In agriculture hybrids are used extensively due to their superior performance in a number of traits such as seed yield, yet the molecular mechanisms underpinning their performance are not understood. Recent evidence has suggested that a decrease in basal defence response genes regulated by reduced levels of the phytohormone salicylic acid (SA) may be important for hybrid vigour. Decreasing levels of SA in the Arabidopsis accession C24 through the introduction of the SA catabolic enzyme NahG results in increased plant size phenocopying the increased size of C24/Ler F1 hybrids. The increased growth of C24 NahG occurs during late development. Our transcriptome analysis of F1 hybrids and C24 NahG identified shared pathways associated with their size increase including decreased expression of defence response genes, SA biosynthetic genes, and SA response genes. In both C24 NahG and F1 hybrids we found decreased expression of key senescence transcription factors WRKY53, NAP and ORE1 and delayed senescence compared to C24. The delay in senescence resulted in an extension of active photosynthesis in the leaves of F1 hybrids compared to the parental lines allowing each leaf to produce more resources for the growth process. Overall design: Samples analyzed included C24, C24B, Ler, C24 x Ler, C24 NahG 2, C24 NahG 3 and Ler NahG. C24, Ler, Ler NahG and C24xLer all had three biological replicates. C24B, C24 NahG 2 and C24 NahG 3 had two biological replicates. C24 NahG 2 and C24 NahG 3 are two indepedent transgenic lines which ere sequenced at a later stage than the other samples. These and were analyzed separately (eg. C24 B versus C24 NahG 2). All biological replicates were made from 6-10 plants.