1-Methylcyclopropene: a review

We investigated the feedback regulation of ethylene biosynthesis in tomato (Lycopersicon esculentum) fruit with respect to the transition from system 1 to system 2 ethylene production. The abundance of LE-ACS2, LE-ACS4, and NR mRNAs increased in the ripening fruit concomitant with a burst in ethylene production. These increases in mRNAs with ripening were prevented to a large extent by treatment with 1-methylcyclopropene (MCP), an ethylene action inhibitor. Transcripts for the LE-ACS6 gene, which accumulated in preclimacteric fruit but not in untreated ripening fruit, did accumulate in ripening fruit treated with MCP. Treatment of young fruit with propylene prevented the accumulation of transcripts for this gene. LE-ACS1A, LE-ACS3, and TAE1 genes were expressed constitutively in the fruit throughout development and ripening irrespective of whether the fruit was treated with MCP or propylene. The transcripts for LE-ACO1 and LE-ACO4 genes already existed in preclimacteric fruit and increased greatly when ripening commenced. These increases in LE-ACO mRNA with ripening were also prevented by treatment with MCP. The results suggest that in tomato fruit the preclimacteric system 1 ethylene is possibly mediated via constitutively expressed LE-ACS1A and LE-ACS3 and negatively feedback-regulated LE-ACS6 genes with preexisting LE-ACO1 and LE-ACO4 mRNAs. At the onset of the climacteric stage, it shifts to system 2 ethylene, with a large accumulation of LE-ACS2, LE-ACS4, LE-ACO1, and LE-ACO4 mRNAs as a result of a positive feedback regulation. This transition from system 1 to system 2 ethylene production might be related to the accumulated level of NR mRNA.

Ethylene, jasmonate, and salicylate play important roles in plant defense responses to pathogens. To investigate the contributions of these compounds in resistance of tomato (Lycopersicon esculentum) to the fungal pathogen Botrytis cinerea, three types of experiments were conducted: (a) quantitative disease assays with plants pretreated with ethylene, inhibitors of ethylene perception, or salicylate; (b) quantitative disease assays with mutants or transgenes affected in the production of or the response to either ethylene or jasmonate; and (c) expression analysis of defense-related genes before and after inoculation of plants with B. cinerea. Plants pretreated with ethylene showed a decreased susceptibility toward B. cinerea, whereas pretreatment with 1-methylcyclopropene, an inhibitor of ethylene perception, resulted in increased susceptibility. Ethylene pretreatment induced expression of several pathogenesis-related protein genes before B. cinerea infection. Proteinase inhibitor I expression was repressed by ethylene and induced by 1-methylcyclopropene. Ethylene also induced resistance in the mutant Never ripe. RNA analysis showed that Never ripe retained some ethylene sensitivity. The mutant Epinastic, constitutively activated in a subset of ethylene responses, and a transgenic line producing negligible ethylene were also tested. The results confirmed that ethylene responses are important for resistance of tomato to B. cinerea. The mutant Defenseless, impaired in jasmonate biosynthesis, showed increased susceptibility to B. cinerea. A transgenic line with reduced prosystemin expression showed similar susceptibility as Defenseless, whereas a prosystemin-overexpressing transgene was highly resistant. Ethylene and wound signaling acted independently on resistance. Salicylate and ethylene acted synergistically on defense gene expression, but antagonistically on resistance.