Maternally-controlled ovule abortion results from cosuppression of dihydroflavonol-4-reductase or flavonoid-3′,5′-hydroxylase genes in Petunia hybrida

Transgenes designed to overexpress anthocyanin genes An6 (encoding dihydroflavonol-4-reductase) or Hf1 (encoding flavonoid-3',5'-hydroxylase) in Petunia hybrida L. produced flower colour phenotypes similar to those caused by sense cosuppression of chalcone synthase (Chs) genes. However, unlike Chs, sense cosuppression of An6 and Hf1 resulted in female infertility in transgenotes exhibiting complete phenotypic suppression of anthocyanins. Female sterility appeared to be due to embryo abortion, with discolouration of ovules first appearing about 4 d post-fertilization, followed by gradual collapse of the ovule. Pollen from cosuppressed, female-sterile transgenotes placed on wild-type stigmas produced normal seed set, indicating that sterility of cosuppressed plants was maternally controlled. We suggest an hypothesis that cosuppression of An6 and Hf1 leads to accumulation of dihydroflavonols in the seed coat, a maternal tissue, and that this accumulation inhibits embryo growth, either directly or indirectly. In this hypothesis, direct inhibition of embryo growth would require that dihydroflavonols diffuse from the seed coat into the embryo and act there, whereas indirect inhibition would require that dihydroflavonols interfere with some capacity of the seed coat to promote embryo growth.