Role of the tomato JOINTLESS gene in inflorescence develoment

Tomato is an important crop worldwide and knowledge about the genetic determinants of its yield is highly valuable for further improvement. The plant has a sympodial shoot architecture, producing about 6 to 12 leaves on the primary stem before the shoot apical meristem (SAM) terminates with an inflorescence; further growth then occurs from a sympodial shoot meristem (SYM) hosted at the axil of the uppermost leaf that forms a sympodial unit of usually 3 leaves and the next inflorescence. The inflorescence of tomato is a monochasial cyme: the apical meristem (the SAM in the primary shoot and the SYM in the sympodial units) is converted to a flower meristem (FM) and inflorescence growth continues from a lateral meristem (the inflorescence sympodial meristem, SIM), which “maturates” towards FM identity and forms a subsequent lateral SIM that repeats the process.
Understanding the regulation of flowering time and inflorescence development in tomato has much progressed thanks to the characterization of loss-of function-mutants and functional analyses of the identified genes. This thesis is focused on one of these mutants: jointless (j), which was initially isolated because it lacks the abscission zone (AZ) in fruit pedicel. However, the inflorescences of j mutants return to leaf initiation after the formation of few flowers, indicating that the J gene also plays a role in the development of the inflorescence. J encodes a MADS-box protein of the AGAMOUS LIKE 24 (AGL24)/SHORT VEGETATIVE PHASE (SVP) subfamily.
Previous research led to the hypothesis that J plays a dual role in the development of the inflorescence, avoiding the reversion of SIMs to a vegetative fate, which would lead to leaf initiation as observed in j mutant, and preventing their early maturation to FM fate, which would lead to fast termination of the inflorescence. However the molecular bases of J function remained to be elucidated. The aim of this thesis was to unravel the roles of J, trying to clarify its mode of action, the genes impacted by its expression and how its suppression leads to a leafy inflorescence phenotype. To achieve this goal, we used different approaches.
First of all, a detailed phenotyping of j mutants was performed. The genetic backgrounds of the mutants differed in their sympodial shoot growth that was either indeterminate (due to the expression of SELF PRUNING (SP) gene, a repressor of flowering, in the SYM) or determinate (sp mutants). We observed that the leafy inflorescence phenotype of the j mutants was stronger in indeterminate backgrounds, indicating a functional link between J and SP. All mutants showed a slight delay of flowering (1 or 2 additional leaves).
Secondly, a transcriptomic analysis (RNA-seq) of the ultimate and penultimate meristems of young inflorescences of j mutants in an indeterminate (Ailsa Craig, AC) and a determinate (Heinz, Hz) backgrounds confirmed the functional link between J and SP. We indeed found that j mutation causes very little transcriptomic changes in sp background (Hz) whereas j mutation leads to SP activation in the inflorescences of j(AC) mutant, indicating that its leafy phenotype is due to the activation of SYM fate in place of a SIM. The transcriptome of j(AC) also revealed the up-regulation of MADS-box genes involved in flower organ identity (homeotic genes of B- and C-classes). These changes might explain that, in addition to leafy inflorescences, j(AC) mutants have reduced inflorescences terminating early after the formation of 2-3 flowers.
Most interestingly, the RNA-seq analysis also revealed that a regulatory network involved in shoot branching, comprising BRANCHED1 (BRC1) and components of physiological signaling by trehalose phosphate and hormones, is active in the inflorescence of tomato.