Differential accumulation of maysin and rhamnosylisoorientin in leaves of high-altitude landraces of maize after UV-B exposure

Flavonoid induction by UV-B was investigated in five maize landraces from high altitudes and a W23 inbred line lacking the B1 and Pl1 transcription factors required for anthocyanin synthesis. In their natural habitats these landraces receive much higher UV-B fluence than plants at lower altitudes at similar latitudes and would be predicted to have UV-B tolerance by recurrent selection against UV-B stress. We identified two flavones that are induced by UV-B in leaves of high-altitude lines: maysin and its biosynthetic precursor rhamnosylisoorientin. Accumulation is controlled by a p-homologous transcription factor expressed in leaves, and this factor is regulated by UV-B. The levels of either maysin or rhamnosylisoorientin are higher in seedling leaves than in subsequent leaves; the highest flavone concentration was detected in silks. Some landraces have only rhamnosylisoorientin; this likely reflects a mutation in salmon silk1 (sm1) or in a duplicate locus, as genetic crosses with W23 restore the production of maysin in heterozygous F-1 plants. In conclusion, it is demonstrated that maize plants from high altitudes respond to UV-B radiation by accumulating UV-absorbing flavones in leaves; in contrast, these compounds are present at only very low levels in inbred lines such as W23 and are not regulated by UV-B.