2,4-D-induced parthenocarpy in pear is mediated by enhancement of GA4 biosynthesis

Parthenocarpy, the productions of seedless fruit without pollination or fertilization, is a potentially desirable trait in many commercially grown fruits, especially in pear, which is self-incompatible. Phytohormones play important roles in fruit set, a process crucial for parthenocarpy. In this study, 2,4-dichlorophenoxyacetic acid (2,4-D), an artificially synthesized plant growth regulator with functions similar to auxin, was found to induce parthenocarpy in pear. Histological observations revealed that 2,4-D promoted cell division and expansion, which increased cortex thickness, but the effect was weakened by paclobutrazol (PAC), a gibberellin (GA) biosynthesis inhibitor. Phenotypic differences in pear may therefore be due to different GA contents. Hormone testing indicated that 2,4-D mainly induced the production of bioactive GA(4), rather than GA(3.) Three key oxidase genes function in the GA biosynthetic pathway: GA20ox, GA3ox and GA2ox. In a pear group treated with only 2,4-D, PbGA20ox2-like and PbGA3ox-1 were significantly upregulated. When treated with 2,4-D supplemented with PAC, however, expression levels of these genes were significantly downregulated. Additionally, PbGA2ox1-like and PbGA2ox2-like expression levels were significantly downregulated in pear treated with either 2,4-D only or 2,4-D supplemented with PAC. We thus hypothesize that 2,4-D can induce parthenocarpy by enhancing GA(4) biosynthesis.