Submitted to: Rice Technical Working Group Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 3/25/2016
Publication Date: 9/12/2017
Citation: Heuschele, D.J., Pinson, S.R.M., Smith, A. 2017. Differences in how rice plants processes arsenic in their cells. Rice Technical Working Group Meeting Proceedings. p. 173. March 1-4, 2016. CDROM.
Technical Abstract: Arsenic (As), a carcinogenic heavy metal, is a problem in some drinking water and staple food supplies around the world. Rice plants readily uptake arsenic and transport a portion of it into the grain. Arsenic is also toxic to plants; therefore mechanisms that reduce toxicity or accumulation have evolved. Plants, such as rice, may regulate uptake, transport, sequestration/tolerance or a combination of all three to prevent toxic effects of heavy metals. A previous study of 1700 international accessions found some rice lines with high As concentrations in their grain (a.k.a. “grain-accumulators”) and others with low concentrations of As in their grain (a.k.a. “grain-excluders”). This study investigated which physiological responses, uptake, transport, or sequestration/tolerance explained the grain As concentration difference of these two groups. Rice seedlings of six varieties, 3 grain-accumulators and 3 grain-excluders from the above noted study, were grown hydroponically to V-3 then exposed to 12.5 ppm AsIII for 0, 24, 48 and 72 hours. Rice root and leaf tissues were sampled for As concentrations and key products known to be involved in tolerance and sequestration pathways for other heavy metals within plants. Arsenic uptake and transport rates between tissue types were similar for grain-accumulators and grain-excluders, therefore uptake and transport do not explain the difference between the two groups. In general however, grain-accumulators died more rapidly from As-exposure suggesting that As-tolerance or sequestration may have an important role in determining grain-As concentrations. Heavy metal tolerance and sequestration both use the same key metabolic compound, glutathione (GSH). GSH is utilized differently depending which type of mechanism is activated. In heavy metal tolerance, GSH aids in the breakdown of toxic secondary compounds such as reactive oxygen species (ROS) and methylglyoxal (MG), whereas in As-sequestration, GSH either directly binds to As or forms phytochelates (PC) that bind to As for transport to the vacuole or outside of the cell. In order to differentiate between As-tolerance and As-sequestration, we measured the key products within each pathway: ascorbic acid (AsA), lactic acid (LA), glutathione (GSH), cysteine (used in GSH production), and four types of PC. AsA and LA concentrations, key products in As-tolerance, did not differ between the groups suggesting that As-tolerance is not the process that distinguishes the grain-accumulators from grain-excluders. However, AsA concentrations in both groups did reduce over time suggesting that ROS produced from initial As toxicity are being neutralized via the breakdown of AsA, but AsA is not being recycled. Lactic acid increased in one variety indicating that MG metabolism, another mechanism of heavy metal tolerance, occurs in rice in response to As stress. This is one of the first times As activation of MG metabolism has been documented in rice. Leaf GSH concentrations were significantly different between grain-accumulators and grain-excluders. Grain-accumulators had more GSH than grain-excluders, but the levels remained the same over time, while grain excluders increased in GSH over time. GSH can neutralize ROS like AsA and the increasing concentration of GSH found over time alone does not differentiate between tolerance and sequestration. Preliminary data indicates PC synthesis is also significantly different for grain-accumulators and grain-excluders. While PC are being produced in both groups, grain-excluders appear to continually increase the production of specific types of PC, whereas grain- accumulators have an initial increase without continued production. Our results suggest that grain-excluders are utilizing AsA and GSH to implement As-tolerance and substantial As-sequestration, unlike grain-accumulators which are employing mainly As-tole