Getting to the root of legume fertility
A conservative estimate puts the value of legumes to agriculture in WA at WA grows legumes for two main purposes. There are grain crops such as lupins, peas, chickpeas and lentils, and forage legumes such as clovers, medics, and serradellas which are used as pasture for animals.Both kinds of legumes are critical to soil fertility -- they increase soil nitrogen so that subsequent cereal crops can be grown with sustainable yields.Unfortunately, some potentially useful pasture legumes -- such as the medics -- show little tolerance for the acid soils which predominate in the Eastern Wheatbelt of WA. This is because most types of the root nodule bacteria necessary to nodulate the legumes die from the acid.This is not necessarily a trait exhibited by all root nodule bacteria. Today there are hundreds of thousands of hectares of the medic (Medicago) species of legumes growing in WA pastures, courtesy of acid tolerant root nodule bacteria which have been introduced from Mediterranean isolates.These strains of bacteria are produced commercially in peat cultures which are simply coated onto the medic seed before sowing.The success of the medic species has led scientists at the Centre for Rhizobium Studies (which involves Murdoch University, Agriculture WA, the Centre for Legumes in Mediterranean Agriculture and the Grains Research and Development Corporation) to pose the simple question: Why are some strains of legumes acid-sensitive and why are some acid tolerant?
One approach to answering this question was to take acid-tolerant strains and mutate them into acid-sensitive strains and then find out what characteristic had been altered, explained Professor Mike Dilworth, Deputy Director of the Centre."If we can isolate that characteristic then we know that something or other must be very important for the acid tolerance process," said Professor Dilworth."The idea behind all this is that if you know enough about this maybe you can tweak these strains ever more to make them even more acid-tolerant."This application would have potential benefit for WA agriculture given that a lot of Australian soils are acidifying rapidly as a result of farming practices, thus leading to a decline in legume productivity.Professor Dilworth explained that the mutation process involved the insertion of a "jumping gene", a piece of DNA which was inserted at random into bacterial DNA. This ruined the normal function of the gene. When the correct gene was reinserted, the recovery of acid tolerance verified that the scientists were looking at a gene important for acid resistance."It's a way of locating where the important genes for acid tolerance are and what they do," said Professor Dilworth.Murdoch University has been carrying out research into root nodule bacteria since the mid 1980s. The research was initially funded by the Australian Wool Corporation which was concerned about the acidification threat to productivity of legume pastures for sheep.Since 1990, the research has been funded by the Australian Research Council with the current three-year grant of approximately $330,000 set to continue till 1999.While mindful of the potential applications for industry benefit, Professor Dilworth is also enthusiastic about some of the questions and discoveries emerging from this basic science research.For example, the research has discovered that the root nodule bacteria become far more sensitive to heavy metals under acid conditions.Copper in the medium became much more toxic under acidic conditions than under neutral conditions, Professor Dilworth said.Other experiments had shown that sensitivity to acid could be reversed by the addition of calcium."We don't understand why that is -- there is a mystery here. All we know is that if we stack a lot of calcium into the medium then even these acid-sensitive mutants will still grow under acid conditions," said Professor Dilworth.There are currently several research programs at the Centre looking at the effect of calcium.Another significant area of research focused on habituating root nodule bacteria to acid conditions.Experiments had shown that if bacteria were grown in slightly acidic conditions and then exposed to acid (as they would be if put into acid soils) they died much more slowly than they normally would have."The question then is should you in fact be growing these plants in a semi-acid pH if you if you want to use them in acid soil," said Professor Dilworth."We don't know if it is true but we are investigating that application with industry. We want the bacteria to last long enough to at least inoculate that set of plants and ideally to colonise the soil for the next time you want to grow that legume." |
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| Nodules on Medicago murex |
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Professor Mike Dilworth Centre for Rhizobium Studies Telephone (08) 9360 2112 email dilworth@central.murdoch.edu.au |
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