DR. DIVYASHREE NAGESWARAN
TGI, United Kingdom
This piece of writing is an extension from the last edition’s topic on “Plant hormones in stress signaling, responses and tolerance”. A new class of phytohormones called “Strigolactones” and its role in plant stress signaling and arbuscular mycorrhizal (AM) symbiosis mechanisms will be discussed.
Strigolactones (SLs) are a small group of isoprenoid-derived compounds that were first identified as “Strigol”, a germination stimulant of the parasitic plant Striga lutea. This Strigol was isolated from the root exudates of cotton. Later, new SLs have been found in exudates of different plant species such as sorgolactone (Sorghum), Orobanchol (red clover), alectrol (cowpea) and solanacol (tobacco). Likewise, nearly 30 SLs have been identified from the root exudates of various plant species, and > 20 SLs have been well characterised in Arabidopsis, rice, tomato, petunia and tobacco. These SLs have been demonstrated to act as germination stimulators in root parasitic weeds, including witchweed (Striga spp.), broomrape (Orobanche spp.and Phelipanche spp.) and Alectra belonging to the family Orobanchaceae. Parasitic weeds, also are flowering plants, whose seeds germinate only in response to SLs or germination stimulants, present in the rhizosphere of host plants. They depend on hosts for photosynthetic assimilates, water and nutrients as they are incapable of surviving on their own. Seed germination begins when the suitable host root comes in contact. Other than SLs that are stimulants, other plant hormones help in making a vascular connection with the host and parasite through a specialised organ called “haustorium”. Upon its establishment, the parasites grow on host plants, especially on crops like sorghum, maize, millet, rapeseed, legumes and tomato. As a consequence of host-parasite relationship, agricultural crop yield is severely affected. This condition is majorly witnessed in the countries of Asia and Africa.
In the recent decade, novel bio-properties of SLs have been found with the existence of natural SLs. SLs have been identified to stimulate fungal hyphal branching in arbuscular mycorrhiza (AM) to initiate a symbiotic relationship with their hosts. Nearly 80% of the terrestrial plants involve in symbiosis with fungi, in which fixed carbon from host plants is swapped for minerals absorbed by the fungal species through a widespread hyphal network. Symbiotic relationships are very beneficial in the case of mineral nutrients with low mobility like phosphate in soil.
SLs have demonstrated to regulate root and shoot architecture, a complex agronomic trait affecting crop yield. SLs are majorly produced in the roots, transported to shoots or released into the rhizosphere where they play a pivotal role as hormones or external signals. Most studies were conducted on increased shoot branching or tillering mutant phenotypes of different species such as pea (ramosus/rms), Arabidopsis (more axillary growth/max), Petunia (decreased apical dominance/dad) and rice (dwarf or high-tillering dwarf/htd). External application of synthetic SL, GR24 led to the inhibition of shoot branching, axillary bud outgrowth, inducing secondary growth, stimulation of internode growth, enhanced root hair elongation, enhanced growth of primary roots, acceleration of leaf senescence, inhibition of lateral and adventitious root formation and other visible changes in plant morphology. This implied that the presence of SL, a root-derived and upward-moving signaling molecule regulates shoot branching and plant architecture.
SLs are apocarotenoids that are carotenoid-derivatives produced from the precursor, beta-carotene. Both plastids and cytosol are required for its biosynthesis. The biosynthetic pathway has been well explained from characterization of SL-deficient mutants which show highly branched phenotype. Carotenoid Cleavage Dioxygenases (CCDs), specifically CCD7 and CCD8 (MAX3 and MAX4 in Arabidopsis, D17/HTD1 and D10 in rice and RMS5 and RMS1 in pea), DWARF27/D27 (rice), MAX1 (Arabidopsis), lateral branching oxidoreductase (LBO) are essential for the SL production. The enzyme D27 isomerises trans–beta-carotene to cis configuration and subsequent steps lead to the formation of a 9-cis-aldehyde by CCD7 and Carlactone is produced by CCD8 from 9-cis-aldehyde. The compound Carlactone acts as a precursor for SLs. Followed by biosynthesis, SL receptor proteins (MAX2 in Arabidopsis, D3 and D14 in rice, RMS4 in pea and DAD2 in petunia) mediate signal transduction/perception. D14 is the first identified receptor, an alpha/beta hydrolase binds to the F-box protein, AtMAX2/D3 and the transcriptional repressor D53 (rice) forming a complex to trigger SL signaling and SL-mediated plant responses. Certain biosynthetic pathway genes have been identified to improve plant architecture especially in modern rice varieties.
DWARF27/D27 that encodes CCD7 was found to increase tiller number and improve grain yield in rice. Understanding the mechanistic role of pathway genes and its exploitation in breeding programs would serve as a base in improving crop yield.
Apart from its role in plant growth & development, SLs show positive resistance to abiotic stresses. SLs have been positively associated to improved drought tolerance. SL mutants of Arabidopsis had rapid water loss and decreased survival due to enlarged stomatal aperture. Exogenous application of SL, GR24 diminished the stomatal aperture thereby reducing transpiration-associated water loss in Arabidopsis. Thus, exogenous application of SLs as agrochemicals, use of modern and modified crop varieties that manipulate SL signaling, have a great prospective in increasing crop productivity in terms of yield, improving nutrient gain, resisting parasitic plants and improving on abiotic stress tolerance via multiple mechanisms.