How is plant growth controlled, and what changes in growth patterns made the new strains of wheat and rice successful?

In normal wheat plants, gibberellins stimulate stem elongation. But in the semi-dwarf plants, a mutation affects the signal transduction mechanism for gibberellins so that the stem cells do not respond to it and growth is reduced. In rice, the mutation is in the gene for an enzyme in the biochemical pathway for the synthesis of gibberellins. Without the hormone, the stem does not elongate. The lives of countless people have been saved by intentional disruptions of hormone signaling. These high-yielding crops respond to added fertilizer with greatly increased growth and production of grain compared with conventional varieties of wheat and rice.

Future directions

Unlike animal hormones, which are made in one tissue and often have specific effects on a limited range of targets, plant hormones are widespread in their sites of synthesis and their effects. It is not surprising that with several hormones in a given tissue at a particular time, they interact to produce a physiological effect, a phenomenon called crosstalk. For example, as a seed germinates underground, the growth of the embryonic stem, or hypocotyl (see photo on p. 776, right column), is stimulated by auxin and gibberellins, and inhibited by ethylene and brassinosteroids. Studies on model plants such as Arabidopsis are revealing proteins common to the signaling pathways of these hormones that have a role in their interacting effects.