Photocatalysis as the ‘master switch’ of photomorphogenesis in early plant development

Enzymatic photocatalysis is seldom used in biology. Photocatalysis by protochlorophyllide oxidoreductase (LPOR) – one of only a few natural light-dependent enzymes – is an exception, and is responsible for the conversion of protochlorophyllide to chlorophyllide in chlorophyll biosynthesis. Photocatalysis by LPOR not only regulates biosynthesis of the most abundant pigment on Earth, it is also a ‘master switch’ in photomorphogenesis in early plant development. Upon illumination, LPOR promotes chlorophyll production, plastid membranes are transformed and the photosynthetic apparatus is established. Given these remarkable, light-induced pigment and morphological changes, the LPOR-catalyzed reaction has been studied extensively from catalytic, physiological and plant development perspectives, highlighting vital, and multiple, cellular roles of this intriguing enzyme. Here, we offer a perspective in which the link between LPOR photocatalysis and plant photomorphogenesis is explored. Notable have been breakthroughs in LPOR structural biology that have uncovered the structural-mechanistic basis of photocatalysis. These studies have clarified how photon absorption by the pigment protochlorophyllide – bound in a ternary LPOR–protochlorophyllide–NADPH complex – triggers photocatalysis, and a cascade of complex molecular and cellular events that lead to plant morphological change. Photocatalysis is therefore the ‘master switch’ responsible for early stage plant development, and ultimately life on Earth.