Flavonoid 3MODIFIER LETTER PRIME-hydroxylase reduces Ca and Mg content in soybean seeds, resulting in decreased hardness of cooked beans

Flavonoid 3MODIFIER LETTER PRIME-hydroxylase (F3MODIFIER LETTER PRIMEH) catalyzes hydroxylation at the 3MODIFIER LETTER PRIME position of the B ring of flavonoids. F3MODIFIER LETTER PRIMEH 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 F3MODIFIER LETTER PRIMEH 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 F3MODIFIER LETTER PRIMEH (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 F3MODIFIER LETTER PRIMEH than from soybean with inactive F3MODIFIER LETTER PRIMEH. 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 F3MODIFIER LETTER PRIMEH activity. The hardness of cooked beans from soybean with active F3MODIFIER LETTER PRIMEH was lower than that from soybean with inactive F3MODIFIER LETTER PRIMEH, 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.