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   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    Dry days trigger leaves to send a surprising growth signal telling roots   
   to keep growing    
      
     Date:   
         June 26, 2023   
     Source:   
         University of Cambridge   
     Summary:   
         Scientists have discovered a new molecular signalling pathway,   
         triggered when leaves are exposed to low humidity, that ensures   
         plant roots keep growing towards water. A new study has found that   
         when the leaves of a plant are exposed to dry air (low humidity),   
         they send a shoot-to-root signal, using abscisic acid (ABA),   
         to tell the roots to keep growing.   
      
         This is a surprising finding as ABA is usually thought to be a   
         growth inhibitor, not a growth promoter.   
      
      
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   FULL STORY   
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   Scientists at the Sainsbury Laboratory Cambridge University (SLCU) have   
   discovered a new molecular signalling pathway, triggered when leaves   
   are exposed to low humidity, that ensures plant roots keep growing   
   towards water.   
      
   In dry soil conditions, plants take action to try and conserve water by   
   producing the drought stress hormone abscisic acid (ABA). For decades   
   plant scientists thought that in response to dry soil, ABA was made   
   in the roots and then transported to the leaves. In this root-to-shoot   
   signalling pathway, ABA closes microscopic leaf pores, called stomata,   
   to prevent water loss from leaves. In recent years, scientists have   
   shown that other root-to-shoot signals also tell leaves to make their   
   own ABA to conserve water.   
      
   A new study has found that when the leaves of a plant are exposed to dry   
   air (low humidity), they send a signal in the opposite direction to the   
   plant's roots -- also using ABA -- to tell them to keep growing. This is   
   a surprising finding as ABA is usually thought to be a growth inhibitor,   
   not a growth promoter.   
      
   This shoot-to-root signalling pathway potentially helps the plant to   
   search for water deeper down into the soil, giving it a greater chance   
   of survival.   
      
   With the drought response now well documented in roots and not so much   
   research done in shoot-to-root signalling, Dr Alexander Jones' research   
   team wanted to find out more about what was happening above-ground in   
   the leaves under dry air conditions. The low humidity shoot-to-root   
   signalling they discovered could be acting as an early warning for the   
   plant to prepare for future water shortages.   
      
   The revelations, published in Nature Plants, were made possible through   
   the Jones team's design and re-engineering of a next-generation biosensor,   
   ABACUS2.   
      
   "We've known for several years that, at low humidity, plants prioritise   
   root growth. In many species when the humidity decreases, even though   
   photosynthesis and shoot growth is reduced, the root growth is maintained   
   or even increased," said Dr James Rowe, first author of the study.   
      
   "The molecular mechanisms behind this phenomenon have been a mystery until   
   ABACUS2 allowed us to measure ABA concentrations at the cellular level in   
   Arabidopsis thaliana seedlings. We saw that when the leaves experience   
   low humidity stress that ABA accumulates in the root tips. The leaves   
   are reacting to the dry air and telling the roots to continue growing,   
   enabling plants to maintain foraging of deeper soil for water."  What is   
   perhaps even more surprising is that ABA is historically thought of as a   
   growth inhibitor. "Even some plant scientists are surprised to discover   
   that ABA can promote root growth," Rowe said, "but it's actually really   
   important so that plants can keep searching for water under the ground   
   during water stress."  ABA concentration levels are key -- just the right   
   amount of ABA maintains root growth, but too much ABA and the roots will   
   stop growing.   
      
   Dr Alexander Jones says this sensitivity to ABA concentration means the   
   plant does not overreact: "The root ABA comes from the phloem, which   
   transports sugars and hormones from the shoot and is unloaded in the   
   root tip. ABA signalling can fine-tune root growth as humidity varies,"   
   Jones said. "Low humidity at the leaves regulates ABA accumulation in the   
   roots, and vice-versa, low soil moisture at the roots regulates ABA in   
   the leaves. This indicates that the root and shoot can each systemically   
   regulate each other's responses to stresses that may only be experienced   
   locally, thus providing a robust system to overcome water stress."   
   "This is useful fundamental information to help in understanding the   
   physiological changes happening to crops grown under irrigation where the   
   air may be dry, but the roots are growing in wet soil -- an increasingly   
   prevalent condition with climate change."  Rowe hypothesises that there   
   could possibly be two signals in play and future research will look at   
   identifying what signalling is happening between the leaves and roots   
   under drought and low humidity stress.   
      
   This research was funded by The Gatsby Charitable Foundation and   
   Biotechnology and Biological Sciences Research Council (BBSRC).   
      
       * RELATED_TOPICS   
             o Plants_&_Animals   
                   # Endangered_Plants # Drought # Soil_Types # Botany #   
                   Organic # Agriculture_and_Food # Nature # Trees   
       * RELATED_TERMS   
             o Root_vegetable o Nicotine o Ginger o Hydroponics o Fertilizer   
             o Herbal_tea o Plant_cell o Purple_loosestrife   
      
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   Story Source: Materials provided by University_of_Cambridge. Note:   
   Content may be edited for style and length.   
      
      
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   Journal Reference:   
      1. James Rowe, Mathieu Grange'-Guermente, Marino Exposito-Rodriguez,   
         Rinukshi Wimalasekera, Martin O. Lenz, Kartika N. Shetty, Sean   
         R. Cutler, Alexander M. Jones. Next-generation ABACUS biosensors   
         reveal cellular ABA dynamics driving root growth at low aerial   
         humidity. Nature Plants, 2023; DOI: 10.1038/s41477-023-01447-4   
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   Link to news story:   
   https://www.sciencedaily.com/releases/2023/06/230626163430.htm   
      
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