The Quest for Drought Tolerant Crops

Whether seeking to address concerns about climate change or trying to grow crops in areas that are otherwise too hostile, plants able to survive in times of low water availability would be significantly beneficial.

To this end, researchers are discovering the genes and pathways that enable some plants to resist death from dehydration better than others. A study published in June in the Plant Cell journal has seemingly found one such gene and discovered how it impacts on the plant’s susceptibility to dehydration. The article is called “The Arabidopsis Transcription Factor NAC016 Promotes Stress Responses by Repressing AREB1 Transcription through a Trifurcate Feed-Forward Regulatory Loop Involving NAP” (catchy title I know – they had at Transcription Factor NAC016).

Confidence grows in the findings of an article when the researchers confirm one result by using a different method that will give an answer to the same question. In this article the same question was asked and then answered using different methods a number of times instead of trusting the result of just one test. Probably part of the reason it is published in a journal with a high impact factor.

In short, this article found that a certain gene encodes a protein that interacts with the DNA (to either activate or repress transcription of a certain gene – the protein is called a transcription factor) and that this particular transcription factor is activated under certain stressors such as dehydration.

The researchers performed a number of tests to connect the dots between the transcription factor (NAC016) and how it effects the plant cell, if at all. What they found is that expression of NAC016 is increased under drought stress and represses the transcription of another gene (AREB1). AREB1 has a positive effect on the ability of the plant to withstand dehydration. Therefore, they found that mutants that under-expressed or this gene tolerated drought stress better than the normal (wild type) plant.

To make these conclusions they performed a number of tests:

  1. Placing plants under different stressors they looked for increased expression levels in NAC016; finding such an increase indicates that it has some effect on the plant whilst being dehydrated and this increase was observed.
  2. To test whether the increased expression of NAC016 was good or bad for the plant in its response to drought, they compared the effect of dehydration stress on a wild type plant, a plant that over-expresses the transcription factor (NAC016 -OX) and a mutant that doesn’t express or under-expresses the transcription factor (NAC016-1). In testing, the mutant survived dehydration and was able to recover when rehydrated in contrast to the wild and the over-expressing plants which didn’t survive or failed to recover.
  3. A certain type of acid (phytohormone abscisic acid or “ABA”) is known to assist plant survival from water deficit. When treated with ABA, the mutant was more sensitive to the treatment and the diameter of the stromata (holes in leaves) reduced, which assists drought resistance in plants. The same effect wasn’t seen in the wild type, suggesting that the decreased NAC016 transcription increased the plant’s sensitivity to ABA.
  4. Where the transcription factor is located in the cell, how it reacts when dehydrated and what effect that reaction has was tested. Using a mutated form of the transcription factor they found that the transcription factor is found away from the nucleus under normal conditions but, under drought stress, part of the transcription factor is cut, allowing to move into the nucleus to act on the gene.
  5. To discover what effect this gene was having on other genes, the researchers performed a microarray analysis to look at differences in gene expression between the NAC016 mutant and wild type, identifying 3 drought responsive genes that were upregulated by more than 2 to 1 in mutants compared to the wild type plant. This signals that a reduced transcription of NAC016 is increasing transcription of other drought tolerant genes, including AREB1.
  6. Testing against another known drought-resistant transcription factor, the researchers over-expressed the AREB1 gene in a plant with either an over-expressed NAC016 transcription factor or the earlier discovered transcription factor. They found that the plants showed the same drought resistance as a plant with just AREB1 over-expressed. This indicated that NAC016 and the earlier found transcription are both are required to have an effect on the plant.

 ArabidopsisFigure from article showing difference in response by Wild Type, mutant and Over-expressed NAC016 Arabidopsis plants to 12 hours drying treatment (HDT).

The article is detailed. It shows the persistence and logical thinking needed to identify with confidence the processes alive in plants that could be used to help us help crop plants to survive such challenges.  As the authors stated:

Identification and functional analysis of drought-responsive genes may help plant breeders to develop drought tolerant plants..

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