Instructive roles for hormones in plant development

Abstract

Plants, like animals, construct their body following modular sets of instructions that determine cell fate, morphogenesis and patterning, among other building requirements. Hormones regulate plant growth in different ways, and there is increasing evidence for a decisive function of certain hormones in the establishment of developmental programs, equivalent to the role of peptidic molecules and signals of another nature in animal embryo development. Here, we review this role of hormones as instructive agents, and illustrate it with examples such as the generation of morphogenetic gradients by auxin (which determine organ patterning and phyllotaxis), the specification of cell fate at the shoot meristem by gibberellins and cytokinins, the switch between alternative developmental programs (photo-and skotomorphogenesis) by gibberellins and brassinosteroids, and the decision between pistil senescence or fruit growth after anthesis. KEY WORDS: gibberellin, auxin, cytokinin, patterning, differentiation Hormones in plant developmentSince the discovery of the first few plant hormones, these compounds have been attributed numerous roles regulating the physiology, growth responses caused by stress and development of plants. However, they do not always act as their metazoan counterparts, for which the term "hormone" was coined. One of the basic differences is that, unlike in animals, plant hormones are not synthesized in a particular and specialized organ and then transported to the target organs. Rather, the ability to synthesize hormones is general to all plant tissues, and very often hormones act in the same cells where they are produced (Bishopp et al., 2006). Plant hormones are also referred to with the generic name of plant growth regulators, to stress the fact that their function as a group is not restricted to precise physiological processes. For instance, auxins have been shown to increase cell division rate; gibberellins and brassinosteroids promote cell expansion; abscisic acid induces the physiological state of dormancy; and ethylene is required for the maturation of fleshy fruits. Nevertheless, there is increasing evidence that hormones act in many cases as an "instructive" signal in plant development, and this particular role is surprisingly similar to the action driven by peptide hormones and proteins during animal development, such as Notch and BMP (Ehebauer et al., 2006, Raftery andSutherland, 2003). The key issue is that, in this instructive role, plant hormones do not act as mere transducers or messengers of other higher rank signals, nor do they promote growth in Int. J. Dev. Biol. 53: 1597-1608 (2009) general. Instead, the information provided by these hormones is the basis for deciding between two alternative developmental programs, or establishing domains of identity within a given tissue.In this review, we wish to underscore the role of hormones as instructive signals in plant development, illustrating it with only a few examples: how auxin acts as a bona fide morphogen in establishing s...

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