Strigolactone: Promising compound for plant endurance
Authors: Nitin Kumar Garg¹, Ashish Marathe¹ , Rakesh Kumar Prajapat^2
1Ph.D Research Scholar, Division of Biochemistry, IARI, New Delhi-110012
²Ph.D Research Scholar, NRCPB, IARI, New Delhi-110012


Strigolactones are signalling compounds thought to be derived from carotenoids, made by plants. Strigolactones were discovered in root exudates due to their ability to stimulate germination of seeds of the parasitic plant Striga, the ‘witchweed’. So, the strigo part of the strigolactones refers to Striga species which are members of the broomrape (Family-Orobanchaceae), most members of which are parasitic on other plants and lactone refers to the chemical structure which is a cyclic ester. Strigolactones, previously identified as active stimuli of seed germination in parasitic plants, are now recognized as a new group of plant hormones that are active in both shoot and root. Plant hormones are invariably detected by a receptor protein, which triggers interaction of that protein with other proteins to elicit a signal transduction cascade leading to changes in cell activity. The strigolactone receptor was identified through studies on mutants that are insensitive to strigolactone treatment, including the rice dwarf 14 (d14) and petunia deceased apical dominance 2 (dad2) mutants. Isolation of the D14 and DAD2 genes showed that they encode members of the α/β-barrel family of proteins with strong similarity to esterases. Strigolactone has been found to trigger depletion of the auxin transporter PIN1 from the plasma membrane of xylem parenchyma cells in the stem within 10 minutes of treatment, before any changes in gene expression.

One of the physiological phenomenon exhibited by strigolactones is reduce shoot branching, leading to reduced lateral growth. Lateral bud growth is inhibited by auxin transported down from the shoot apex and by strigolactone transported upwards from the root. However, cytokinin, also transported from root to shoot, can promote bud outgrowth. These different signals are modulated in response to different environmental factors, such as light and nutrients, and are integrated through crosstalk between biosynthesis and signalling pathways.

Environmental stresses, such as drought and high salinity, adversely affect plant growth and productivity. Strigalactones (SL) are known to be involved in regulation of plant stress responses including drought and salinity. Mutant studies have also revealed the role of Strigolactones in stomatal closure. Strigolactone deficient mutant (max- more axillary growth) of Arabidopsis exhibited increased leaf stomatal density relative to WT and slower abscisic acid (ABA) induced stomatal closure. In addition to their presence in plants, receptor proteins like d14 are also found in some bacteria like Bacillus. However no such proteins have been reported in fungi yet. But studies using artificial strigolactones (GR24) revealed a change in mitochondrial function in the arbuscular mycorrhizae, which is an important area of future research. This is agriculturally important since plant-fungi symbioses is a pre-requisite for ecosystem wellbeing.

Therefore, using Biotechnological applications, increased Strigolactone synthesis and enhanced signalling can prove beneficial for plant system due to the plethora of functions that strigolactones offer.

References:

1. Gao, Z., Qian, Q., Liu, X., Yan, M., Feng, Q., Dong, G., Liu, J. and Han, B. 2009. Dwarf 88, a novel putative esterase gene affecting architecture of rice plant. Plant Molecular Biology, 71:265â€"276.

2. Hamiaux, C., Drummond, R.S., Janssen, B.J., Ledger, S.E., Cooney, J.M., Newcomb, R.D. and Snowden, K.C. 2012. DAD2 is an α/β hydrolase likely to be involved in the perception of the plant branching hormone, strigolactone. Current Biology, 22:2032â€"2036.

3. Shinohara, N., Taylor, C. and Leyser, O. 2013. Strigolactone can promote or inhibit shoot branching by triggering rapid depletion of the auxin efflux protein PIN1 from the plasma membrane. PLoS Biology, 11:e1001474.

4. Chien, V. H., Marco, A. L., Yuriko, O., Uyen, T. T., Rie, N. and et, al. 2014. Positive regulatory role of strigolactone in plant responses to drought and salt stress. PNAS, 111: 851â€"856


About Author / Additional Info:
I am a first year Ph.D research scholar in division of biochemistry, IARI, New Delhi