It is known that plant roots, whose primary role is to supply water and nutrients to the above-ground portions of the plant, may also provide growth signals that regulate shoot growth, but researchers are only beginning to understand the ways in which roots are able to effect this developmental control from afar. Several characterized plant hormones, such as cytokinin and abscisic acid, are thought to be transmitted from root to shoots, yet these hormones have no clearly established role in coordinating shoot growth.
In the new work, University of Utah researchers Jamie Van Norman, Rebecca Frederick, and Leslie Sieburth identify a gene, called BYPASS1, which appears to function in the coordination of root and shoot growth by regulating amounts of a growth-inhibiting substance that is transmitted from roots to shoots. The study shows that the roots of mutant Arabidopsis plants that lacked BYPASS1 function constitutively produced this growth-inhibiting substance. If the mutant roots were removed, the mutant plant's shoot was able to resume normal development, and if the mutant roots were grafted to the shoot portion of a normal plant, the mutant roots were sufficient to stop the normal plant's shoot development. Although the growth-inhibiting substance controlled by BYPASS1 has yet to be identified, the authors show that it is different from previously identified plant hormones or signaling molecules and report evidence that it may be a novel carotenoid.