Title

Role of hormones and signal transduction through inositol 1,4,5-trisphosphate in hormone-induced tracheary element differentiation

Date of Completion

January 1999

Keywords

Biology, Molecular|Biology, Plant Physiology

Degree

Ph.D.

Abstract

Mesophyll cells of Zinnia elegans L. cv. Envy, when cultured in the presence of auxin and cytokinin, differentiate into tracheary elements (TEs). Differentiation involves production of secondary cell wall and programmed cell death as mature TEs are hollow conducting vessels of the xylem. My results support that extracellular Ca2+ is required early in TE differentiation. If 300 μM Ca2+ was added to the medium at different times after establishing the culture, the maximum number of TEs could be obtained when Ca2+ was added in the first 6 hour of culturing. Levels greater than 300 μM did trigger differentiation, but the process was delayed. If cultures were initiated in 300 μM extracellular Ca2+ and then transferred to 0.5 μM extracellular Ca2+, TE differentiation was inhibited, and the earlier the transfer, the fewer the TEs which could be recovered, particularly the first 6 hours of culturing. If cells were cultured in plasticware to reduce leaching of Ca2+ into the medium no TEs developed when cells were transferred from 300 μm Ca2+ to Ca2+-free medium, even if the transfer was after 48 hours, suggesting that a minimal level of extracellular Ca2+ is required throughout the differentiation process. An increase in phospholipase C (PLC) activity and inositol 1,4,5-trisphosphate (IP3) levels was found 4 hours after application of auxin and cytokinin to the culture. The PLC inhibitors U73122 and U73343 prevented the increase in IP3 levels and inhibited TE differentiation, with similar dose-response curves. Furthermore, pertussis toxin, a G-protein inhibitor, both blocked IP3 accumulation and inhibited TE differentiation. ^ It was thought auxin was the major force to cause TE formation, however, I found that cytokinin alone can stimulate TE differentiation, but in a much slower rate and a far lower level than in the presence of both auxin and cytokinin. Auxin cannot trigger TE formation by itself. Therefore, I propose that auxin synergistically enhance cytokinin-induced TE formation. Either auxin or cytokinin is capable of expanding cells. Cytokinin is only required in the beginning of cell culture. Moreover, it is found that cytokinin can stimulate IP 3 production, whereas auxin is not able to cause great change in IP 3 levels. It has also been found that cytokinin can remain cells to be competent to differentiate longer than auxin. ^