Chlorophyll Breakdown in Senescent Arabidopsis Leaves. Characterization of Chlorophyll Catabolites and of Chlorophyll Catabolic Enzymes Involved in the Degreening Reaction

Abstract

During senescence, chlorophyll (chl) is metabolized to colorless nonfluorescent chl catabolites (NCCs). A central reaction of the breakdown pathway is the ring cleavage of pheophorbide (pheide) a to a primary fluorescent chl catabolite. Two enzymes catalyze this reaction, pheide a oxygenase (PAO) and red chl catabolite reductase. Five NCCs and three fluorescent chl catabolites (FCCs) accumulated during dark-induced chl breakdown in Arabidopsis (Arabidopsis thaliana). Three of these NCCs and one FCC (primary fluorescent chl catabolite-1) were identical to known catabolites from canola (Brassica napus). The presence in Arabidopsis of two modified FCCs supports the hypothesis that modifications, as present in NCCs, occur at the level of FCC. Chl degradation in Arabidopsis correlated with the accumulation of FCCs and NCCs, as well as with an increase in PAO activity. This increase was due to an up-regulation of Pao gene expression. In contrast, red chl catabolite reductase is not regulated during leaf development and senescence. A pao1 knockout mutant was identified and analyzed. The mutant showed an age-and light-dependent cell death phenotype on leaves and in flowers caused by the accumulation of photoreactive pheide a. In the dark, pao1 exhibited a stay-green phenotype. The key role of PAO in chl breakdown is discussed.Chlorophyll (chl) degradation is an integral part of leaf senescence and fruit ripening. The fate of chl during senescence has been well established in recent years (for review, see Matile et al., 1999; Hörtensteiner, 1999; Hö rtensteiner and Kräutler, 2000;Kräutler, 2003;Eckhardt et al., 2004). Thereby, chl is converted to colorless nonfluorescent chl catabolites (NCCs; Fig. 1) in a pathway that is probably active in all higher plants (Pružinská et al., 2003;Gray et al., 2004). Structure elucidation of NCCs from different species has unraveled a common tetrapyrrolic skeleton with an oxygenolytically opened porphyrin macrocycle (Kräutler, 2003). Peripheral modifications at several side chains within different NCCs (Fig. 1, R 1 -R 3 ) are species specific (Berghold et al., 2002(Berghold et al., , 2004, and hence have been proposed to occur rather late in the pathway (Hö rtensteiner, 1999). Indeed, a primary chl breakdown product (primary fluorescent chl catabolite-1 [pFCC-1]), which exhibits a blue fluorescence, could be identified as a common product of porphyrin ring cleavage ( Fig. 1; Mü hlecker et al., 1997). Thus, the sequence of reactions is the removal of phytol and magnesium (Mg) by chlorophyllase and Mg-dechelatase, respectively, followed by the conversion of pheophorbide (pheide) a to pFCC-1, which requires the activity of two enzymes, pheide a oxygenase (PAO) and red chl catabolite (RCC) reductase (RCCR; Rodoni et al., 1997;Hö rtensteiner, 1999).PAO is a chloroplast envelope-bound Rieske-type iron-sulfur oxygenase, which is identical to lethal leaf spot 1 (LLS1) from maize (Zea mays) and accelerated cell death 1 (ACD1) from Arabidopsis (Arabidopsis thaliana; Pružinská et al., 2003;Yang e...

Stay-green protein, defective in Mendel’s green cotyledon mutant, acts independent and upstream of pheophorbide a oxygenase in the chlorophyll catabolic pathway

“…Hence, the gene mutations of type C mutants were considered to affect the pathway of chl breakdown directly. This has been confirmed and is obvious for pao1 and nyc1, which are affected in catabolic steps of the pathway (Pružinská et al, 2003;Pružinská et al, 2005;Kusaba et al, 2007). The situation is different for sgr/sid mutants, since so far the function of SGR/SID could not unequivocally be demonstrated.…”

“…pao1 was transformed by a floral dip method and homozygous pao1 atsgr1i lines selected by kanamycin resistance. This was possible, since pao1 is not resistant to kanamycin (Alonso et al, 2003;Pružinská et al, 2005). Production of the AtSGR1-silencing lines, atsgr1i, has been described (Armstead et al, 2007).…”

“…Genomic DNA and cDNA was isolated from both JI4 and JI2775 leaves as described (Murray and Thompson, 1980;Pružinská et al, 2005). PsSGR was amplified with primers PsSGR-LP (5'-CGGGATCCATGGATACTCTAACGAGTGCTC-3') and PsSGR-RP (5'-TTCTGCAGTTACAAGTTACCATGTTGGGTTC-3') using the proofreading Pfu polymerase (Promega).…”

“…For RT-PCR analysis, total RNA was prepared from Pisum leaves and reverse transcribed as described (Pružinská et al, 2005). Quantitative PCR was performed in a LightCycler (Roche Diagnostics) using the QuantiTect SYBR Green PCR kit (Qiagen).…”

“…Like NYC1, PAO expression is senescence-specific and absence of PAO in a respective mutant, pao1, leads to a staygreen phenotype in the dark. Due to the accumulation of phototoxic pheide a, pao1 plants show a light-dependent lesion mimic phenotype (Pružinská et al, 2005).…”