SlMBP3 Knockout/down in Tomato: Normal-Sized Fruit with Increased Dry Matter Content through Non-Liquefied Locular Tissue by Altered Cell Wall Formation

Abstract The phenotypic effect of the knockdown/out of AGAMOUS clade MADS-box gene SlMBP3 in tomato was evaluated using a transferred DNA (T-DNA)-tagged mutant of SlMBP3 and SlMBP3-RNA interference lines. SlMBP3 was preferentially expressed in the locular tissue of fruit and the seed coat combined with the endoderm. Consistent with where SlMBP3 is expressed, the SlMBP3-knockout/down lines showed non-liquefied locular tissues and increased number of seed hairs than the wild type (WT). The early cell degradation of the locular tissue was not observed in the fruits of the SlMBP3-knockout/down lines, and the cells were elongated like placental cells resulting in non-liquefied locular tissues. As the result, the fruits of the SlMBP3-knockout/down lines exhibited higher dry matter contents and titratable acidity than those of the WT. During locular tissue cell development under the SlMBP3 knockout/down, the expression of cell-enlargement-related genes (beta-expansin gene SlEXPB1 and endo-beta-1,4-D-glucanase gene Cel8) and pectinase-inhibitor-related genes (pectin esterase inhibitor gene PE inhibitor and polygalacturonase inhibitor gene PG inhibitor) was upregulated and that of pectinase-encoding genes (polygalacturonase gene QRT3-like and pectin lyase gene PL2) was downregulated. In the seed coat of the SlMBP3-knockout/down lines, tomato trichome-formation-related genes such as MYB genes containing R2 and R3 repeats (R2R3-MYB) transcription factor SlMYB75, B-type cyclin SlCycB2 and Homeodomain Leucine Zipper (HD-Zip) IV transcription factor Woolly were downregulated. Our results demonstrate that SlMBP3 is involved in the liquefaction of the locular tissue through the modification of cell development and degradation processes and seed hair formation in tomato fruits, and the SlMBP3 knockout/down results in normal-sized fruit with increased dry matter content.