The Physiology of Flowering Plants - KHAM PHA MOI

310 REPRODUCTIVE DEVELOPMENTOne of the first genetically modified plants to be brought to marketwas the Flavr-Savr tomato. These plants had been modified so that theyexpressed less polygalacturonase, an important enzyme involved incell wall degradation and fruit softening. This modification did notprevent the fruit from ripening but it did limit the degree of softening.Soft tomatoes are difficult to transport and so tomatoes are normallypicked under-ripe. They are ripened artificially (see below) but do notdevelop the same flavour as vine-ripened tomatoes. The Flavr-Savrtomato could be picked later, thus improving the flavour (hence thename, although this is no excuse for the spelling!). Perhaps moreimportant, from a commercial point of view, was the increase in thesolids content of the tomatoes. This increased the yield of tomatopastes and purees – a market worth $2 billion annually in the USA.Pastes made from genetically modified tomatoes captured up to 60%of the market in the UK even though they were clearly identified asGM, but they were eventually withdrawn from sale in 1999 as a resultof public and commercial attitudes towards GM foods.Many aspects of fruit ripening are regulated by the plant hormoneethylene. Ethylene synthesis is controlled by two key enzymes, ACCsynthase and ACC oxidase, the activities of which increase early infruit ripening, leading to a massive increase in ethylene production.The fruits of tomato plants which have been genetically modified toprevent the expression of ACC synthase do not ripen and the neverripemutant of tomato has been identified as having a lesion in anethylene receptor. Ethylene is thought to trigger many of the subsequentstages of ripening, including the climacteric, and thus placingripe fruit in close proximity to unripe fruit stimulates the latter toripen. Commercially many fruit are picked unripe and stored in lowoxygen, CO 2 -enriched atmospheres which prevent ethylene-inducedripening. The fruit can then be transported long distances andripened on demand by the addition of ethylene-generating chemicals.It is this control of the ripening process which allows manyfruits to be economically shipped around the world, providing a yearroundsupply.11.14 Seed dormancy11.14.1 Late embryogenesis and the onset of dormancyOne of the principal reasons for the success of the angiosperms is theirproduction of desiccation-tolerant seed. This allows survival in a metabolicallyinactive state during periods of unfavourable environmentalconditions and facilitates widespread dispersal. The majority of angiospermspecies produce desiccation-tolerant seed but, as discussedbelow, some do not, including many commercially important species.A developing seed grows rapidly following fertilization as theembryo and surrounding tissues develop. This growth phase is associatedwith high concentrations of plant growth regulators such as