associated triacylglycerol lipase AtOBL1

“…LDs are foundat least to a small extentin most cells, and are most prominent in seeds and tapetosomes (Pyc et al, 2017) as well as in pollen (Ischebeck, 2016). Here, the LDs enter the pollen tube after germination (M€ uller et al, 2017) and, at least in olive pollen tubes, they are also degraded (Zienkiewicz et al, 2013) and an inhibition of TAG degradation impaired olive pollen tube growth. Interestingly, tobacco pollen tubes form TAG de novo from radioactively labelled ethanol and glucose (Mellema et al, 2002), indicating that LDs might not only be a source, but also a sink for acyl chains.…”

Degradation of Lipid Droplets in Plants and Algae—Right Time, Many Paths, One Goal

“…LD-associated lipase activities were also found in vitro and in situ in germinating pollen and growing pollen tubes of olive (Olea europaea L.) (Rejon et al, 2012;Zienkiewicz et al, 2013). Their direct role in proper pollen tube growth has been confirmed by in vitro olive pollen germination in medium with and without sucrose, with higher lipase activity resulting in the latter case (Zienkiewicz et al, 2013). In these studies, TAG lipase activity was found only on LDs isolated from germinating pollen tubes, and not from the mature pollen grain.…”