Microtopography and Shoot-bud Formation of Rice (Oryza sativa) Callus

Abstract

The scutellar epithelium of rice seeds shows a variety of activities under different physiological conditions. The epithelial cells cease activity soon after the start of germination, but in the seminal root culture system, they retain their haustorial property for a long period. Furthermore, on solid medium with added 2, 4-D, they divide to form callus masses that lack a cuticular layer. The two-step method of callus culture results in a higher frequency of shoot-bud formation that is related to topographical variation on the callus surface. The development of a compact callus with white and green patches is a morphological prerequisite for shoot-bud formation. The cells constituting a callus mass behave differently according to the positional information determined by the internal and external environment. The external appearance of the callus largely depends on the extracellular matrix. The callus surface consists of a fragile slime-like membrane, firm fibrils, and a thin epicuticular layer. Finally, it is suggested that the newly formed cuticular layer on the callus surface is essential for shoot-bud formation. Consequently, we propose that the topographical characteristics of the callus mass play an important role in shoot-bud formation.