Local response and systemic regulation in plant roots

Background/Question/Methods: Soil nutrients are often spatiotemporally variable in nature. Plant roots respond to soil variability through plasticity not only involving the influence of local stimuli, but also affected by whole plant mechanisms regulating local hormones that may be affected by different nutrient elements. We hypothesized: 1) different nutrient elements in different soil patches will affect root elongation and proliferation differently, and 2) the contents of phytohormones controlling root growth in root tips and leaves discrepantly. To examine the hypotheses, we designed greenhouse split-root experiments using Callistephus chinensis and Solidago canadensis. The root system of each plant was equally divided and grown in two hydroponic devices with different nutrient treatments. The hydroponic nutrient solution is revised Hogland solution. The treatments included: 1) low NO₃^- vs. low NH₄⁺; 2) low P (nitrogen source: NO₃^-) vs. low NO₃^-; 3) low P (nitrogen source: NH₄⁺) vs. low NH₄⁺; 4) both low P (nitrogen source: NO₃^-); 5) both low P (nitrogen source: NH₄⁺); and 6) both low NO₃^-; and 7) both low NH₄⁺. Thirty days later, the roots were carefully harvested and scanned. Root samples were selected and froze in -80^oC before hormone content analysis using HPLC. The results were compared in ANOVA.

Results/Conclusions: We found that low homogeneous NO₃^- and low homogeneous P both promote root growth in terms of mass, first order root elongation, and branching, and non-significant between the two treatment in root mass. While low P (nitrogen source: NO₃^-) vs. low NO₃^- treatment reduced root mass compared to homogeneous NO₃^- and P. Both NH₄⁺ treatment reduced root growth compared to NO₃^-, but have more root mass than both low P (nitrogen source: NH₄⁺) treatment. And high P promote root branching than P deficiency. The contents of phytohormone (IAA, ABA and CK) in roots differed significantly among the treatments with rather different responses to local treatments of nutrient elements. Thus lead to different proportions of phytohormone in roots and leaves indicating the complex hormonal regulation mechanisms. In addition, the two species also demonstrated different responses in root hormones as well as root growth responding to the treatments. Our results also showed relationships in hormone contents between root and leaves, indicating local responses and whole plant control in root growth.