PARENT SESSION
Plenary Lectures 2
Tuesday August 31st, 2004 8:30 AM Room 210A

Chlorophyll metabolism in higher plants: its relation to acclimation and cell death. Ayumi Tanaka^*,1, ¹ The Institute of Low Temperature Science, Sapporo, Japan

ABSTRACT- Chlorophyll is a molecule that harvests light energy in antenna systems and drives electron transfer in the reaction centers. Chlorophylls are synthesized during the process of greening in leaves and degraded during senescence as a mechanism for recovery of carbon and nitrogen resources. In addition to these functions, chlorophyll metabolism is closely related to many processes such as acclimation of photosynthesis, expression of nuclear genes, the stay green phenomenon and also programmed cell death. Chlorophyll b is a major pigment of peripheral antenna complexes in green plants and is involved in the regulation of antenna size. Chlorophyll b is synthesized by chlorophyllide a oxygenase (CAO) and CAO mRNA levels are known to change in response to variations in light intensities. In a transgenic study, photosystems of plants overexpressing CAO gene failed to acclimate to the variation of light intensities and had large antenna under any tested light conditions. In addition to regulation at the mRNA level, CAO is also post-translationally regulated. The CAO enzyme consists of three domains. Although the N-terminal domain destabilized CAO proteins in the presence of chlorophyll b, this destabilization does not occur when chlorophyll b is lacking. As a result, this mode of post translational regulation enables the plants to maintain the CAO protein at the required levels. An additional function of chlorophyll metabolism that I will discuss is programmed cell death which is induced by pheophorbide a; which is a degradation product of chlorophyll. Under light conditions, the accumulation of pheophorbide a induced cell death which was most likely related to the generation of singlet oxygen. Interestingly, pheophorobide a also induced cell death under complete darkness. As the result of this observation, it is unlikely in this case that pheophorbide a have induced cell death via the formation of singlet oxygen.

KEY WORDS: metabolisms