Salt Solution

Rice plants can be induced to boost their filtering system and reduce salt uptake
Saturday, June 25th, 2011
Root Barriers

Rice plants (above) and their roots (below) stained with berberine aniline blue. Rice images from WikiCommons; Root images from paper cited in article. Design: Aathira Perinchery.

As in many parts of the world, soil salinity is a chronic condition in cultivable lands across most of India. Water provided by irrigation is saltier than rain, and so increased dependence on irrigation, and inadequate drainage systems, are causing soil salt to build up to toxic levels. Plant scientists are trying to develop crop varieties and management strategies that lead to higher salt tolerance. Pannaga Krishnamurthy and M.K. Matthew, from NCBS and their international colleagues* recently developed an elegant and practical technique to induce salt tolerance in rice, reported in the Journal of Experimental Botany.
Casparian Bands

Before-and-after-conditioning photos of roots of the rice cultivar IR20. Exposure to a mild level of salt causes the walls of certain root cells to become impregnated with a waterproof wax (arrows). Water must now enter salt-filtering cells, which stop the salt reaching the plant's plumbing system (the xylem, the bright rings). Images from the cited paper.

Their method, developed using the salt-intolerant cultivar IR20, simply requires growing young rice plants for a week in a moderately high level of salt, about half that of the concentration that is toxic. This brief exposure, which they refer to as conditioning, "vaccinated" the plants: they would now survive if grown in what were previously toxic levels of salt.

The authors were also able to show how the tolerance to salt is increased. Conditioning shifts the balance between the two ways in which soil water can enter the plant's plumbing system (the xylem). Prior to conditioning much of the soil water that enters the xylem does so directly - it is thus unfiltered, and carries with it all the soil's dissolved compounds, both good and bad. After conditioning, a much greater proportion of soil water can only enter the xylem after having first crossed the plant's living tissues. These can filter out unwanted compounds - like salt. Shunting of water into the living tissues is primarily the job of a network of waxy, waterproof barriers (Casparian Bands) the most important of which are handily located around the inner core of the root, where the xylem is concentrated. Conditioning boosts the extent and effectiveness of this network, and this means that more of the plant's water ends up being filtered.

Filtering comes at a cost though. After conditioning, water moves through the plant more slowly, and the authors found that rice plants compensate by sprouting more roots. Whether these responses affect productivity is not yet known. But more importantly, the team was able to confirm that the roots that are triggered by conditioning are not leaky: they keep salt out too.

The authors also studied a second rice cultivar, Pokkali, which has a long history of success in areas suffering from salinity. They were able to show that its high salt-tolerance was due to it already having, without any need for conditioning, a filtering network of similar effectiveness to that which they saw after conditioning in IR20.

Overall, the study indicates that farmers will soon have much more choice in the cultivars they can grow on India's increasingly saline soils. And it tells scientists the characteristics they should focus on when they try to breed cultivars that are even more salt-tolerant.

Pannaga Krishnamurthy

Pannaga Krishnamurthy

* The article: "Root apoplastic barriers block Na+ transport to shoots in rice", (2011) J. Exp. Bot. doi: 10.1093/jxb/err135
Pannaga Krishnamurthy1, Kosala Ranathunge2, Shraddha Nayak1, Lukas Schreiber2 and M.K. Mathew1
1. NCBS, Bangalore.
2. University of Bonn, Germany


Hello, The problem is

Hello, The problem is interesting. Not going into the details I like to remark: Many possible solutions as in 2 links below. First thing which occurs to mind is a cylic crop which obsorbs salts to keep the salt % under check. Secondly, Use of salt desolving or converting media mixed with the fertilisers. ... Does Salt tolerance mean gentically modified? What happens to a salt tolerant plant in a salt free soil? Is it true? A lemon tree yields sour lemons once planted in any type of soil. If so, how? Best wishes RaoSN

There are plants which

There are plants which accumulate Na+. The Central Salt & Marine Chemicals Research Institute works on some such plants. I am not sure what desolving or converting media are. Salt tolerance does not necessarily mean genetically modified. There are many traditional crop varieties that are very salt tolernat - including the Pokkali cultivar of rice that we studied in the paper. What we showed is that some salt sensitive (and high yielding) varieties also have the ability to mount defenses against salinity if challenged to do so. The issue now is to find out ways of eliciting those responses when needed. Behaviour of salt tolerant varieties in salt-free soil. That is a good point. The way we think of this is that Pokkali (whihc is salt tolerant) has its defenses turned ON all the time. This is useful for Pokkali as it is grown in coastal regions and often gets inundated with sea water. The inland varieties are capable of the same defenses but turn them on only when forced to do so. The difference could well be in terms of yield as it costs energy to mount these defenses.

Kahara Lana is a rain fed

Kahara Lana is a rain fed crop that grows under hot arid conditions of north western Rajasthan. It is widely known for growing in soils having high salt concentration. salt tolerance, not necessarily being a genetically modified crop here but definitively it does includes the inbuilt capacity of a crop to withstand the damaging effect of high salt concentrations. studies done in our laboratory has shown it's high salt tolerance level when being grown in test tubes at different salt concentrations. salt tolerance like other abiotic stresses is a complex trait involving differential regulation of various stress related transcripts especially those involved in maintenance osmotic and ionic homeostasis. one such system; owing to their high tolerance levels can be studied to understand the expression of genes inducing signalling cascade there by conferring salt stress tolerance.

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