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Strigolactones: The First Members of a New Family of “Shoot Branching Hormones” in Plants?

Plants show an incredible wealth of habits and statures ranging from less ramified to highly branched, from small bushes to big trees. Although the architecture is largely determined by genetic factors, an individual plant is also able to adapt its growth in response to the local environment and biotic effects. Thus, shoot branching depends on local nutrients, especially phosphate, but also on biotic factors such as herbivory and infection. An endogenous factor is the growth hormone auxin, which is released by the apical bud and suppresses the growth of subapical lateral shoots, causing apical dominance. Lower concentrations of auxin or removal of the apical bud lead to multiple shoot tips, and the plant becomes highly branched. The effect is impressively documented by newly developing shoots on a recently cut tree. Accordingly, branching and growth of lateral stems is a process that is highly regulated by genetic, hormonal, and environmental factors. Previously only auxins, abscisic acid, and cytokinins have been assumed to control shoot branching. 3] However, recent studies on a series of mutants of Arabidopsis thaliana, rice (Oryza sativa), and pea (Pisum sativum) with enhanced shoot branching suggested the existence of a novel type of hormone(s) that might be derived from carotenoids. These mutants with similar phenotypes include ramosus (rms) in pea, more axillary growth (max) in Arabidopsis, dwarf (d) or high-tillering dwarf (hdt) in rice, and decreased apical dominance (dad) in petunia. In particular, the genetically well characterized Arabidopsis mutants show substantial branching variation, thus providing an ideal platform for determining the genetic and molecular factors of the shootbranching regulatory network. Two recently published papers by GomezRoldan et al. and Umehara et al. have now presented the first evidence that the long-known strigolactones and related compounds (Scheme 1) might be the first members of a family of apocarotenoid signals that are involved in the regulation of the branching process. 5] Strigolactones were previously known as plant-derived isoprenoids occurring in the rhizosphere. They stimulate the seed germination of obligate parasitic plants like Striga or Orobanche and enhance the seed germination of certain nonparasitic plants. Moreover, upon phosphate depletion, they function as root-derived signals causing elevated hyphal branching and spore germination of symbiotic mycorrhizal fungi, to enhance the uptake of soil nutrients. They are present in the root exudates of monoand dicotyledonous plants (for example, rice and pea) and have additionally been detected in their shoots. 11] Since they are also found in the nonmycotrophic plant A. thaliana, a pivotal role for these apocarotenoids in coordinating plant growth below as well as above ground has been suggested. 13] In particular, work with mutants of pea, petunia, rice, and, especially, Arabidopsis combining enhanced shoot branching with a reduced stature, the max1 to max4 mutants, with defects in carotene degradation and signal transduction (vide infra), revealed the importance of carotene-dioxygenases (CCDs) for the generation of an unknown end product that affects branching. Nine CCDs have been identified in Arabidopsis, of which five are involved in abscisic acid (ABA) synthesis, the others (MAX) have a different target complex. In the mutants rms5/max3/htd1/d17-1 and rms1/max4/d10-1 from pea, Arabidopsis, and rice, 17] the two dioxygenases, namely CCD7 and CCD8, encode for enzymes that degrade carotenoids into a variety of products (Scheme 2). CCD8 can cleave the product of the CCD7-catalyzed reaction, and MAX1, which encodes a cytochrome P450, is assumed