Phenotypic changes in T-cyt-transformed potato plants are consistent with enhanced sensitivity of specific cell types to normal regulation by root-derived cytokinin.

From over forty independently isolated potato lines transformed with wild-type and promoter-mutated T-cyt genes, a number of lines were selected for examination of phenotypic changes in growth and development for plants grown in soil in a controlled environment. The three lines chosen for most detailed examination showed a wide spectrum of phenotypic changes. In comparisons with control potato cv. Désirée, the plants of one line had a two- to three-fold increase in biomass production during early vegetative growth, advanced senescence and a shortened plant life-span. Another line showed abnormal cellulytic senescence. In two lines there were increases in tuber numbers and more skewed tuber size distributions which correlated with reduced shoot apical dominance and shortened dormancy of the stored tubers. None of the lines showed altered timing of onset of tuberization or flowering, although tuberization was consistently delayed when expressed as a function of increasing total plant weight. A hypothesis is proposed to explain the diverse phenotypes which postulates that (1) T-cyt transformation causes enhanced sensitivity to cytokinins in specific types of shoot cells which are already targets for regulation by normal root-derived cytokinins; (2) two distinct types of shoot target cells are present, one in shoot meristems and one in leaves; (3) the two types can acquire enhanced sensitivity, either separately or in combination depending on the particular T-cyt transformation event. The scope for using the transformed plants in subsequent physiological, biochemical and molecular studies, aimed at examining the molecular basis of the model or selected consequences of T-cyt transformation in altering regulation of potato plant growth and development, is discussed. The attention is drawn to the possible involvement at the subcellular level of sucrose phosphate synthase in mediating the phenotypic effects caused by T-cyt transformation.