|Bird, David Mck|
Submitted to: Plant Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/12/2004
Publication Date: 4/1/2004
Citation: Prasad, D., Schaff, J.E., Laskey, J.G., Kieber, J.J., Bilyeu, K.D., Bird, D. 2004. Cytokinins play opposite roles in lateral root formation and nematode and rhizobial symbioses. Plant Journal. 38(2):203. Interpretive Summary: Scientists have known for many years that plants utilize hormone signal molecules to regulate plant development and respond to the environment. Historically, direct evaluation of the role of hormones in various plant signaling pathways was hampered by extremely low levels of hormones present in any tissue. New technologies based on the discovery that certain plant proteins can be used as markers for the presence of the group of plant hormones known as cytokinins are allowing the investigation of the role of cytokinins in plant development and responses to the environment at the molecular and plant physiology levels. The objective of this work was to identify and correlate the presence of endogenous cytokinins in different regions of developing roots and roots exposed to symbiotic nodulating bacteria or pathogenic root knot nematodes. As assayed by protein expression for cytokinin responsiveness, the hormones appeared to regulated spatially and temporally in the formation of lateral roots. A role for cytokinins was implicated in the formation of nitrogen-fixing nodules, and it was found that a cytokinin response is present subsequent to nematode root penetration and migration. In addition, experiments with roots engineered to destroy the cytokinin signal revealed that those roots have increased amounts of lateral roots but fewer nodules and nematode galls compared to control roots. Taken together, the results demonstrate an intricate balance of cytokinin signals in different cells of root tissues and a role for cytokinins in the formation of nitrogen fixing nodules and nematode induced galls. This information is important to researchers seeking to understand the relation of root hormones and nitrogen-fixing bacteria toward improving birdsfoot trefoil cultivars for pasture and livestock.
Technical Abstract: We used a cytokinin responsive gene (ARR5) promoter fused to a b-glucuronidase reporter gene, and cytokinin oxidase (CKX) genes from Arabidopsis thaliana (AtCKX3) and maize (ZmCKX1) to investigate the roles of endogenous cytokinins in lateral root formation, and symbiosis in Lotus japonicus. We detected no cytokinin response at the early stage of lateral root formation in dividing initial cells. However, a cytokinin response was later induced in the base of the lateral root primordium. The root tip continues to respond to cytokinin during subsequent development of the lateral root. In contrast, we observed a cytokinin response in curled/deformed root hairs, and also in nodule primordia in response to inoculation by rhizobia. The cytokinin response was weak or absent in the nodule once it emerged from the parent root. Root penetration and migration of root knot nematode (RKN) second stage larvae (L2) did not induce a cytokinin response. However, a strong cytokinin response was induced when L2 reached the differentiating vascular bundle and during early stages of the nematode-plant interaction. Cytokinin response was specifically absent in mature giant cells, although dividing cells around the giant cells remained cytokinin responsive. The same pattern was observed using a green fluorescent protein reporter driven by the ARR5 promoter in tomato. Over expression of CKX genes rendered the transgenic hairy roots resistant to exogenous application of the cytokinin N6-(D2 isopentenyl) adenine riboside. Cytokinin oxidase roots have significantly more lateral roots but fewer nodules and RKN induced root galls per plant than control hairy roots.