How does the plant make the fruit?

First, lets define what fruit is.

Fruit Definition

noun, plural: fruits

(1) (botany) The seed-bearing structure in angiosperms formed from the ovary after flowering.

(2) The edible, usually fleshy and sweet smelling part of a plant that may or may not contain seed(s).

(3) The offspring from a sexual union.

http://www.biology-online.org/dictionary/Fruit


Now for how they grow: (Link to full article is at the bottom of this necessarily long answer.)

Fruit Development

Contributions of different flower parts to the fruit

Most fruit develops from the ovary. In fact some schemes classify fruit derived from a single ovary as “true fruits” while “false fruits” are composed of tissues derived from flower parts other than the ovary or from more than one ovary.

In “true fruits” the outside of the fruit is called the pericarp and develops from the ovary wall. The pericarp can be dry and papery, like in maple or dandelions, woody like in nuts or fleshy as in berries (grapes and tomatoes) and stone fruits (cherries and peaches). These pericarp differences reflect adaptations to different dispersal mechanisms (eg. wind for papery pericarps, animal consumption for fleshy fruits). The fruit can contain a single seed as in corn, or many seeds like a pea pod or pumpkin. The pericarp of some fruits is further differentiated into specialized layers called exocarp, meso- and endocarp. For example in citrus the rind is the exocarp, the white covering is the mesocarp and the juice sacs are the endocarp.

Many fruits we consider berries, such as raspberries and strawberries, are botanically not classified as berries. Raspberries are examples of aggregate fruits. Each juicy little sphere is actually an individual fruit of the same class as cherries, and what we consider as the fruit is really an aggregation of fruits.

Strawberries and apples are examples of accessory fruits, where some of the fleshy tissue is derived from flower parts other than the ovary. Strawberry fruits are actually what we consider the seeds. They are called achenes, which are dry fruits in the same category as dandelions. The fleshy part that we eat develops from the receptacle. Most of the fleshy tissue in apples develops from the hypanthium which is a region of the flower where sepals, petals and stamens are all fused to the ovary. Thus all floral organs contribute to the fleshy portion of apples.

Phases of fruit development

Fruit development can generally be considered to occur in four phases: fruit set, a period of rapid cell division, a cell expansion phase, and ripening/maturation.

Fruit set involves the decision whether to abort the ovary or proceed with fruit development. Fruit set is normally dependent on pollination. Pollen triggers fruit development indicating that positive signals are generated during pollination. In the absence of these signals, the flowers abscise. Growing pollen produces GA and application of GA can induce parthenocarpic fruit, therefore it is believed that GA is a triggering signal. Lagging slightly behind the growing pollen tube is a wave of increased auxin production by the style and then the ovary. Auxin application can also induce parthenocarpy and so it is thought that GA acts by inducing auxin production. However, most GA deficient mutants are able to produce fruit indicating that this is not the sole mechanism to induce fruit development and in an auxin insensitive tomato mutant, fruit growth is normal.

Continued fruit development usually relies on the continued presence of developing seeds. Seed abortion or removal causes fruit abortion, which can be reversed with auxin application. For example. removal of strawberry “seeds” prevents the development of the receptacle as a “fruit” but if auxin is applied following seed removal, fruit development continues. Commercial crops that produce parthenocarpic (seedless) fruits, such as bananna, often show quantitaive or qualitative differences in GA or auxin content in the ovary when compared to nonparthenocarpic varieties.

The phase of rapid cell division involves all growing parts of the fruit. This is thought to be controlled by the developing seeds. The number of fertilized ovules in a fruit is correlated with both the initial cell division rate and the final size of the fruit. Also, fruits with an uneven distribution of seeds are often lopsided. There is a correlation between cytokinin levels in developing embryos and cell division in surrounding tissues but there is no direct evidence that embryo cytokinin in fact regulates fruit cell division. It is difficult to reconcile the complete development of parthenocarpic fruit with the requirement of embryos for cell division except to say that parthenocarpy represents an abnormal situation.

The cell division phase gradually shifts into the cell expansion phase. The rate and duration of cell division varies among fruits and also among tissues within a fruit. Tissues made up of many small cells at maturity continue dividing while tissues composed of large cells have begun expanding. In tomato the cell division phase lasts approximately 7-10 days while cell expansion lasts 6-7 weeks. Cell expansion accounts for the largest increase in fruit volume, often contributing in excess of a 100 fold size increase. Gibberellins are also associated with fruit expansion and removal of the seeds from pea pods inhibited GA biosynthesis in the pericarp. Many believe that auxins from seeds regulate cell expansion of the pericarp, but auxin application does not always compensate for seed removal, and in an auxin insensitive tomato mutant, fruit growth is normal.

Fruit ripening

Ripening represents the shift from the protective function to dispersal function of the fruit. Ripening occurs synchronously with seed and embryo maturation, as described in the lecture on embryo development. In dry fruits (cereals, nuts, dandelions) ripening consists of desiccation and is considered maturation. Ripening in fleshy fruits is designed to make the fruit appealing to animals that eat the fruit as a means for seed dispersal. Ripening involves the softening, increased juiciness and sweetness, and color changes of the fruit. Fleshy fruits are either climacteric or non-climacteric. Climacteric fruits produce a respirative burst with a concomitant burst in ethylene synthesis, as the fruits ripen. These include fruits with high degrees of flesh softening, like tomato, banana, avacado, peach etc.

Ripening has been most intensively studied in tomato. Ethylene is a major regulator of the ripening process. Inhibitioin of ethylene with inhibitors, transgenic approaches or mutants blocks ripening. Exogenous ethylene accelerates ripening. There are also developmental factors involved because fruit does not attain competence to respond to ethylene until near the end of the cell expansion phase (the mature green stage). Several genes associated with ripening are ethylene inducible. This occurs transcriptionally in most genes but at least one is known where mRNA accumulation is regulated post-transcriptionally. None of these genes are induced until competence for ethylene response is attained.

The tomato never-ripe mutation blocks fruit ripening and is insensitive to ethylene. The mutated gene is similar to the ethylene receptor isolated from arabidopsis, suggesting that never-ripe is an ethylene receptor mutant. NR mRNA is not expressed until the mature green stage, suggesting that lack of this ethylene receptor might be related to the lack of competence to respond to ethylene at earlier stages.

Ethylene production is autocatalytic. That is, exposure to ethylene stimulates the synthesis of more ethylene. This occurs because the genes for the biosynthetic enzymes (e.g. ACC SYNTHASE) are ethylene inducible. The result is a positive feedback loop. Furthermore, the Never-ripe gene is ethylene inducible, resulting in a positive feedback loop for ethylene sensitivity as well. Both these factors contribute to the dramatic burst of ethylene production during ripening.

Fruit softening involves a partial breakdown of cell walls. Several enzymes are known to be involved in this process. Polygalacturonase hydrolyzes bonds in pectins. The gene for this enzyme is ethylene inducible.

Changes in fruit color involve changes in the expression of pigment biosynthetic genes. The major pigment in tomato is a carotenoid. The first committed step in carotenoid biosynthesis is catalyzed by phytoene synthase, and the gene for this enzyme is induced by ethylene.

http://www.public.iastate.edu/~bot.512/lectures/seed&fruit.htm