Strigolactones are thought to be carotenoid originated plant hormones. Their role in germination of parasitic plants and stimulating symbiotic association with fungi was well known. But off late they were found to be involved in inhibition of branching. Two mutants namely rms1 and dad1, increased branching mutant in garden pea and petunia respectively were instrumental in the discovery. It was revealed that, in these mutants a mobile signal produced in shoot and root inhibits shoot branching. This signal was referred to as SMS for shoot multiplication signal. Cytokinin and auxin regulate the apical dominance; cytokinin promotes the growth of axillary buds while auxin inhibits the growth. It was believed that the ratio between auxin and cytokinin regulates the growth and branching of the plants. So researchers presumed that cytokinin must be in excess in the increased branching mutants. In order to know the substance involved in the SMS, the indigenous plant hormones like auxin, cytokinin and abscisic acid were measured. It was presumed that indigenous cytokinin concentration would be high as cytokinin promotes the growth of axillary buds. But the result was opposite. Similar measurements were also conducted in other mutants and it was concluded that SMS was a novel plant hormone. SMS moves acropetally in shoot and inhibits lateral bud outgrowth. When mutant plant was grafted onto normal plant, shoot branching returned to the normal state. Since then, increased shoot branching mutants were observed in other plants. As the same phenomenon was observed in other plants also it was highly possible that the new plant hormone was involved in the process. The shoot branching process involves the formation of axillary buds in the axil of leaves and subsequent growth of bud. Earlier studies using recessive mutants have shown that an unidentified hormone was inhibiting the outgrowth of axillary buds. These mutants were ramosus (rms) of pea (Pisum sativum), more axillary growth (max) of Arabidopsis, decreased apical dominance (dad) of petunia (Petunia hybrida) and dwarf (d) or high-tillering dwarf (htd) of rice (Oryza sativa). Researchers used three increased branching mutants each with respective deficiencies of D17, D10 and D3 genes. Genes D17 and D10 were shown to produce carotenoid cleavage dioxygenase (CCD), which cleaves pigment called carotenoids. This suggested that branch inhibiting hormone was produced when carotenoids were cleaved. As branch inhibiting hormone was not produced in the mutants D17 and D10, they were showing excess branching. Gene D3 was considered to be the receptor for the shoot branching hormone. It was concluded that D3 gene deficient mutant plant produced branching inhibiting hormones but failed to respond to them leading to excess shoot branching. In order to identify the branching inhibiting hormone researchers conducted bioassays. They administered the mutant plants with the liquid extracts from the tissues of plants that were believed to contain the branching inhibiting hormone. If the extract elicited the expected response, the components of the extract were isolated and administered separately. The cycle of separation, administration and examination was repeated time and again. Still researchers had hard time zeroing in. The deadlock was finally broken when a paper on the root-parasitic plant Striga was published in October 2005.

Striga (Striga asiatica) is a common weed plant in dry region of South Asia and Africa. It is a parasitic plant that attacks the roots of monocotyledons plants like Sorghum. The striking feature of Striga is its seed germinate only when there is a presence of a compound called Strigolactone in the medium. Strigolactone is secreted from the roots of the host plants. The secretion of Strigolactone was known from decades but 2005 paper showed that Strigolactone was produced from caroteniods like that of branching inhibiting hormones which were produced when caroteniods were cleaved. The researchers correlated the two stories and thus they formed the hypothesis that Strigolactone was the branching inhibiting hormone. To confirm the hypothesis researchers administered Strigolactone to mutant plants and observed that the excess shoot branching was returned to normal. Thus they have confirmed that Strigolactone was a new plant hormone responsible for inhibition of branching.

1. Gomez-Roldan, V. et al. (2008) Strigolactone inhibition of shoot branching. Nature 455, 189-194
2. Umehara, M. et al. (2008) Inhibition of shoot branching by new terpenoid plant hormones. Nature 455, 195-200

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