Embryogenic induction in pollen grains of Nicotiana tabacum L.

Abstract

The process of embryogenic induction in pollen grains of tobacco was investigated through direct pollen culture. Pollen grains at the late unicellular to middle bicellular stages could develop into seedlings through a process of embryogenesis when they were cultured in sucrose-free medium for several days and then transferred into Nitsch's liquid medium containing sucrose, as described by GHANDIMATHI (1982) and IMAMURA et al. (1982). Based on microscopic observations of cultured pollen grains and experiments where 3H-thymidine was incorporated into the pollen grains, it was revealed that pollen grains accumulated starch granules resulting in pollen germination and that DNA was synthesized only in the generative nucleus, sugbaesting that they continued their gametophytic development in the medium containing sucrose. On the other hand the gametophytic development was markedly suppressed in the sucrose free-medium. These finidings indicate that the initial event of embryogenic induction is the suppression of the gametophytic development of the pollen grains and that embryogenic induction proceeds successfully in the absence of sucrose in the medium. Furthermore, the results of 3H-thymidine incorpora-tion showed that DNA synthesis in the vegetative nucleus, which was assumed to be the DNA synthesis leading to pollen embryogenesis, occurred soon after the suppression of the gametophytic development caused by the absence of sucrose. Since cultured cells generally do not undergo the phase of DNA synthesis when starved of sugar, it is inferred that pollen grains of tobacco have the inherent ability to synthesize DNA in the absence of sugar. It is concluded that the process of embryogenic induction in pollen embryogenesis consists of two phases, namely, the suppression of the gametophytic development caused by sugar starvation and DNA synthesis leading to pollen embryoogenesis which proceeds depending on the inherent properties of pollen grains.