Iron deficiency induces changes in metabolism of citrate in lateral roots and cluster roots of Lupinus albus

This paper describes the first measurement of enzyme activities in cluster roots under –Fe stress, at different stages of cluster root development and function. In Lupinus albus L., Cluster roots are produced both under iron- and phosphorus-deficient conditions. In both cases the structure is similar, but the level of exudation is much greater in iron-deficient plants. Much work has been done on the enzyme kinetics of P-deficient cluster roots, but none on enzyme activities of Fe-deficient cluster roots. The enzymes investigated were citrate synthase (EC 4.1.3.7), aconitase (EC 4.2.1.3), isocitrate dehydrogenase [IDH(NAD) (EC 1.1.1.41) and IDH (NADP) (EC 1.1.1.42)] and lactate dehydrogenase (LDH) (EC 1.1.1.27). In cluster roots, citrate synthase activity was initially lower than in lateral roots but, after 5 days, recovered to the lateral root level. Cluster root aconitase levels initially increased, but fell sharply on day 3, and no activity was detected after day 5. IDH (NAD) levels were much lower in cluster roots than in laterals, dropping to a low on day 3, and then rising throughout development. IDH (NADP) levels were always higher in cluster roots than in lateral roots, increasing throughout development. LDH levels in cluster roots fell throughout development. Internal tissue concentrations of citrate were markedly higher in –Fe laterals than in +Fe lateral roots and in cluster roots. Cluster root levels of citrate increased dramatically after day 3. Results are discussed within the context of previous work on enzyme kinetics under –P, and the importance of a block in aconitase activity is highlighted.