STORRS, Conn. -- A team of scientists led by University of Connecticutplant biologist Roberto Gaxiola has discovered an overlooked genetickey to generating plants that are more productive, more droughtresistant and can grow in soils low in nutrients. Their work is thefirst to successfully test in cells a 30-year-old hypothesis thatexplains the movement of a primary growth and development hormonethrough plants and is expected to prompt biology textbooks to berewritten.
The researchers from UConn, Purdue University and Pennsylvania StateUniversity determined that one of three proton pumps found within plantcells, previously believed to have an extremely limited function, playsa critical role in plant root and shoot system growth and developmentby controlling cell division, expansion and hormone transport.Over-expressing the single gene that encodes this particular protonpump significantly enhances the transportation of the primary plantgrowth hormone, auxin, and results in plants with stronger, moreextensive root systems and as much as 60 percent more foliage, theresearchers report in the Oct. 7 issue of the prestigious journalScience.
"This discovery has the potential to revolutionize agricultureworldwide," said Gaxiola, an assistant professor-in-residence inUConn's plant science department. "This over-expression regulates thedevelopment of one of the most important parts of the plant, the roots.A plant with larger roots is a healthier and more productive plant,because, with a larger root system, the plant is able to get water andnutrients from larger soil areas.
"Biology textbooks tell you there are three pumps inside a plant's cellbut one is less important. Our research shows that is not the case,"Gaxiola said. "As it turns out, that tiny pump is required to shuttlethe master pump, the plant's major engine, to the plasma membrane.That, in turn, allows the master pump to facilitate the transport ofmore of the growth hormone, auxin, through the plant's plasma membraneand through the plant's root and shoot systems, resulting in enhancedcell division and growth."
All plants contain three proton pumps -- a master pump, known as theP-type H+-ATPase, that facilitates transport of nutrients in and out ofplant cells, and two other pumps that work within plant cells.Biologists have shown that only the P-type H+-ATPase pumps protons intothe space outside the cell to create changes that drive the transportof small molecules in and out of cells. Until now, they believed theAVP1 H+-PPase that Gaxiola's group over-expressed merely controlled pHlevels within plant vacuoles, or large storage areas inside plantcells, and served primarily as a back-up pump to a larger vacuolar pumpknown as V-ATPase. Scientists believed that the larger vacuolar pumpwas the only one to help shuttle the master pump to and from the plantcell's plasma membrane.
In collaboration with scientists at the Massachusetts Institute ofTechnology and Harvard University, Gaxiola previously had createdplants in which the AVP1 gene was over-expressed using the researchplant Arabidopsis thaliana. As Gaxiola predicted, these plants weresalt- and drought-resistant and sequestered more salt ions in theirvacuoles. Surprisingly the plants also had abnormally large root andshoot systems.
Simon Gilroy, a Pennsylvania State University cell biologist, providedanother piece to the puzzle when he discovered that the pH, whichindicates proton concentration, was unchanged inside the cells. But theextra-cellular pH was lower, meaning it was more acidic and had ahigher proton concentration.
The next clue came from plant cell biologist Angus Murphy and his colleagues at Purdue University.
"When Simon reported the acidity and the proton gradient was increasedbetween the inside and outside of plant cells in Roberto's overexpression lines, we saw an opportunity to test the model that had beenused to explain the transport of the plant hormone auxin for the last30 years," Murphy said. "This model predicts that an increased protongradient should result in a faster rate of auxin transport. This theorynever had been tested directly tested in plants where the protongradient had been manipulated by molecular genetic techniques. When wedetermined that the rate of transport was increased, but the overallauxin content was not, the auxin transport model was validated."
They determined AVP1's critical role by comparing the transgenic plantsto both ordinary Arabidopsis plants and mutant versions of the plantthat were devoid of AVP1. They discovered that the AVP1 mutants didn'tdevelop functional root systems and their shoots were tiny anddeformed.
Gaxiola specializes in manipulating plant proton pumps for cropimprovement and relied on Murphy and Purdue colleague Wendy Peer, forexpertise in auxin transport in plants, and Gilroy for expertise inplant cell biology with an emphasis on roots.
Additional authors are UConn doctoral students Jisheng Li, HaibingYang, Soledad Undurraga and Mariya Khodakovskaya; Purdue doctoralstudents Joshua Blakeslee, Anindita Bandyopadhyay, BoosareeTiapiwantakun, Elizabeth Richards; Penn State doctoral student GregoryRichter; and University of South Carolina Biology Professor Beth Krizek.
Gaxiola said that early experiments to duplicate the Arabidopsisresults in other crops, such as tomatoes, rice, cotton and poplartrees, indicate the team's discovery could have implications forincreasing the world's food production and aiding global reforestationefforts. He predicts that within the next five years there will be a"boom" of crops genetically engineered using his team's approach.The research team's findings are likely to be particularly significantfor farmers in developing countries, including Gaxiola's native Mexico,because many live in arid regions and lack irrigation systems and moneyfor the amount of expensive fertilizers needed to feed plants with lessexpansive root systems.
U.S. patents currently are pending and a research licensing agreement with an international company has been signed.
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