USE OF A TOMATO MUTANT CONSTRUCTED WITH REVERSE GENETICS TO STUDY FRUIT RIPENING, A COMPLEX DEVELOPMENTAL PROCESS

Fruit ripening is one of the most dramatic developmental transitions a ssociated with extensive alteration in gene expression. The plant horm one ethylene is considered to be the causative ripening agent. Transge nic tomato plants were constructed expressing antisense or sense RNA t o the key enzyme in the ethylene (C2H4) biosynthetic pathway, 1-aminoc yclopropane-1-carboxylate (ACC) synthase using the constitutive CaMV 3 5S and fruit specific E8 promoters. Fruits expressing antisense LE-ACS 2 RNA produce less ethylene and fail to ripen only when ethylene produ ction is suppressed by more than 99% (>0.1 nl/g fresh weight). Ethylen e production is considerably inhibited (50%) in fruits expressing sens e LE-ACS2 RNA. Antisense fruits accumulate normal levels of polygalact uronase (PG), ACC oxidase (pTOM 13), E8, E17, J49, and phytoene desatu rase (D2) mRNAs which were previously thought to be ethylene-inducible . E4 gene expression is inhibited in antisense fruits and its expressi on is not restored by treatment with exogenous propylene (C3H6). Antis ense fruits accumulate PG mRNA, but it is not translated. Immunoblotti ng experiments indicate that the PG protein is not expressed in antise nse fruits but its accumulation is restored by propylene (C3H6) treatm ent. The results suggest that at least two signal-transduction pathway s are operating during tomato fruit ripening. The independent (develop mental) pathway is responsible for the transcriptional activation of g enes such as PG, ACC oxidase, E8, E17, D2, and J49. The ethylene-depen dent pathway is responsible for the transcriptional and posttranscript ional regulation of genes involved in lycopene, aroma biosynthesis, an d the translatability of developmentally regulated genes such as PG. ( C) 1993 Wiley-Liss, Inc.