Evidence for reversible phosphorylation as a post-translational regulatory mechanism in the chlorophyll catabolic pathway

Senescence, the final developmental stage in plants, is a highly regulated degenerative process, which ultimately allows for the remobilization of nutrients within the plant from older to younger developing tissues and organs. Among the events characteristic of senescence in aerial plant tissues is the disassembly of the photosynthetic apparatus, accompanied by the release and degradation of chlorophyll molecules. As unbound chlorophyll and several of its catabolites react readily with light to form damaging reactive oxygen species, chlorophyll degradation functions as a critical detoxification process in senescing plants. Chlorophyll is rapidly catabolized into a set of linear, photodynamically safe compounds through a series of enzyme-mediated steps. Although these steps have all been identified, little is known about their regulation, particularly at the post-translational level. A better understanding of the post-translational mechanisms governing activity of one enzyme in particular, pheophorbide a oxygenase (PaO), is especially desirable as PaO catalyzes the oxidative opening of the porphyrin ring structure, considered to be the key regulatory point in chlorophyll degradation. Through site-directed mutagenesis of the Arabidopsis thaliana gene encoding PaO, the in vivo effects of the phosphorylation state at a defined site on this enzyme were explored. The mutant plant line made to mimic the dephosphorylated state of PaO at the specified position exhibited slightly accelerated rates of chlorophyll degradation and was fully capable of catabolizing pheophorbide a, the substrate of PaO. The plant line mimicking the phosphorylated state at this same position behaved very differently; chlorophyll degradation was not noticeably affected, however, the ability of PaO to catabolize pheophorbide a was drastically reduced, as indicated by quantification of this catabolite in senescing leaf tissue. Thusly, it could be concluded from the analyses of chlorophyll content and catabolite pools throughout senescence in these mutant Arabidopsis lines, that phosphorylation at a defined position on this enzyme greatly attenuates PaO activity, indicating a role for reversible phosphorylation in regulating PaO.

Name thesis:degree_name

M.S.

Level thesis:degree_level

Thesis

Discipline thesis:degree_discipline

Plant Biology

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2013

Author dc:creator

• Hall, Pamela K.

Contributors dc:contributor

• Ort, Donald R.

• chlorophyll catabolism
• senescence
• phosphorylation
• enzyme regulation
• pheophorbide a oxygenase

Language dc:language

en

Handle dc:identifier

http://hdl.handle.net/2142/44018

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/44018