Flavonoid 3ʹ‐hydroxylase reduces Ca and Mg content in soybean seeds, resulting in decreased hardness of cooked beans

Abstract

Flavonoid 3ʹ-hydroxylase (F3ʹH) catalyzes hydroxylation at the 3ʹ position of the B ring of flavonoids. F3ʹH is associated with the seed coat and pubescence color, antioxidant properties, and chilling stress tolerance of soybean [Glycine max (L.) Merr.]. In this study, we found that soybean F3ʹH increases the calcium (Ca) and magnesium (Mg) contents in pods and reduces the levels of these elements in seeds. Inductively coupled plasma mass spectrometry (ICP-MS) analysis using a near-isogenic line (NIL) derived from Japanese cultivars 'Natto-shoryu' and 'Hyoukei-kuro 3' for F3ʹH (Glyma.06g202300), also known as the T gene, indicated that the contents of Ca and Mg were higher in the pods but lower in the seed coats and cotyledons from soybean with active F3ʹH than from soybean with inactive F3ʹH. No such clear tendency was observed for other inorganic ions measured in this study. These data suggested that the transport of Ca and Mg from the pod to the seed was inhibited by flavonoid produced through F3ʹH activity. The hardness of cooked beans from soybean with active F3ʹH was lower than that from soybean with inactive F3ʹH, which was likely related to the increase in heat-labile pectin caused by fewer cross-linked divalent ions. These results were supported by ICP-MS analysis and the hardness of cooked beans of other NILs for the T gene. Our findings demonstrate a novel and unexpected effect of flavonoids on the processing quality of soybean.