I only just learned about Arctic® apples, marketed as non-browning apples. They achieve this by adding extra copies of the polyphenol oxidase gene, thereby inducing gene silencing.
Polyphenol oxidase is the enzyme responsible for turning the white flesh of an apple brown after exposure to oxygen, by catalyzing the oxidation of polyphenols to o-quinone, which can autopolymerize and/or cross-link with other molecules that contain phenol rings including the amino acid like tyrosine to form melanin.
Physicochemical Properties and Function of Plant Polyphenol Oxidase: A Review • 2003 Jan 23 • Ruhiye Yoruk and Maurice R. Marshall • Journal of Food Biochemistry
Why do apple slices turn brown after being cut? • 2007 Jul 30 • Lynne McLandsborough • Scientific American
Aesthetics aside (I know lots of people don’t like eating brown apples even if they’re otherwise fine), I do wonder if this really extends shelf-life, since the mushiness/mealiness is due primarily to the degradation of cell walls.