Endogenous cytokinins in shoots of Aloe polyphylla cultured in vitro in relation to hyperhydricity, exogenous cytokinins and gelling agents

The process of hyperhydricity in tissue cultured plants of Aloe polyphylla is affected by both applied cytokinins (CKs) and the type of gelling agent used to solidify the medium. Shoots were grown on media with agar or gelrite and supplemented with different concentrations of N-6-benzyladenine (BA) or zeatin (0, 5 and 15 mu M). Endogenous CKs were measured in in vitro regenerants after an 8-weeks cycle to examine whether the hyperhydricity-inducing effect of exogenous CKs and gelling agents is associated with changes in the endogenous CK content. On media with agar a reduction in hyperhydricity occurred, while the gelrite treatment produced both normal and hyperhydric shoots (HS). The content of endogenous CKs, determined by HPLC-mass spectrometry, in the shoots grown on CK-free media comprised isopentenyladenine-, trans-zeatin- and cis-zeatin-type CKs. The application of exogenous CKs resulted in an increase in the CK content of the shoots. Following application of zeatin, dihydrozeatin-type CKs were also detected in the newly-formed shoots. Application of BA to the media led to a transition from isoprenoid CKs to aromatic CKs in the shoots. Shoots grown on gelrite media contained higher levels of endogenous CKs compared to those on agar media. Total CK content of HS was higher than that of normal shoots grown on the same medium. We suggest that the ability of exogenous CKs and gelrite to induce hyperhydricity in shoots of Aloe polyphylla is at least partially due to up-regulation of endogenous CK levels. However, hyperhydricity is a multifactor process in which different factors intervene.