Polyamines Alter Fruit Size and Shape By Regulating Cell Division and Expansion in Tomato

Several genes regulating fruit size and shape have been identified, but the signals regulating these processes in fruit are not yet known. Polyamines (mainly putrescine, spermidine, and spermine) are small ubiquitous biogenic amines that play an essential role in eukaryotic cell division, but their role in determining plant architecture is not yet investigated. We have developed transgenic tomato lines by expressing yeast spermidine synthase under constitutive or fruit-specific promoters that exhibit higher spermidine and spermine levels. Fruits from these genetically altered tomato lines were digitally characterized for 40 different morphometric attributes, including fruit shape, blockiness, and pericarp thickness. High polyamines fruits exhibited increase in pericarp thickness ratio and height-to-width ratio, but reduced perimeter and proximal fruit-end blockiness, imparting more obovoid fruit shape compared to wild type (WT) fruits. Microscopic analysis of ovaries with high PAs indicated that polyamines alter fruit size and shape during early fruit development. The qRT-PCR showed that transcript levels of SUN and OVATE, two fruit architecture-related genes were up- and down-regulated respectively, during 10 days post-anthesis (DPA) by polyamines. Transcripts of FW2.2, a negative regulator of cell division, were not detected. Among the genes involved in cell cycle progression, CDKA1 was upregulated whereas CYCD3 and CYCA2 were down-regulated from 5 days before anthesis to 20 DPA in PA accumulating fruits. The transgenic fruits also showed transient increase in transcript levels of CDKB2 and CYCB2 at 2 and 5 DPA, respectively. CDK-inhibitor KRP1 was up regulated at 2 DPA before down regulation thereafter in high PA fruits. An increase in transcripts for WEE1, an endoreduplication associated gene, was also observed at 5DPA in one of the transgenic lines suggesting a possible role of endo-reduplication in this phenotype. Collectively, our results suggest that PAs influence fruit development by regulating both cell division and expansion.