A Comparison of Sun, Ovate, Fs8.1 and Auxin Application on Tomato Fruit Shape and Gene Expression

Elongated tomato fruit shape is the result of the action of the fruit shape genes possibly in coordination with the phytohormone auxin. To investigate the possible link between auxin and the fruit shape genes, a series of auxin (2,4-D) treatments were performed on the wild-type and the fruit shape near-isogenic lines (NILs) in Solanum pimpinellifolium accession LA1589 background. Morphological and histological analyses indicated that auxin application approximately 3 weeks before anthesis led to elongated pear-shaped ovaries and fruits, which was mainly attributed to the increase of ovary/fruit proximal end caused by the increase of both cell number and cell size. Fruit shape changes caused by SUN, OVATE and fs8.1 were primarily due to the alterations of cell number along different growth axes. Particularly, SUN caused elongation by extending cell number along the entire proximal-distal axis, whereas OVATE caused fruit elongation in the proximal area, which was most similar to the effect of auxin on ovary shape. Expression analysis of flower buds at different stages in fruit shape NILs indicated that SUN had a stronger impact on the transcriptome than OVATE and fs8.1. The sun NIL differentially expressed genes were enriched in several biological processes, such as lipid metabolism, ion transmembrane and actin cytoskeleton organization. Additionally, SUN also shifted the expression of the auxin-related genes, including those involved in auxin biosynthesis, homeostasis, signal transduction and polar transport, indicating that SUN may regulate ovary/fruit shape through modifying the expression of auxin-related genes very early during the formation of the ovary in the developing flower.