Developmental Analysis on Compensatory Growth of Lateral Roots Responding to Excision of Seminal Root Tip in Rice

Development of lateral roots (LRs) is promoted when parental root elongation is suppressed. Promotion of LR growth compensates for parental root growth, which contributes to the maintenance of total root length. However, the developmental processes underlying the compensatory growth of LRs are still unclear. In this study, we induced LR compensatory growth in rice by surgical excision of a parent (seminal) root tip, and analyzed the morphological and anatomical changes in LRs. Our analysis revealed that seminal root-tip excision increased the diameter of first-order LRs by increasing in the number of ground-tissue layers and the stele diameter and also promoted elongation, with higher order LRs emerging in the remaining proximal portions. We also found that LR primordia in early developmental stages increased in diameter markedly and then produced higher order LRs pronouncedly in response to seminal-root cutting. Furthermore, the degree of change in LR morphology differed depending on the distance from the cut site, with changes in morphologies weakening further from the cut site. Taken together, the morphological and anatomical changes of LRs that are induced by the root-tip excision of the parent vary continuously, and are dependent on both the developmental stage of LR primordia at the site of root-cutting and the distance from the cut site. Based on these findings, we are now planning to undertake several experiments to further our understanding of the molecular mechanisms underlying compensatory LR growth.