Yet the molecular identities of such output components, which are likely to encode proteins directly involved in growth control, have remained elusive. Here we report the discovery of a suite of tropic stimulus-induced genes in Brassica oleracea that are responsive to an auxin gradient and exhibit morphologically graded expression concomitant with, or before, observable curvature responses. These results provide compelling molecular support for the Cholodny-Went theory and suggest that morphologically graded transcription represents an important mechanism for interpreting tropically stimulated gradients of auxin. Intriguingly, two of the tropic stimulus-induced genes, EXPA1 and EXPA8, encode enzymes involved in cell wall extension, a response prerequisite for differential growth leading to curvatures, and are up-regulated before curvature in the flank that will elongate. This observation suggests that morphologically graded transcription likely leads to the graded expression of proteins whose activities can directly regulate the establishment and modulation of tropic curvatures.gravitropism ͉ NPH4͞ARF7 ͉ phototropism

In an attempt to compensate for their sessile nature, plants have developed growth responses to deal with the copious and rapid changes in their environment. These responses are known as tropisms and they are marked by a directional growth response that is the result of differential cellular growth and development in response to an external stimulation such as light, gravity or touch. While the mechanics of tropic growth and subsequent development have been the topic of debate for more than a hundred years, only recently have researchers been able to make strides in understanding how plants perceive and respond to tropic stimulations, thanks in large part to mutant analysis and recent advances in genomics. This paper focuses on the recent advances in four of the best-understood tropic responses and how each affects plant growth and development: phototropism, gravitropism, thigmotropism and hydrotropism. While progress has been made in deciphering the events between tropic stimulation signal perception and each characteristic growth response, there are many areas that remain unclear, some of which will be discussed herein. As has become evident, each tropic response pathway exhibits distinguishing characteristics. However, these pathways of tropic perception and response also have overlapping components -a fact that is certainly related to the necessity for pathway integration given the everchanging environment that surrounds every plant.

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