Submitted to: Planta
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/12/2004
Publication Date: 5/1/2004
Citation: Hacisalihoglu, G., Hart, J.J., Vallejos, C.E., Kochian, L.V. 2004. The role of shoot-localized processes in the mechanism of zn efficiency in common bean. Planta. 218:704-711. Interpretive Summary: Crop yields are often limited by low soil levels of mineral micronutrients such as zinc (Zn). It has been estimated that 30% of the world's arable soils are Zn deficient. There is significant genetic variation both within and between plant species in their ability to maintain significant growth and yield under Zn deficiency conditions; this has been termed Zn efficiency (ZE). In recent years, there has been considerable interest in elucidating the physiological mechanisms that confer ZE, as an understanding of these mechanisms is necessary to help develop more Zn efficient crops. We previously had shown that root Zn uptake does not play a role in differential ZE. In the present study, we studied other potential ZE mechanisms in Zn efficient and inefficient cultivars of common bean. Grafting experiments were carried out where the shoots of the efficient cultivar were grafted onto roots of the inefficient cultivar and the reciprocal graft was also made. When these different grafted seedlings were grown on low Zn media, only those grafts that had the Zn efficient shoot showed no Zn deficiency symptoms. These experiments clearly showed that ZE is a shoot-localized phenomenon. Several shoot related Zn transport processes were studied (Zn translocation to shoot and Zn compartmentation within leaf cells) and no differences were seen between the efficient and inefficient cultivars. However, we did find that when bean seedlings were grown under low Zn conditions, the activities of several shoot-localized Zn-requiring enzymes were considerably higher in the Zn efficient genotypes. These findings suggest that a general mechanism for ZE in higher plants could involve biochemical Zn utilization in leaf cells. This will be the focus of future investigations.
Technical Abstract: Zn efficiency (ZE) is the ability of plants to maintain high yield under Zn deficiency stress in the soil. Two bean (Phaseolus vulgaris L.) genotypes that differed in ZE, Voyager (Zn-efficient) and Avanti (Zn-inefficient) were used for this investigation. To investigate the relative contribution of the root versus the shoot to ZE, observations of Zn-deficiency symptoms in reciprocal grafts of the two genotypes were made. After growth under low Zn conditions, plants of nongrafted Avanti, self-grafted Avanti and reciprocal grafts that had the Avanti shoot scion exhibited Zn-deficiency symptoms. However nongrafted and self-grafted Voyager as well as reciprocal grafts with the Voyager shoot scion were healthy with no visible Zn deficiency symptoms under the same growth conditions. More detailed investigations into putative shoot-localized ZE mechanisms were conducted. No differences were found in shoot tissue Zn concentrations between the Zn-inefficient and Zn-efficient genotypes grown under the low Zn conditions where differences in ZE were exhibited. Shoot Zn compartmentation was investigated using radiotracer (65Zn) efflux analysis and indicated that the zinc efficient genotype maintains slightly higher cytoplasmic Zn concentrations and less Zn in the leaf-cell vacuole, under Zn deficiency. Analysis of Zn-requiring enzymes in bean leaves revealed that the zinc efficient genotype maintains significantly higher levels of carbonic anhydrase (CA) and Cu/ZnSOD activity under Zn deficiency. While these data are not sufficient to allow us to determine the specific mechanisms underlying Zn efficiency, they certainly point to the shoot as a key site where Zn efficiency mechanisms are functioning, and could involve processes associated with biochemical Zn utilization.