octopine A rare amino acid derivative produced by a certain type of crown gall tissue. The octopine synthase gene is located in the T-DNA of the Ti plasmid of Agrobacterium tumefaciens.

off-type (1) Plants or seeds that do not conform to the characteristics of a variety as described by the breeder. (2) Plants or seeds that are not necessarily of any variety. (3) Plants or seeds resulting from (i) cross-pollination by other kinds or varieties, (ii) uncontrolled self-pollination during production of hybrid seed, or (ii) segregates from any of the above plants. See also roguing.

opaque-2 An endosperm mutant of maize associated with suppressed prolamine production in the endosperm which results in increased lysine content of the protein fraction.

open continuous culture A continuous culture in which inflow of fresh medium is balanced by outflow of a corresponding volume of culture. Cells are constantly washed out with the outflowing liquid. In a steady state, the rate of cell washout equals the rate of formation of new cells in the system. See also turbidostat.

open storage Storage (e.g., of seed) with free access to normal atmospheric conditions.

operator The region of DNA preceding the coding sequence of a gene to which a repressor molecule will bind. The operator comes just after, or overlaps with, the promoter of a gene.

opine The class name for rare derivatives (e.g., octopine, nopaline) from certain amino acids and sugars found in some crown gall tumors.

organ culture The maintenance or growth of organ primordia or the whole or parts of an organ in vitro, so as to allow differentiation and preservation of the architecture and/or function.

organelle genome The DNA of chloroplasts or mitochondria. The chloroplast genome ranges in size from 120 to 200 kb, and the mitochondrial genome in higher plants ranges from ~200 to >2200 kb. These genomes exist as circular chromosomes lacking structural proteins and may be present in many copies per cell. Small linear or circular plasmids may also be present in mitochondria.

organogenesis The process of initiation and development of a structure that shows natural organ form and/or function (e.g., roots or shoots), the initiation of which is removed in time from the initiation of other organs. Such organs may be adventitious or de novo in origin. See also embryogenesis.

organogenesis 2 The process of initiation and development of callus into one distinct organ (e.g., roots or shoots). Initiation is removed in time and space from the initiation of other organs. Such organs may be adventitious or de novo in origin.

origin of replication The site on a DNA molecule at which DNA replication is initiated. Abbr.: ORI.

ortet The original plant from which the members of a clone have descended. Compare ramet.

outcrossing The contamination of a seed production field resulting from pollen of an undesirable plant strain pollinating the stigma of the parent seed plant.

overseeding Seeding into an existing turf. See also winter overseeding.