Title

A genetic approach to understanding the plant hormone cytokinin: Characterization of cytokinin response and shoot-organogenesis-deficient mutant of Arabidopsis thaliana

Date of Completion

January 2001

Keywords

Biology, Genetics|Biology, Plant Physiology

Degree

Ph.D.

Abstract

Cytokinins affect almost all aspects of plant growth and development. However, cytokitnins are probably the least understood among the major plant hormones. In this research, we have used a genetic approach to study the role of cytokinin in plant growth and development. ^ The cytokinin and light response mutant (clr) has been identified in Arabidopsis thaliana as a cytokinin-resistant line with alterations in skotomorphogenesis, chlorophyll accumulation and leaf senescence. Cytokinin resistance was observed in primary root elongation, lateral root initiation, anthocyanin accumulation and shoot organogenesis. The resistance was not caused by changes in exogenous cytokinin uptake or metabolism, or changes in endogenous cytokinin levels. There was no cross-resistance to auxin, ethylene, abscisic acid or gibberellin. Darkgrown clr seedlings were partially de-etiolated, showing longer primary roots, more lateral roots, short hypocotyls, expanded cotyledons, no apical look and anthocyanin accumulation. In clr plants, chlorophyll levels were lower than wild type plants, and mature leaves did not rapidly lose chlorophyll in the dark. The clr phenotype was produced by a novel, recessive and monogenic mutation on chromosome 5. Examination of the clr phenotype indicated that one mutation can affect both light-regulated responses and cytokinin signal transduction. ^ Shoot-organogenesis-deficient mutants (sod) of Arabidopsis were selected in a different screen to work towards identifying the genetic basis of de novo shoot organogenesis. The defects in mutants sod1–sod5 may be in the early stages of cell competence and determination prior to shoot apical meristem formation, rather than in general hormone perception, signal transduction, cell division and/or organ differentiation stage. The mutants sod3 and sod5 carried monogenic recessive mutations, sod1 and sod2 carried digenic recessive mutations, and sod4 may have dominant mutation(s). The quantitative nature of the trait of shoot organogenesis has been a barrier to research. The sod mutant lines, especially the two lines (sod3 and sod5) carrying a single mutation, may be very useful in future studies dissecting the genetic basis of shoot organogenesis. ^