Auxin's role as an example of the mechanisms of shoot/root relations

The development of the shoot and the roots exhibits a dynamic balance that is characteristic of the plant and the local conditions. Consideration of all aspects of these organ relations, including plant regeneration, suggest that information about organ size, performance and developmental rates is communicated over considerable distances. The movement of substrates, sucrose and essential ions, could not suffice to account for common situations in which they originate in storage tissues, nor for the essential correlation between organ development and vascular differentiation. More generally, the temporal and spatial separation of developmental processes on the one hand and mature functions on the other means that the information exchanged must include the potential for future and not only immediate function. In contrast to substrates, special signals could be selected to depend on development and to elicit varied responses throughout the plant. An example is the concrete evidence that auxin originates in shoot tissues and is essential for root initiation. Further, this same auxin can influence and thus integrate different processes throughout the plant. Mature organs and tissues participate in auxin relations and influence development, with the result that responses to auxin are in the context of the organs present on the plant and missing structures are added as needed. The acquisition of essential substrates feeds back to hormones by limiting development and influencing auxin distribution. A possibility that requires further study is whether feedback could also be due to direct coupling between functional performance and auxin relations. In more general terms, the discussion demonstrates the importance of the mechanisms that integrate plant development and ways they could mask mutant expression.