An important lesson about the significance of Chl breakdown is taught by the various stay-green mutants. In all of them a deficiency in the catabolic system causes a marked retention not only of Chl but also of apoproteins of pigment-protein complexes in the thylakoids (Thomas, 1982; Guiamet etal., 1991; Cheung etal., 1993; Guiamet and Giannibelli, 1994; Bachmann etal., 1994). Hence, Chl appears to protect its apoproteins from proteolysis. The apoproteins account for a substantial proportion of the total Cpl protein. In other words, stay-green genotypes pay a penalty (Hauck et al., 1997) as some 30% of plastidic N is excluded from remobilization during senescence and recycling to other parts of the plant. It is not surprising, therefore, that many of the known stay-green cultivars are diazotrophic legumes which are less dependent on N recycling than are plants which depend on nitrate. The famous stay-green mutant of Festucapratensis, Bf 993, has not been discovered in a natural meadow but coincidentally in a breeding program (Thomas and Stoddart, 1975). Natural stay-greens are perennials in which nutrient recycling includes microbial decomposition of leaves in litter and soil and reabsorption of nutrients by the roots. In Alnus (a diazotrophic genus again), Chl is not degraded in autumn and the entire senescence program is reduced to the differentiation of abscission layers (Bortlik et al., 1987).
The protection of complexed Chl-binding proteins from proteolytic degradation does not explain why Chl is degraded at all. Theoretically, dismantling of the complexes with only the protein part being broken down is imaginable all the more as the four N atoms of the Chl molecule are not available for recycling. And yet, breakdown is indispensable because Chl is a photodynamically active pigment and, hence, a hazard if not protected as is the case in the thylakoids. In the complexed state, the photodynamism of Chl is compensated for by carotenoids. They represent efficient quenchers of triplet state activated Chl as well as of singlet oxygen which occur when the harvesting of quanta is not balanced by deactivation of Chl through photosynthetic electron transport. Hence, free Chl produced upon dismantling of complexes must immediately be inactivated in order to prevent photodynamic damage in the senescing cells. This vitally important process must be attributed primarily to macrocycle cleavage catalyzed by the PaO/RCCR system. In land plants, the abolition of photodynamism is particularly important as catabolites are stored intracellularly until the very end of the senescence period. It is justified, therefore, to apostrophize Chl breakdown as a kind of detoxification process.
It is understandable that in an alga such as Chlorella, Chl breakdown is less elaborated; enforced Chl breakdown in darkness and under heterotrophic conditions proceeds only to the level of macrocycle cleavage and the first product, RCC, is excreted into the surrounding medium (Oshio and Hase, 1969; Engel et al., 1991). In the evolution of Chl catabolism, further photodynamic inactivation of RCC by RCCR probably coincides with the transition to terrestrial life and to intracellular disposal of catabolites. Indeed, the activity of RCCR has been detected in several taxa of pteridophytes including primitive ferns (Matile group, unpublished results).
Although there are good reasons to believe that in senescing leaves Chl is catabolized exclusively via the PaO pathway, alternative mechanisms of breakdown have been, and still are, considered.
Under appropriate conditions Chl is readily bleached, for example by peroxidases or, in the presence of free unsaturated fatty acids, by "Chl oxidase" associated with the photosystems (for references see Matile et al., 1999). The evidence favoring peroxidative and oxidative Chl bleaching is largely based on correlations between activities and rates of degreening or on effects of inhibitors. Perhaps the strongest argument against a role of these bleaching reactions in Chl catabolism during leaf senescence is provided by results obtained with stay-green mutants: so far none of them has turned out to be deficient with regard to peroxidase or Chl oxidase.
Nevertheless, alternative mechanisms of Chl breakdown almost certainly play a role at stages of development other than senescence. Thus, turnover of Chl in developing and mature leaves (Stobart and Hendry, 1984) is not associated with the accumulation of NCCs (Matile group, unpublished results) suggesting that the PaO pathway is not involved. The stabilization of newly synthesized Chl requires the presence of apoproteins; uncomplexed Chl molecules are rapidly degraded and thereby photodynamically inactivated. Such continuous detoxification is also important with regard to photodynamically active porphyrinic intermediary products of Chl biosynthesis. Indeed, oxygen requiring enzymes catalyzing pigment bleaching in vitro have been demonstrated for example in developing leaves of cucumber (Whyte and Castelfranco, 1993). The properties of these enzymes as described so far are clearly different from those of the PaO/RCCR system. Unfortunately, catabolites are unknown and the work has not been pursued recently.
Convincing evidence for the existence of alternative mechanisms underlying the loss of green color is also provided by observations in the stay-green mutant of meadow fescue. During leaf senescence, the Di protein of the reaction center of photosystem II is retained in the mutant as are all other proteins complexed with Chl. Turnover of D1 in the light, however, is normal suggesting that two different processes are responsible for breakdown during light-dependent turnover and senescence, respectively (Hilditch et al., 1986). Whereas high Chl retention in senescing leaves of the mutant is caused by deficient PaO activity (Vicentini et al., 1995a) the mechanism of bleaching in the light is unknown.
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