Mutation of class C: No stamens or carpels; petals and sepals instead

Mutation of class B: No petals or stamens; sepals and carpels instead

Mutation of class C: No stamens or carpels; petals and sepals instead

A gene called leafy codes for a protein that controls the transcription of the ABC genes. Plants with a mutation that causes the underexpression of leafy are just that—they make leaves, but no flowers. The protein product of this gene acts as a transcription factor stimulating genes A, B, and C so that they produce flowers (Figure 19.13). This finding, too, has practical applications. It usually takes 6-20 years before a citrus tree produces flowers, and thus the fruits we eat. Scientists have made an orange tree transgenic for leafy coupled to a strongly expressed promoter, which flowers and fruits years earlier than a normal tree.

Morphogen gradients provide positional information

During development, cells often need to "know" where they are with respect to the body as a whole, as in the case of the whorls in the flowers just described. This spatial "sense" is called positional information. Positional information usually comes in the form of a signal, called a morphogen, that diffuses from one group of cells down a body axis, setting up a concentration gradient. There are two requirements for a signal to be considered a morphogen:

► It must directly affect target cells, rather than triggering a secondary signal that affects target cells.

► Different concentrations of the signal must cause different effects.

19.13 A Nonflowering Mutant

Mutations in the leafy gene of Arabidopsis prevent the transcription of the organ identity genes, and the resulting plant does not produce any flowers.

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