The Non-Mendelian Green Cotyledon Gene in Soybean Encodes a Small Subunit of Photosystem II

Abstract

Chlorophyll degradation plays important roles in leaf senescence including regulation of degradation of chlorophyll-binding proteins. Although most genes encoding enzymes of the chlorophyll degradation pathway have been identified, the regulation of their activity has not been fully understood. Green cotyledon mutants in legume are stay-green mutants, in which chlorophyll degradation is impaired during leaf senescence and seed maturation. Among them, the soybean (<i>Glycine max</i>) green cotyledon gene <i>cytG</i> is unique because it is maternally inherited. To isolate <i>cytG</i>, we extensively sequenced the soybean chloroplast genome, and detected a 5-bp insertion causing a frame-shift in <i>psbM</i>, which encodes one of the small subunits of photosystem II. Mutant tobacco plants (<i>Nicotiana tabacum</i>) with a disrupted <i>psbM</i> generated using a chloroplast transformation technique had green senescent leaves, confirming that <i>cytG</i> encodes PsbM. The phenotype of <i>cytG</i> was very similar to that of mutant of chlorophyll <i>b</i> reductase catalyzing the first step of chlorophyll <i>b</i> degradation. In fact, chlorophyll <i>b</i>-degrading activity in dark-grown <i>cytG</i> and <i>psbM</i>-knockout seedlings was significantly lower than that of wild-type plants. Our results suggest that PsbM is a unique protein linking photosynthesis in presenescent leaves with chlorophyll degradation during leaf senescence and seed maturation. Additionally, we discuss the origin of <i>cytG</i>, which may have been selected during domestication of soybean.