High-throughput sequencing reveals that pale green lethal disorder in apple (Malus) stimulates stress responses and affects senescence

Pale green lethal (PGL) is a recessive genetic disorder of apple (Malus) characterized by severe chlorophyll deficiency and seedling lethality. Following germination, seedlings cannot photosynthesize and die at the cotyledon stage. We previously reported that the genetic and biochemical basis of PGL is due to a loss-of-function mutation in a gene required for the biosynthesis of phylloquinone (vitamin K1), a molecule essential for photosynthesis. For the present study, we used Illumina high-throughput RNA sequencing to identify genes differentially regulated between wild-type and PGL cotyledons. Changes in the expression of chlorophyll-related genes alone cannot explain the reduced chlorophyll content of PGL seedlings. However, genes putatively responding to numerous stress-related conditions including carbohydrate starvation, water deficit, and senescence were differentially regulated. This pattern of transcript accumulation suggests PGL seedlings alter many physiological and metabolic processes such as sorbitol metabolism, osmoprotectant production, and abscisic acid activity. The functions of individual genes relating to specific stresses are discussed. These findings provide insight into possible mechanisms PGL seedlings employ during stress response. Pale green lethal disorder may be a useful model for studying abiotic stress and senescence in rosaceous fruit tree species.