Interplay of MYB factors in differential cell expansion, and consequences for tomato fruit development.

We previously identified SlFSM1 as an early fruit-specific gene encoding a short protein harboring a non-canonical SANT/MYB-like domain. Here, we investigated the role of FSM1 during fruit development in tomato and its mode of action. By analyzing tomato plants ectopically expressing FSM1, we established that it negatively affects cell expansion, particularly of those cells with the highest potential to expand, such as those residing inner to the vascular bundles in the fruit pericarp. This function of FSM1 differs from that of the snapdragon FSM1-like gene, RAD, which through an antagonistic activity with DIV participates in establishing floral asymmetry. Revealing an additional component of the FSM1/RAD regulatory complex, we show here that FSM1 physically interacts with FSB1, a previously uncharacterized factor harboring an atypical MYB repeat. We also demonstrate that FSB1 physically interacts with the transcription factor MYBI, a homolog of DIV. Our results show that the formation of the FSB1-MYBI complex is competed by FSM1, which recognizes in FSB1 the same region as MYBI does. Taken together, these studies expose a function for the FSM1/FSB1/MYBI complex in controlling tomato cell expansion, while revealing a mechanism by which competing MYB-MYB interactions could participate in the control of gene expression.

The tomato early fruit specific gene Lefsm1 defines a novel class of plant-specific SANT/MYB domain proteins.

We describe here a novel plant-specific gene, Lefsm1 (fruit SANT/MYB-like 1) harboring a single SANT/MYB domain. The expression of Lefsm1 is specific to the very early stages of tomato (Lycopersicon esculentum) fruit development. Ectopic expression of Lefsm1 results in severe developmental alterations manifested in retarded growth, and reduced apical dominance during tomato and Arabidopsis seedling development. A promoter sequence residing 1.0 kb upstream to the translation initiation codon confers the organ-specific expression of the gene. Lefsm1 belongs to a novel small gene family consisting of five to six members in tomato, Arabidopsis and rice. The SANT/MYB domain of LeFSM1 and its orthologs in Arabidopsis and rice differs from that of all other plant or animal MYB proteins and from the SANT domains found in part of the chromatin remodeling proteins. Together, our results indicate that Lefsm1 is a founding member of a small family of proteins containing a novel MYB/SANT domain which is likely to participate in the regulation of a plant-specific developmental program.