Seedless fruit is welcomed by many consumers of fruits and vegetables, and can be obtained through parthenocarpy if fruit develops without fertilization and by stenospermocarpy if the seeds abort after fertilization. SlTIR1 up-regulation lines exhibited parthenocarpy and stenospermocarpy phenotypes in Micro-Tom and Ailsa Craig, respectively. The same genes up-regulated in the two cultivars exhibited different seedless fruit phenotypes, indicating that the phenotypic differences may be due to the genetic backgrounds of the two cultivars. Micro-Tom is a dwarf variety of tomato, and has been proposed as a preferred cultivar for molecular research in tomato. The miniature dwarf phenotype of Micro-Tom is caused by three major recessive gene mutations together. Two mutant genes have been identified: SELF-PRUNING (SP) and DWARF (D). The SP gene is associated with the transformation of vegetative and reproductive growth of the apical meristem and the D gene is involved in biosynthesis of a key enzyme producing brassinosteroids. MNT (miniature) gene, associated with the GA signal transduction, may be the third mutated gene [25] and [29]. Thus, it is possible that up-regulation of SlTIR1 resulted in different phenotypes of Micro-Tom and Ailsa Craig, especially in fruit development, mainly because of differences in SP, D and MNT genes in the two cultivars. Further study is necessary to elucidate the differences in seedless fruit formation between Micro-Tom and Ailsa Craig.
Our work demonstrates that SlTIR1 regulated auxin-induced root growth and stimulated stenospermocarpic fruit formation in tomato. Furthermore, SlTIR1 may be a key mediator of the crosstalk among auxin and other hormones to co-regulate plant growth and development.