Suppression of a hexokinase gene SlHXK1 in tomato affects fruit setting and seed quality

Hexokinase is considered to be the key molecule in sugar signaling and metabolism. Here, we reported that silencing SlHXK1 resulted in a decrease in flower number, increased rate of flower dropping, abnormal thickening of the anther wall, and reduced pollen and seed viability. An anatomical analysis revealed the loss of small cells and abnormal thickening of anther walls in SlHXK1-RNAi lines. Treatment with auxin and 1-methylcyclopropene inhibited flower dropping from the pedicel abscission zone. qRT-PCR analysis revealed that the effect of SlHXK1 on abscission was associated with the expression levels of genes related to key meristem, auxin, ethylene, cell wall metabolism and programmed cell death. Pollen germination and pollen staining experiments showed that pollen viability was significantly reduced in the SlHXK1-RNAi lines. Physiological and biochemical analyses showed that hexokinase activity and starch content were markedly decreased in the transgenic lines. The expression of genes related to tomato pollen development was also suppressed in the transgenic lines. Although the RNAi lines eventually produced some viable seeds, the yield and quality of the seeds was lower than that of wild-type plants. Yeast two-hybrid and bimolecular fluorescence complementation assays showed that SlHXK1 interacted with SlKINγ. Furthermore, SlPIF4 inhibited the transcriptional expression of SlHXK1. In conclusion, our results demonstrate that SlHXK1 may play important roles in pollen, anther, seed and the pedicel abscission zone by affecting starch accumulation or cell wall synthesis, as well as by regulating the number of the transcripts of genes that are involved in auxin, ethylene and cell wall degradation.