Too Much Boron

Tim -   Boron is a naturally-carrying salt soil element that is naturally quite high in concentration in soils of marine origin and that covers most of Southern Australia.  So in South Australia, for example, in our grain growing regions, we think about 30% of our soils have levels of boron above about 15 mg/kg in the top 100 cm soil, just generally consider the threshold for boron toxicity.

Chris -   So when you grow a plant in a soil like that, what impact does it have on the plant itself?  Is it directly toxic in some way?

Tim - It is.  The boron in the soil is virtually immediately taken up by the plant roots.  It gets into the transpiration stream.  It then accumulates in the leaves.  Once it gets to a certain threshold concentration in the leaves, it affects metabolic processes.  The plant cells die.  The leaves eventually die off and you get a plant that's really struggling to survive with that amount of boron in its leaves.  And the history of this is quite interesting.  Boron toxicity was really only discovered as a problem in Southern Australia in the early 1980s.  There was an association made between boron content in leaves and boron content in soil samples taken across sites in Southern Australia.  And so, this started the whole process of traditional plant breeding at the Waite Campus whereby, one of the first tips was the screening for wild relatives of barley.  They were naturally highly tolerant to boron toxicity.  And one of the most tolerant lines that was discovered in their early work was a line called Sahara.  It's an Algerian land race.  It comes from obviously West Africa and it's highly boron tolerant.  It grows very heavily on concentrations of boron in the soil that far exceed anything we see in an agronomic situation.  You wouldn't grow them as a farmer because you wouldn't get a good crop out of them.

Chris -   And so your plan would be of course, to identify strains like that and then cross those with high yielding varieties so you get the best of both worlds.  You get the yield plus you get the ability to grow well on a soil that's got a lot of boron in it.

Tim -   That's correct and that is the fundamental of modern plant breeding.  So the line Sahara has been used extensively in breeding programs across southern Australia as a donor linefor the tolerants to boron trait and that has been integrated into more agronomic lands of barley using traditional breeding approaches.

Chris -   How do you know that you've got the boron resistant trait into the progeny, into the new hybrid that you make by crossing the resistant form with the high yielding variety?

Tim -   We can look at the progeny from those crosses and we can phenotypically assess them.  Are they boron tolerant or are they not?  If they are, they obviously have the genes or genetic loci are required for tolerance or we can use more modern techniques, utilising molecular markers, or we can use a DNA tag, we know is linked to the boron tolerance trait of interest, and we can screen progeny, resulting from crosses for that molecular marker.

Chris -   So do you actually know then, what genes are responsible for making plants grow well even when there is an overdose of boron in the soil?  Have you got that gene that does that?

Tim -   Yes, we do.  We have. We know of at least four of the major genes in barley that was identified through traditional mapping work in the late 90s of this campus.  We've since cloned about three of those, three of the four major transporters.  So we're getting a good grip on the molecular bases for these tolerance traits in barley.

Chris -   So you say transporters.  So, these genes make some kind of pump that can move boron out  of the plant if it's in a soil, where boron is trying to come in to the plant, it can get rid of it.

Tim -   That's correct.  The main transporter located on chromosome four of barley is a membrane efflux transporter and its role in the plant is to pump boron out, once it gets into the cell.  And the net effect of that is, is there's less accumulation in the leaves of the plant.  The plants are happier.  They don't suffer from toxicity.

Chris -   What about if you put that same plant on a soil that's now not got too much boron?  Is it still happier?

Tim -   It depends which gene you're talking about.  The major one on chromosome four is constitutively expressed and what that means is that it's not induced by the presence of boron.  It's on all the time everywhere.  The downside of that is that under conditions of lower boron supply in the soil, these plants actually suffer from slight symptoms of boron deficiency.  They're not getting enough boron into their leaves to be happy.  And that's something that we're working on trying to modify.  That's a good example of why it's really important to clone these genes.  Identify the exact sequence of these genes.  And once we know that, we can look at the regulator elements upstream of these genes that control their expression.  In other words, when and how much are these things expressed.  Once we know these things, we can then start to modify them and one of the things we could try and do, and we are trying to do indeed is to make this boron transporter inducible to boron so that when it's not needed in the plant, it's not expressed.  But when it is needed under conditions of high boron, the gene comes on, and you get the tolerance phenotype that you're looking for.

Chris -   Which would be incredibly neat because the plant then just responds to the environment according to what it needs.  And you don't have to have more than one type of plant.  If you were to look at the impact of this technology presuming you're going to be successful, what difference could it make to the yield that you'll get, both here in Australia and perhaps to other countries around the world?

Tim -   The numbers that we think occur, at least in southern Australia, we think about 30% of our soils were affected by boron.  That could acquire to $250 million of annual production.  But talking more globally, it's difficult to say because often, these problems don't come in isolation.  Often when you have a boron toxic soil that comes along with things like salinity, aluminium toxicity, a whole range of problems that are difficult to isolate and single out.  That could have a big impact.

Chris -   And how close are you to having those crops ready to go?

Tim -   We're working very hard to get the material to a stage where it could be useful for farmers, and in fact, we're in the stage two or our second year now of a GM field trial in South Australia where we have gene material containing the boron tolerance gene from Sahara.  So we're evaluating that material at the present in this growing season.