Philip White writes: Recently, I compiled a list of contemporary Citation Classics in Plant Sciences. My intention was not just to identify key papers in Plant Sciences but also to discover something about the historical context of these papers, the authors’ motivations, and why the authors believed their paper had become so well cited. So, I have asked Tim Flowers (University of Sussex, UK) to comment on his paper from 2004. This is one of my favourite papers from the last quarter century and one I frequently include on reading lists for my students. Tim contributed to the original Citation Classics series published in Current Contents describing how his studies of the mechanisms of salt tolerance in halophytes began and the following commentary reaches back through his longstanding interest in the subject.
Commentary by Tim Flowers
Since my undergraduate days (when I shaved the hairs off leaves of Verbascum thapsus to look at the effects on transpiration), I have been interested in plant water relations, although at the time I felt more at ease with biochemistry than physiology. Fortunately, I had the chance to combine these interests in my PhD with Professor F L Milthorpe at the University of Nottingham and then with Professor J B Hanson at the University of Illinois. It was in Illinois, where I looked at the effects of solutes on respiratory enzymes, that the idea of testing the response of enzymes of salt-tolerant plants to salt first arose. In 1968, I moved to a lectureship at the University of Sussex, where I was able to look at the properties of enzymes extracted from halophytes. Although my work attracted Research-Council funding and generated an Annual Review, it was clear to me that this topic would not provide research funds over the long-term. After various enquiries, I was awarded a grant from what is now the Department for International Development to look at whether anything could be done to enhance salt-tolerance in rice. Tony Yeo, one of the co-authors of the Annual Review, joined me in this research and we spent many years trying to unravel the response of rice to salt. In 1995, we published a paper in which we explored how best resistance to salt might be bred into crops. This was at a time when transgenic plants were beginning to be produced and Hans Bohnert and Richard Jensen pointed out that we had neglected the role of plant transformation in generating salt tolerance. They advocated the use of transgenic plants and argued “…that successful releases of tolerant crops will require large scale ‘metabolic engineering’ which must include the transfer of many genes”. We responded that we thought this approach might be viable in the future, but not as the ‘next step’ in breeding for tolerance and that we did not agree with the expressed view that “tolerance breeding must be accompanied by plant transformation”. However, in the decade from 1993, more and more papers were published with the intended goal of generating salt-resistant genotypes through transformation and I became alarmed at the way the salt tolerance was being evaluated. Salt-tolerance is a trait that depends on the environmental conditions and tests of tolerance often neglected this fact. Consequently, in 2003, I set out to evaluate published papers where transformation was reported to enhance salt tolerance and to comment on the validity of the methods of assessment.
The 2004 review is set in the context of a world where the human population was predicted to grow from 6.1 billion to 9.3 billion and with 800 million people being chronically undernourished. Consequently, crop production needed to rise but it was not easy to see where more crops could be planted as approximately half of the world’s land surface was arid or semi-arid. Irrigation would be required, but because of a strong link between irrigation and land salinisation, enhancing the salt tolerance of crops was necessary. The review set out the historical context of attempts to raise salt tolerance in crops and explored the genetic and physiological bases of tolerance, establishing the trait as multigenic. The review then returned to the issue raised by Bohnert and Jensen and showed that 13 species had been transformed with 40 genes in experiments reported between 1993 and 2003. However, only 19 of 68 reports provided quantitative data, with 35 papers evaluating tolerance in the absence of transpiration. My message was that tolerance had to be evaluated quantitatively on genetically stable material and compared with a parental line under saline and non-saline conditions – where plants transpired. Although there were limited data, there was a surprise in that changing the expression of single genes could affect salt tolerance. If single-gene transformants could alter tolerance, I argued that changing a key gene or process could affect the overall trait, clearly, an important result in terms of our ability to manipulate complex traits. I concluded, “Transgenic technology will undoubtedly continue to aid the search for the cellular mechanisms that underlie tolerance, but the complexity of the trait is likely to mean that the road to engineering such tolerance into sensitive species will be long. In the meantime, it would be expedient to continue to invest in other avenues such as the manipulation of ion excretion from leaves through salt glands and the domestication of halophytes.”.
So why has this review been cited so often? Part of the reason might be because of continued population growth and the need to feed ten or so million people when the climate is changing in such a way that aridity is likely to increase in many areas; aridity generates salinity through both the evaporation of natural water-resources and the use of irrigation. Since virtually all of our crops are salt-sensitive, salinity could clearly limit productivity. While I like to think the review had a message to molecular biologists about how to evaluate the fruits of their labours, I am not confident this has been received. Recently, I looked again at the salt tolerance transgenic plants. By early March 2013, more than 430 papers involving transgenic plants had been published with the expressed aim of enhancing salt-tolerance; however, only 17 of these involved field trials and only in wheat was there good evidence of enhanced tolerance in a transgenic line. Overall, these results are rather disappointing and suggests to me that where plant transformation is part of a plant-breeding programme, the molecular biology may be the simpler part of the overall process. There is a long path from transformant to crop in the field.