Effects of sucrose depletion on pollen tube growth

Pollen tubes are rapidly growing, photosynthetically inactive cells that need high rates of energy to support growth. Energy can derive from internal and external storage sources. The lack of carbon sources can cause various problems during pollen tube growth, which in turn could affect the reproduction of plants.

Parotta and colleagues analysed the effects of energy deficiency on the development of Nicotiana tabacum (Solanaceae) pollen tubes by replacing sucrose with glycerol in the growth medium. They focused on cell growth and related processes, such as metabolite composition and cell wall synthesis.

The team found that the lack of sucrose affects pollen germination and pollen tube length during a specific growth period. Both sugar metabolism and ATP concentration were affected by sucrose shortage when pollen tubes were grown in glycerol-based media; this was related to decreases in the concentrations of glucose, fructose and UDP-glucose. The intracellular pH and ROS levels also showed a different distribution in pollen tubes grown in sucrose-depleted media. Changes were also observed at the cell wall level, particularly in the content and distribution of two enzymes related to cell wall synthesis (sucrose synthase and callose synthase). Furthermore, both callose and newly secreted cell wall material (mainly pectins) showed an altered distribution corresponding to the lack of oscillatory growth in pollen tubes. Growth in glycerol-based media also temporarily affected the movement of generative cells and, in parallel, the deposition of callose plugs.

Pollen tubes represent an ideal model system for studying metabolic pathways during the growth of plant cells. Protta et al. found evidence that glycerol, a less energetic source for cell growth than sucrose, causes critical changes in cell wall deposition. The evidence that different aspects of pollen tube growth are affected is an indication that pollen tubes adapt to metabolic stress.

The authors add: “[W]e did not analyse the organization of microtubules following metabolic stress. We obtained some preliminary indications in actin filaments, which, in pollen tubes grown in BKG, seemed to be organized exactly as in the BKS controls. Monitoring the cytoskeleton of pollen tubes in glycerol-based medium could be an interesting research topic to focus on later.”