Differential expression of a chimeric CaMV-tomato proteinase Inhibitor I gene in leaves of transformed nightshade, tobacco and alfalfa plants.

The open reading frame and terminator region of a wound-inducible tomato Inhibitor I gene, regulated by the CaMV 35S promoter, was stably integrated into the genomes of nightshade (Solanum nigrum), tobacco (Nicotiana tabacum), and alfalfa (Medicago sativa), using an Agrobacterium-mediated transformation system. The expression of the foreign Inhibitor I gene in leaves of each species was studied at the mRNA and protein levels. The levels of Inhibitor I protein present in leaves of each species correlated with the levels of mRNA. The average levels of both mRNA and Inhibitor I protein were highest in leaves of transgenic nightshade plants (over 125 micrograms of Inhibitor I per g tissue), less in tobacco plants (about 75 micrograms/g tissue), and lowest in leaves of transgenic alfalfa plants (below 20 micrograms/g tissue). Inhibitor I protein was observed in all tissues throughout transgenic plant species, but inhibitor concentration per gram of tissue was 2-3 times higher in young developing leaf tissues and floral organs. The differences in the expression of the CaMV-tomato Inhibitor I gene among the different plant genera suggests that either the rate of transcription of the foreign gene or the rate of degradation of the nascent Inhibitor I mRNA varies among genera. Using electron microscopy techniques, the newly synthesized pre-pro-Inhibitor I protein was shown to be correctly processed and stored as a mature Inhibitor I protein within the central vacuoles of leaves of transgenic nightshade and alfalfa. The results of these experiments suggest that maximal expression of foreign proteinase inhibitor genes, and perhaps other foreign defense genes, may require gene constructs that are fashioned with promoters and terminators that allow maximum expression in the selected plant species.