Chlorophyllide a Oxygenase mRNA and Protein Levels Correlate with the Chlorophyll a/b Ratio in Arabidopsis thaliana

Abstract

Plants can change the size of their light harvesting complexes in response to growth at different light intensities. Although these changes are small compared to those observed in algae, their conservation in many plant species suggest they play an important role in photoacclimation. A polyclonal antibody to the C-terminus of the Arabidopsis thaliana chlorophyllide a oxygenase (CAO) protein was used to determine if CAO protein levels change under three conditions which perturb chlorophyll levels. These conditions were: (1) transfer to shaded light intensity; (2) limited chlorophyll synthesis, and (3) during photoinhibition. Transfer of wild-type plants from moderate to shaded light intensity resulted in a slight reduction in the Chl a/b ratio, and increases in both CAO and Lhcb1 mRNA levels as well as CAO protein levels. CAO protein levels were also measured in the cch1 mutant, a P642L missense mutation in the H subunit of Mg-chelatase. This mutant has reduced total Chl levels and an increased Chl a/b ratio when transferred to moderate light intensity. After transfer to moderate light intensity, CAO mRNA levels decreased in the cch1 mutant, and a concomitant decrease in CAO protein levels was also observed. Measurements of tetrapyrrole intermediates suggested that decreased Chl synthesis in the cch1 mutant was not a result of increased feedback inhibition at higher light intensity. When wild-type plants were exposed to photoinhibitory light intensity for 3 h, total Chl levels decreased and both CAO mRNA and CAO protein levels were also reduced. These results indicate that CAO protein levels correlate with CAO mRNA levels, and suggest that changes in Chl b levels in vascular plants, are regulated, in part, at the CAO mRNA level.

Involvement of AtNAP1 in the regulation of chlorophyll degradation in Arabidopsis thaliana

“…In this process, the chlorophyll b-to-chlorophyll a conversion most likely takes place to balance the chlorophyll a/b ratios (Masuda et al 2003;Tanaka and Tanaka 2005). In contrast, chlorophyll b synthesis is accelerated (Harper et al 2004) when plants acclimate to low light intensities in order to increase LHCII levels. As mentioned above, the chlorophyll b-to-chlorophyll a conversion is the initial step of chlorophyll b degradation (Hörtensteiner 2006;Kusaba et al 2007).…”

“…Among various photosynthetic components, core chlorophyll-protein complexes of photosystem II (D1) rapidly turn over during photosynthesis (Vasilikiotis and Melis 1994). Chlorophyll metabolic activity is also observed during acclimation to changes in light intensity in order to control the amount of absorbed light energy (Masuda et al 2002;Harper et al 2004;Pattanayak et al 2005).…”

The N-Terminal Domain of Chlorophyllide a Oxygenase Confers Protein Instability in Response to Chlorophyll b Accumulation in Arabidopsis

“…These results suggest that CAO enzyme activity is important for chlorophyll b synthesis. Chlorophyll a/b ratios are reported to correlate with CAO mRNA levels in Arabidopsis thaliana (Harper et al, 2004) and Dunaliella salina (Masuda et al, 2002(Masuda et al, , 2003 during acclimation to different light intensities, indicating that chlorophyll b synthesis is at least partly regulated by the level of CAO mRNA. However, the chlorophyll a/b ratio was only slightly decreased (from 2.85 to 2.65) in CAOoverexpressing plants (Tanaka et al, 2001), which suggests that CAO activity is regulated at the post-transcriptional level.…”

The Clickable Guard Cell, Version II: Interactive Model of Guard Cell Signal Transduction Mechanisms and Pathways

“…The Mg-chelatase H subunit and GCR2 but not FCA were reported to function in ABA regulation of stomatal movements (Razem et al, 2006;Shen et al, 2006;Liu et al, 2007b), although the GCR2 results could not be produced and have been questioned Johnston et al, 2007;Liu et al, 2007a). In addition, a weak mutant allele in the Mg-chelatase subunit, cch1 (Harper et al, 2004) shows strong recessive ABA insensitivity phenotypes in ABA-induced stomatal closing and in ABA inhibition of seed germination and root elongation (Shen et al, 2006). It is therefore surprising that mutations in this gene have not been identified in conventional forward genetic screens (Koornneef et al, 1984).…”