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JXB Advance Access originally published online on November 29, 2004
Journal of Experimental Botany 2005 56(412):525-536; doi:10.1093/jxb/eri031
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Journal of Experimental Botany, Vol. 56, No. 412, © Society for Experimental Biology 2004; all rights reserved

RESEARCH PAPER

Regulation of lysine catabolism in Arabidopsis through concertedly regulated synthesis of the two distinct gene products of the composite AtLKR/SDH locus

Asya Stepansky, Youli Yao, Guiliang Tang and G. Galili*

Department of Plant Sciences, The Weizmann Institute of Science, Rehovot 76100, Israel

* To whom correspondence should be addressed. Fax: +972 8 9344181. E-mail: gad.galili{at}weizmann.ac.il

Lysine catabolism in plants is initiated by a bifunctional LKR/SDH (lysine-ketoglutarate reductase/saccharopine dehydrogenase) enzyme encoded by a single LKR/SDH gene. Yet, the AtLKR/SDH gene of Arabidopsis also encodes a second gene product, namely a monofunctional SDH. To elucidate the regulation of lysine catabolism in Arabidopsis through these two gene products of the AtLKR/SDH gene, an analysis was carried out on the effects of the hormones, abscisic acid and jasmonate, as well as various metabolic and stress signals, including lysine itself, on their mRNA and protein levels. The response of the two gene products to the various treatments was only partially co-ordinated, but the levels of the monofunctional SDH mRNA and protein were always in excess over their bifunctional LKR/SDH counterparts. These results suggest that lysine catabolism is regulated primarily by the first enzyme LKR, while the excess level of SDH enables efficient flux of lysine catabolism following the LKR step. Analysis of transgenic plants expressing ß-glucoronidase fusion constructs with the AtLKR/SDH and monofunctional AtSDH promoters demonstrated that transcriptional regulation contributes to the modulation of expression of the bifunctional LKR/SDH and monofunctional SDH gene products in response to hormonal and metabolic signals. To test whether the enhanced expression of the LKR/SDH gene under various hormonal and metabolic signals is correlated with enhanced lysine catabolism, wild-type Arabidopsis and a knockout mutant lacking lysine catabolism were exposed to abscisic acid and sugar starvation. Free lysine accumulated to significantly higher levels in this knockout mutant than in the wild-type plants.

Key words: ABA, catabolism, essential amino acids, jasmonate, lysine-ketoglutarate reductase, saccharopine dehydrogenase, stress, sugar starvation


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