Chlorophyll breakdown in higher plants

Chlorophyll is the characteristic pigment of plants. Its metabolism is considered to be the most visible sign of life on earth. The emergence of the fall colours in the foliage of deciduous trees, as well as the change of colour observable during ripening of fruit and cereals are all particularly highly visible manifestations of chlorophyll breakdown. In spite of its obvious importance, only in the last decade has biological and chemical information on chlorophyll breakdown become available. Major and pioneering contributions to this subject were provided by our group. The present project delineates experiments, intended to address basic questions concerning the structure and occurrence of chlorophyll catabolites in higher plants, as well as on the biological chemistry of key steps in chlorophyll breakdown. For this purpose the availability of synthetic pathways for the preparation of chlorophyll catabolites will also be further exploited. As the specific contributions of the planned work, it is proposed (i) to examine by in vitro experiments two of the key transformations in chlorophyll breakdown in higher plants: i) the reduction of the "red chlorophyll catabolite" to the "fluorescent chlorophyll catabolite", and ii) the isomerization of the latter to "nonfluorescent chlorophyll catabolites", which are considered likely to be the "final" products of chlorophyll breakdown in higher plants; (ii) to study the occurrence and structures of colourless chlorophyll catabolites in a selection of senescent higher plants, in order to examine the general validity of the now know pathway of chlorophyll breakdown under natural growth conditions; (iii) to develop synthetic (bio-mimetic) means for the preparation of still hypothetical (tetrapyrrolic) red, fluorescent and nonfluorescent chlorophyll catabolites and to study the biologically relevant chemical reactivity of the synthetically accessible (analogues of) red, fluorescent and nonfluorescent chlorophyll catabolites. This work is planned to contribute to a subject of prime ecological and economical importance and to expand our knowledge on a most fascinating natural phenomenon.

Chlorophyll is the key pigment of plants and its seasonal metabolism is probably the most visible sign of life on earth. The emergence of the fall colours in the foliage of deciduous trees, and the change of colour observable during ripening of fruit, are highly visible and fascinating signs of chlorophyll breakdown. Only in the last sixteen years has structural and biochemical information on chlorophyll breakdown become available. Our group has made major contributions to this subject, which meanwhile has become textbook knowledge. The present project gave specific experimental answers to basic questions on the structure and occurrence in higher plants of chlorophyll catabolites, either as intermediary or as "final" products of chlorophyll breakdown, as well as, on how key steps in chlorophyll breakdown in senescent higher plants take place. In summary, chlorophyll breakdown in higher plants was revealed to be a rather regulated process: In its course, an enzyme bound red intermediate is formed in minute amounts, which is rapidly transformed further into "fluorescent" chlorophyll catabolites (FCCs), and from there to the "nonfluorescent" chlorophyll catabolites (NCCs). Our work showed that FCCs isomerize spontaneously to the corresponding NCCs, which carry a variety of peripheral functional groups. Timing, organellar localization and mode of this transformation during chlorophyll breakdown were still largely unexplored at the outset of this project. We have provided experimental support for it to occur in the acidic vacuoles, the "garbage cans" of the plant cell. NCCs are considered to be the ("final") storage forms of the tetrapyrrolic remains of chlorophyll and are found, indeed, in the vacuoles. Our work also identified two NCCs in ripe fruit and showed specific NCCs to be a previously overlooked component of nutrition. These discoveries may indicate the need to know more about the physiology of NCCs and may eventually also lead to innovative (agro-technological) developments with plants cultivated as sources for nutrition and for other purposes.