The role of FRUITFULL-like genes in the tomato flowering regulatory network

Tomato is economically an important vegetable crop, and its fruit yield is among others determined by the timing of flowering, inflorescence architecture and shoot determinacy. These traits are biological consequences of the activities of reproductive meristems, which are regulated by endogenous conditions, phytohormones and flowering pathway genes. The FUL gene, encoding a pleiotropic MADS domain transcription factor in Arabidopsis, has been implicated in different flowering pathways. Its homologs play conserved roles in flowering and inflorescence development in many other species but had not yet been thoroughly investigated in tomato. The tomato genome carries four FUL-like genes (SlFULs: FUL1, FUL2, MBP10, MBP20), among which only FUL1 and FUL2 are known to be involved in regulation of fruit development. The aim of the studies described in this thesis is to investigate the functions of the SlFUL genes in flowering and inflorescence development, and their redundancy/interaction with MC and SP, as well as their regulation of downstream target genes, such as the SlCKXs. To do so, we employed CRISPR mutagenesis, transcriptome analysis, protein-protein interaction analysis, gene expression analysis, and investigated meristem development in detail. In Chapters 2 and 3, we found that knock-out of both SlFULs and CKXs could lead to additional formation of IMs, but the identity of the specified IMs was not affected. However, previous studies had shown that mutation of the close SlFUL homolog MC did cause inflorescence vegetative reversion after the production of a few flowers. This sparked us to study the unique and overlapping functions of SlFUL and MC in the regulation of meristem phase change and meristem identity specification during reproductive development. In Chapter 4, we show that combined mutations of the AP1/FUL-like genes cause delayed FM maturation that allows additional formation of flanking meristems, which adopt a vegetative fate instead of IM fate, leading to a massively bushy inflorescence. Moreover, the late primary shoot flowering was mildly enhanced, but the sympodial shoot flowering was massively delayed in mc ful2 mbp20 mutants. We concluded that FUL2/MBP20 and MC function redundantly in both the timing of flowering and the specification of FM/IM identity, with a more prominent role for FUL2/MBP20 in the former and for MC in the latter. Based on Chapters 2 and 4, we attempted to make use of the loss of AP1/FUL-like genes to counteract mutations in the flowering repressor SP and achieve an optimal balance of flowering signals to optimize tomato plant architecture for yield improvement. In Chapter 5, we demonstrate that loss of MBP20 restores the indeterminate growth of sp mutants to a large extent, and the faster sympodial cycling gave an enhanced inflorescence complexity and compactness, which resulted in more flowers and improved fruit yield. The observed intermediate phenotypes suggest that SP and MBP20 act in parallel pathways in the determination of sympodial shoot flowering time.