Communication by plant growth regulators in roots and shoots of horticultural crops

Plant growth regulators (PGRs) play important roles in the way plants grow and develop. Myriad processes important to horticultural crops are regulated by PGRs. Changes in the presence, balance, and distribution of PGRs communicate developmental, stress-related, or environmental cues that alter growth. Short-distance communication involves changes in biosynthesis or metabolic conversion, whereas longer-distance communication may also require export and translocation of PGRs, their precursors or metabolites. Examples are presented that demonstrate PGR communication between roots and shoots in horticultural commodities. For example, increased duration and intensity of flooding stress can result in synthesis of 1-aminocyclopropane-1-carboxylic acid (ACC), precursor of the PGR ethylene, in roots. ACC transported to the shoot through the transpiration stream is converted to ethylene and causes leaf epinasty. Roots sense the onset of water stress and can communicate the need to close leaf stomata by altering abscisic acid (ABA) levels in the shoot. Daylength and temperature regulate synthesis and transport of gibberellins, which promote stem elongation and stolon formation and inhibit tuberization in potato. Outgrowth of axillary buds following the decapitation of the apical meristern is dependent on synthesis and transport of cytokinin from root to the axillary buds as well as the balance of indole-3-acetic acid (IAA) cytokinin, and additional messengers. Current research in the field of long-distance communication within plants is uncovering novel messengers and altering our view of the central roles for PGRs in such signaling.