The Control of Lateral Root Development in Cultured Pea Seedlings. I. The Role of Seedling Organs and Plant Growth Regulators

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Abstract
Experiments were conducted to determine the influence of seedling organs on lateral root development in Pisum sativum `Alaska.' The cotyledons, root tip, and epicotyl were excised from seedlings containing roots of less than 3 cm. In some cases, plant growth regulators or nutritional factors were substituted for the excised organs. The number of emerged and initiated lateral roots was counted after 7 days. Cotyledon removal completely inhibited lateral root emergence and reduced the total number of initiated laterals by 40%. Cotyledon removal did not alter the number of laterals initiated per cm of primary root. Epicotyl removal reduced the number of emerged lateral roots but not the total number initiated. Root tip removal slightly increased the total number of lateral roots formed and emerged from the root region remaining after tip excision. Sucrose, nicotinic acid, pyridoxine, and thiamine did not promote lateral root emergence when provided as cotyledon substitutes. Growth regulator substitutions for either the cotyledons or the root tip mimicked the substituted organs. High auxin (10-4 M indolylacetic acid) promoted lateral root initiation and emergence when it was a cotyledon substitute. High cytokinin (10-4 M 6-benzylaminopurine) inhibited lateral root initiation and emergence when supplied as a root tip substitute. Evidence is provided in support of the hypothesis that the cotyledons and root tip act together to establish auxin and cytokinin concentrations along the primary root axis to control lateral root initiation and emergence.