Previous studies have shown that DWARFIO （D10） is a rice ortholog of MAX41RMS1/DAD1, encoding a carotenoid cleavage dioxygenase and functioning in strigolactones/strigolactone-derivatives （SL） biosynthesis. Here we use DIO. RNA interference （RNAi） transgenic plants similar to dlO mutant in phenotypes to investigate the interactions among DIO, auxin and cytokinin in regulating rice shoot branching. Auxin levels in node 1 of both decapitated DIO.RNAi and wild type plants decreased significantly, showing that decapitation does reduce endogenous auxin concentration, but decapitation has no clear effects on auxin levels in node 2 of the same plants. This implies that node 1 may be the location where a possible interaction between auxin and DIO gene would be detected. DIO expression in node 1 is inhibited by decapitation, and this inhibition can be restored by exogenous auxin application, indicating that DIO may play an important role in auxin regulation of SL. The decreased expression of most OsPINs in shoot nodes of DIO-RNAi plants may cause a reduced auxin transport capacity. Furthermore, effects of auxin treatment of decapitated plants on the expression of cytokinin biosynthetic genes suggest that DIO promotes cytokinin biosynthesis by reducing auxin levels. Besides, in DIO-RNAi plants, decreased storage cytokinin levels in the shoot node may partly account for the increased active cytokinin contents, resulting in more tillering phenotypes.
Because plants are sessile organisms,the ability to adapt to a wide range of environmental conditions is critical for their survival.As a consequence,plants use hormones to regulate growth,mitigate biotic and abiotic stresses,and to communicate with other organisms.Many plant hormones function plei-otropically in vivo,and often work in tandem with other hormones that are chemically distinct.A newly-defined class of plant hormones,the strigolactones,cooperate with auxins and cytokinins to control shoot branching and the outgrowth of lateral buds.Strigolactones were originally identified as compounds that stimulated the germination of parasitic plant seeds,and were also demonstrated to induce hyphal branching in arbuscular mycorrhizal(AM) fungi.AM fungi form symbioses with higher plant roots and mainly facilitate the absorption of phosphate from the soil.Conforming to the classical definition of a plant hormone,strigolactones are produced in the roots and translocated to the shoots where they inhibit shoot outgrowth and branching.The biosynthesis of this class of compounds is regulated by soil nutrient availability,i.e.the plant will increase its production of strigolactones when the soil phosphate concentration is limited,and decrease production when phosphates are in ample supply.Strigolactones that affect plant shoot branching,AM fungal hyphal branching,and seed germination in parasitic plants facilitate chemical synthesis of similar compounds to control these and other biological processes by exogenous application.
