Genetic defects in the CDP-choline pathway for phosphatidylcholine biosynthesis cannot be transmitted to offspring via male gametophytes owing to interruption of autophagy-like processes required for pollen germination in Arabidopsis thaliana

Phosphatidylcholine is a major plant membrane phospholipid that contributes to the biogenesis and desaturation of membrane lipids and storage lipids. Thus, to ensure reproductive capacity, any genetic defect that affects phosphatidylcholine biosynthesis must be eliminated before fertilization. In Arabidopsis thaliana, phosphatidylcholine biosynthesis depends on CCT1 and CCT2, both encoding CTP: phosphorylcholine cytidylyltransferase. Using A. thaliana T-DNA-tagged mutants, we demonstrate that neither cct1-3 cct2-3 nor cct1-3 cct2-5 seedlings are viable. Reciprocal crosses of cct2-3/CCT2 or cct2-5/CCT2 plants in the cct1-3 background revealed that neither cct2-3 nor cct2-5 was transmitted via cct1-3 male gametophytes, although each allele was transmitted via cct1-3 female gametophytes. Although all pollen grains on a pollen quartet from qrt1-1 cct1-3 cct2-5/CCT2 plants were viable, none of the cct1-3 cct2-5 pollen grains from cct2-5 cct1-3/CCT1 and cct1-3 cct2-5/CCT2 plants were able to germinate in vitro. Transmission electron microscopy analysis of pollen grains subjected to pollen germination revealed that cct1-3 cct2-5 pollen grains developed unusual ultrafine structures, such as lipid bodies of disproportionate size including extremely enlarged ones, swelling of small granular structures, inhibition of vacuole development, and accumulation of incomplete autophagosome-like bodies enclosing various intracellular compartments. Thus, transmission of cct1-3 cct2-5 to offspring via male gametophytes appears to be strictly prohibited by interruption of autophagic processes required for pollen germination, thereby preventing the widespread dispersal of deleterious mutations among the progeny. By contrast, cct1-3 cct2-5 was partly transmissible via female gametophytes, so the background genome could be rescued by fertilization.