Chlorophyll Breakdown in Senescent Leaves

The biosynthetic pathway of Chl, probably the most important pigment in the biosphere, has been studied in considerable detail and is mentioned in plant physiology textbooks. Oddly enough, however, the breakdown of Chl is barely mentioned. Yet, as pointed out by Hendry et al. (1987), an estimated one billion tons of Chl are broken down every year, seemingly without leaving a trace. The only enzyme that is taught is chlorophyllase, discovered by Arthur Stoll in 1912, which cleaves Chl into phytol and Chlide, the Mg-porphyrin moiety of Chl. For most of the subsequent 80 years, chlorophyllase has been the only known part of the catabolic system. Until quite recently a11 the subsequent steps of Chlide breakdown remained un- discovered because no one was able to identify the break- down products. We now know that the reason these prod- ucts remained undetected is quite trivial: they are colorless. In mammals, the bile pigments biliverdin and bilirubin are colored breakdown products of heme; by analogy, plant biochemists may have expected the breakdown products of Chlide to be colored as well, but this is not the case, which is probably why they were overlooked for so long. The identification of the structures of these colorless catabolites has, in the last few years, led to the elucidation of the pathway of Chl catabolism in the senescing leaves of higher plants.