LANTHANIDE IONS ARE AGONISTS OF TRANSIENT GENE-EXPRESSION IN RICE PROTOPLASTS AND ACT IN SYNERGY WITH ABA TO INCREASE EM GENE-EXPRESSION

Previous work has shown that in rice suspension cells, NaCl at 0.4 M c an induce Em gene expression and act synergistically with ABA, possibl y by potentiating the ABA response pathway through a rate-limiting int ermediate (R.M. Bostock and R.S. Quatrano (1992) Plant Physiol., 98, 1 356-1363). Since calcium is an intermediate in ABA regulation of stoma tal closure, we tested the effect of calcium changes on ABA-inducible Em gene expression in transiently transformed rice protoplasts. We sho w that calcium is required for ABA-inducible Em-GUS expression and can act in synergy with ABA. The trivalent ions lanthanum, gadolinium, an d aluminum, which are known to interact with calcium- and other signal ing pathways, can act at sub-millimolar concentrations to increase GUS reporter gene expression driven by several promoters in transiently t ransformed rice protoplasts. This effect is not specific for the ABA-i nducible Em promoter, but is synergistic with ABA. The lanthanum syner gy with ABA does not require calcium. In rice suspension cells, lantha num alone does not induce Em gene expression, in contrast to transient ly transformed protoplasts, yet can act synergistically with ABA to ef fectively increase the sensitivity to ABA greater than tenfold. Trival ent ions may be a useful tool to study the regulation of gene expressi on. The possible effects of trivalent ions on ABA signal transduction and gene expression are discussed.