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Plant Thermomorphogenesis: Molecular Aspects

Temperature is the key aspect of plant distribution and their growth and development. Being sessile, plants have developed highlyresponsive sensing mechanism to sense minute alterations in temperature and adjust their growth and development accordingly. The term ‘thermomorphogenesis’was described as the eects of temperature on plant morphologycoined by Erwin and colleagues, in analogy to photomorphogenesis (light-mediated growth).Holistic changesin plant morphology and architecture induced by high temperatures, below the heat-stress range, is togethercan be described as thermomorphogenesis.

The year 2015 is on track to surpass 2014 as the warmest year ever recorded since systematic temperature measurements began more than a century ago. In fact, the 10 warmest years on record all occurred after 1998. Such figures are alarming as it is expected that thiswill strongly aect plant distribution and survival, and thereforethreaten biodiversity. Likewise, crop productivity will probably suer greatly fromglobal warming, while food production is required to increase significantly to sustain a growing and more demanding world population. Plant can avoid these by activating mechanisms to adapt growth andmorphology through enhancedevaporative cooling, increased convection and direct avoidance ofheat flux from the Sun. If understood, the underlying molecularprocesses of these so-called thermomorphogenesis responses could be attractive breeding targets for improving crops to withstandclimate warming (American metrological society, 2015).

Elongation of the hypocotyl is one of the earliest thermomorphogenic eects seen in seedlings across Arabidopsis accessions inresponse to high ambient temperature. It has been suggested that hypocotyl elongation moves the sensitivemeristematic and photosynthetically active tissues away from heatabsorbing soil and may promote cooling by allowing better accessto moving air. Changes in plant morphology initiated by high ambient temperatureand by vegetation shade are very similar, indicating the possibilityof shared signaling elements.

This idea led to the identification ofthe bHLH transcription factor PIF4 as a key regulator of thermomorphogenic phenotypes including hyponasty, hypocotyl and petiole elongation. PIF4 (and to a lesserextent PIF5) performs its pivotal function in high-temperature signaling by orchestrating transcriptional changes that subsequentlytrigger primarily phytohormone-induced elongation responses.Recent findings have suggested importantroles for light signaling pathways, the circadian clock, auxin and other phytohormones in PIF4-mediated temperatureinduced growth. Furthermore, epigenetic mechanisms appear at the nexus of induction and attenuation of growth acclimation inresponse to high ambient temperatures.To control thermomorphogenesis, multiple pathways regulate the modulation of PIF4 levels, activity and downstream mechanisms. Thermomorphogenesis is integrally governed by various light signaling pathways, the circadian clock,epigenetic mechanisms and chromatin-level regulation (Nomato et al., 2012).

Understanding the molecular genetic circuitries underlying thermomorphogenesis is particularly relevantin the context of climate change, as this knowledge will be key to rational breeding for thermo-tolerant crop varieties. Untilrecently, the fundamental mechanisms of temperature perception and signaling remained unknown. Our understanding oftemperature signaling is now progressing, mainly by exploiting the model plant Arabidopsis thaliana. The transcription factorPHYTOCHROME INTERACTING FACTOR 4 (PIF4) has emerged as a critical player in regulating phytohormone levels and theiractivity.

In addition to thermomorphogenesis, adaptation to high ambient temperature also involves physiological processes such as photosynthetic acclimation, respiration andchanges in carbon balance. Meeting future challenges to plant productivity imposed by globally increasing temperatures will require basic research in modelplant species as well as applied approaches in crops. Integrationof these ends of the spectrum will require directed eorts fromthe academic plant research community and private companies.Further development of thermomorphogenesis as a research areacould ultimately provide efficient and timely leads for the initiationof appropriate breeding eorts to generate much-needed thermos-tolerant crops.

References

American Meteorological Society. State of the climate in 2014. Bull. Am.Meteorol. Soc. 96 (special suppl.) (2015).

Nomoto, Y. et al. A circadian clock- and PIF4-mediated double coincidencemechanism is implicated in the thermosensitive photoperiodic control of plantarchitectures in Arabidopsis thaliana. Plant Cell Physiol. 53, 1965–1973 (2012).



Writer :: Saurabh Pandey and Sunidhi Mishra      Published on :: 25-Jan-2019


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