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Plant organogenesis remains one of the most essential questions of plant developmental biology. Callus tissue in vitro is a valuable tool for the studies on hormonal aspects of plant organogenesis, especially its early events, and immunohistochemical analysis is one of the few approaches offering information on the localization and role of hormones during organ development. The localization of endogenous zeatin and indole-3-acetic acid was investigated during simultaneous bud and root formation in calluses derived from immature embryos of wheat (Triticum aestivum L.). Calluses were induced on Murashige and Skoog (MS) medium supplemented with 2.0 mg L −1 2,4-dichlorophenoxyacetic acid. To stimulate simultaneous bud and root formation, calluses were transferred onto MS medium supplemented with 0.2 mg L −1 kinetin and 0.2 mg L −1 indoleacetic acid. Strong immunostaining for both hormones was detected in proliferating callus tissue, in developing meristematic centers and meristematic zones (whose cells were shown to be involved in organ formation), and at the sites of shoot and root apex initiation. During further development, shoot apexes with leaf primordia were heavily immunostained for zeatin, while immunostaining for indole-3-acetic acid was more intense at the sites of leaf primordia initiation and incipient primordia themselves. In the developing roots, immunostaining for both hormones reached a maximum in the root apex and gradually declined with increasing distance from the apex. Cells of developing procambial strands were also strongly stained for both zeatin and indole-3-acetic acid. These data suggest considerable similarity between patterns of hormone distribution in organs in vitro and in vivo. Thus, callus culture is a convenient and useful model for the study of fundamental biological questions such as how hormones regulate development.

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