This review focuses on the energy metabolism during pollen maturation and tube growth and updates current knowledge. Pollen tube growth is essential for male reproductive success and extremely fast. Therefore, pollen development and tube growth are high energy-demanding processes. During the last years, various publications (including research papers and reviews) emphasize the importance of mitochondrial respiration and fermentation during male gametogenesis and pollen tube elongation. These pathways obviously contribute to satisfy the high energy demand, and there are many studies which suggest that respiration and fermentation are the only pathways to generate the needed energy. Here, we review data which show for the first time that in addition plastidial glycolysis and the balancing of the ATP/NAD(P)H ratio (by malate valves and NAD(+) biosynthesis) contribute to satisfy the energy demand during pollen development. Although the importance of energy generation by plastids was discounted during the last years (possibly due to the controversial opinion about their existence in pollen grains and pollen tubes), the available data underline their prime role during pollen maturation and tube growth.
Keywords: 2-OG, 2-oxoglutarate; 2-PGA, 2-phosphoglycerate; 3-PGA, 3-phosphoglycerate; ACS, acetyl-CoA synthase; ADH, alcohol dehydrogenase; ALDH, aldehyde dehydrogenase; AOX, alternative oxidase; BPGA, bisphosphoglyceric acid; ENO, enolase; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; GOGAT, glutamate synthase; GPT, G-6-P/phosphate translocators; Gln, glutamine; Glu, glutamate; MDH, malate dehydrogenase; NDP, nucleotide diphosphate kinase; NMNAT, nicotinate/nicotinamide mononucleotide adenyltransferase; NTT, ATP/ADP transporters; OAA, oxaloacetate; OPP, oxidative pentose-phosphate pathway; PDC, pyruvate decarboxylase; PDH, pyruvate dehydrogenase; PEP, phosphoenolpyruvate; PGAM, phosphoglycerate mutase; PGDH, 3-phosphoglycerate dehydrogenase; PK, pyruvate kinase; PPSB, phosphorylated pathway of serine biosynthesis; PPT, phosphoenolpyruvate/phosphate translocator; PSP, phosphoserine phosphatase; RNS, reactive nitrogen species; ROS, reactive oxygen species; RPOT, T3/T7 phage-type RNA polymerases; T, malate/oxaloacetate translocator; TP, triose phosphate.; energy metabolism; malate; plastidial glycolysis; pollen tube growth; respiration.