FREQUENCIES, TIMING, AND SPATIAL PATTERNS OF CO-SUPPRESSION OF NITRATE REDUCTASE AND NITRITE REDUCTASE IN TRANSGENIC TOBACCO PLANTS

Frequencies, timing, and spatial patterns of co-suppression of the nit rate (Nia) and nitrite (Nii) genes were analyzed in transgenic tobacco (Nicotiana tabacum) plants carrying either Nia or Nii cDNAs under the control of the 35S promoter, or a Nii gene with its own regulatory si gnals (promoter, introns, and terminator) cloned downstream of two cop ies of the enhancer of the 35S promoter. We show that (a) the frequenc ies of transgenic lines affected by cosuppression are similar for the three constructs, ranging from 19 to 25%; (b) Nia and Nii co-suppressi on are triggered stochastically during a phenocritical period of 2 wee ks between germination and flowering; (c) the timing of co-suppression (i.e. the percentage of isogenic plants affected by co-suppression re ported as a function of the number of days of culture) differs from on e transgenic line to another; (d) the percentage of isogenic plants af fected by cosuppression is increased by growing the plants in vitro pr ior to their transfer to the greenhouse and to the field; and (e) at t he end of the culture period, plants are either unaffected, completely co-suppressed, or variegated. Suppressed and nonsuppressed parts of t hese variegated plants are separated by a vertical plane through the s tem in Nia co-suppression, and separated by a horizontal plane in Nii co-suppression.