INCREASE OF ENDOGENOUS ZEATIN RIBOSIDE BY INTRODUCTION OF THE IPT GENE IN WILD-TYPE AND THE LATERAL SUPPRESSOR MUTANT OF TOMATO

We studied axillary meristem formation of the lateral suppressor (ls) mutant of tomato after elevating the endogenous cytokinin levels throu gh introduction of the isopentenyltransferase (ipt) gene from Agrobact erium tumefaciens. Growth and development of several transformants wer e examined during in vitro culture. Transformants exhibited phenotypes varying in severity and were divided into four classes. A number of t he ipt transformants had a normal phenotype, as non-transformed plants . Others showed a mild to severe 'cytokinin-like' phenotype. Transform ants with a mild phenotype exhibited reduced internode length and redu ced root development. Transformants with a severe phenotype showed eve n shorter internodes, loss of apical dominance, reduction of leaf size , production of callus at the basis of the shoots and absence of root development or development of green non-branching roots. The severity of the phenotype correlated well with the level of ipt gene expression , as measured by northern analysis. Transformants with a severe phenot ype also exhibited increased levels of zeatin riboside, but zeatin lev els were not elevated. The increase in endogenous zeatin riboside leve ls in the ls mutant did not restore axillary meristem formation, but s ometimes bulbous structures were formed in the initially 'empty' leaf axils. Several adventitious meristems and shoots developed from below the surface of these structures. It is concluded that a reduced level of cytokinins in the ls mutant shoots is not responsible for the absen ce of axillary meristem formation.