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07 September 2012

Making lazy, stupid plants work harder

Plants with larger root systems take up minerals more easily.
Plants these days. They're coddled, entitled, fed with a silver spoon.

Use of man-made fertilizer and traditional breeding, over the years, has selected for traits that led to today's modern-variety plants that grow fat with yields.

But the downside of easy access to nutrients is that it has allowed for the breeding out of desirable traits that has left plants, well, acting like enabled, spoiled children.

"They're lazy," said plant biochemist Roberto Gaxiola, an assistant professor of cellular and molecular biosciences at Arizona State University. Because nutrients are plentiful, they don't bother with growing large root systems. Yet, he explained to me, larger root systems are needed for them to take up more phosphate and nitrogen from the soil.

More now than ever, plants depend on these fertilizers for growth. Wild crop plant varieties, on the other hand, have had to evolve in an environment of everyday nutrient scarcity. It's these wild crop plant root systems that have been the focus of Gaxiola's research for more than a decade.

Root engineering 

Gaxiola told me that these wild varieties have learned to use more efficient pathways of transporting sugars from leaves to roots. They produce larger root systems that do a better job at acidifying soil and taking up minerals like nitrogen, phosphate, potassium.

He's identified genes involved in overexpression of a type of kinase, an H+-PPase, that phosphorylates a process of loading sugar. The localization of this kinase is in cells that surround the vascular tissue, which is used to synthesize and transport sugar.

"It’s the only way that roots can grow and be active," Gaxiola told me. "You cannot grow a root if you don't move sugar from the leaves. What does this kinase regulate? It could be one of the genes, but clearly it's multifactorial. It's upregulating sugar transport for a larger root system."

Last May, Gaxiola and his colleagues published their latest of a series of papers detailing how his lab inserted genes from wild rice, tomatoes and arabidopsis into modern-yield, salt-tolerant varieties (1). These genes, as part of commercial varieties through root engineering, could lead to more efficient use of phosphorus and increase crop biomass and seed yields.

Another paper, recently published in Nature by scientists from the International Rice Research Institute, uses a similar approach (2). These scientists isolated and inserted a gene from a wild variety of rice into commercial strains to produce larger roots that better take up phosphorus, nitrogen, and potassium.

Gaxiola said, "It's interesting work. We have similar results, but with a different gene." Again, a kinase is involved that regulates synthesis of sugar for transport to the roots to become more active, grow, and take up minerals.

Phosphorus and the future

"It's a good example of how crop domestication made crop plants 'stupid' and dependent on heavy external fertilizer inputs. Their plants seem very promising indeed," said biologist James Elser, a subject of a previous post.

Elser, also an ASU professor, is raising more awareness about the need for sustainable phosphorus. His wish, he'd said to me before, is to hear "President Obama say the word 'phosphorus',"as the problem deserves serious attention.

According to Elser, plants that can more efficiently use phosphorus could end up being highly beneficial to poor people, such as in underdeveloped Asian countries. As phosphorus availability becomes depleted and costs for fertilizer rises, farmers in these countries can't afford fertilization of their rice plants. They're also hit with the fact that the roots won't take nutrients. Often the soils have the phosphates, but they're bound to the soil. The roots need to acidify the soil to release the phosphate that's bound.

Plants that efficiently use phosphorus could also reduce eutrophication. The problem is caused when fertilizer run-off enters streams, rivers, lakes, and oceans. These extra nutrients cause algae to bloom and create "dead zones," Elser said, one of the most famous examples of which is the Gulf of Mexico Dead Zone.

Hurdles of funding and anti-GM sentiment

With so much to gain from plants that can grow in phosphorus-poor soil or with less fertilizers, I had to ask Gaxiola, How close are we to seeing salt-tolerant, large-rooted plants being used commercially? Gaxiola's plants, for example, as he described, are "one of the strongest phenotypes" ever produced for the market.

"Oh, my friend," Gaxiola replied gently to my question, in a way I've found to be typical of his native Mexico City, "there are field trials, heavy investment, and then you have to pray that a company like Monsanto gets interested."

Why Monsanto? Because there are few other large biotechnology companies who could jump through the hoops required to commercialize a genetically engineered crops. It's unfortunate, Gaxiola told me, because to reduce the impact of fertilizer overuse on the environment and biodiversity, what we really need is more investment into plant engineering and more commercialization of nutrient-efficient plants.

One is led to wonder: With all it's focus on growing crops organically, is anti-GM sentiment actually hindering development of plants that could help the environment? Also, who is anti-GM sentiment helping if not only Monsanto? If only Monsanto can invest in GM? Wouldn't less regulations on GM help other biotechnology companies get in the game? These are questions worth more exploration.

References

  1. Gaxiola RA, Sanchez CA, Paez-Valencia J, Ayre BG, Elser JJ. Genetic Manipulation of a "Vacuolar" H+-PPase: From Salt Tolerance to Yield Enhancement under Phosphorus-Deficient Soils. Plant Physiol 159, 2012;3-11. doi: 10. 1104/ pp. 112. 195701
  2. Gamuyao R, Chin JH, Pariasca-Tanaka J, Pesaresi P, Catausan S, Dalid C, Slamet-Loedin I, Tecson-Mendoza EM, Wissuwa M, Heuer S. The protein kinase Pstol1 from traditional rice confers tolerance of phosphorus deficiency. Nature 488, 535–539 (23 August 2012) doi: 10.1038/nature113466

Photo credit: ASU

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