Monday, June 15, 2015

Drought Resistant Plants?

Water is an essential component to life on this planet. Dehydration is a serious problem; complete lack of water will kill an adult human in only 3 days. Plants also suffer from dehydration, which can also lead to their death, but more important to agriculture is the loss in seed yield. Susceptibility to dehydration varies between plants, for example, cacti store water and thus can go a very long time without water, while impatiens require frequent watering. Drought, lack of water, is occurring worldwide. Right now California is, and has been, experiencing an exceptional drought. In 2014, 5% of the irrigated cropland and over $1 billion in direct agriculture was lost. That's just the losses in one year for one state, imagine the losses worldwide if droughts continue to spread.

Scientists have been studying drought tolerance properties in plants for decades with the hope of being able to translate the knowledge into drought resistant crops. And this month, a new transcription factor was described that might play a major role in regulating drought tolerance (Sakuraba, 2015). A transcription factor is a protein that binds to DNA to activate specific genes, they are the middle men of the cell, passing a signal to activate the work force. When new environmental condition arises, cells need to change the genes that are expressed rapidly and this is accomplished by transcription factors. Once they are activated, transcription factors then activate a suite of genes which produce the proper proteins for survival. This particular transcription factor is designated NAC016. The NAC family of transcription factors is one of the largest known in plants, with 106 NAC genes in the Arabidopsis (the study organism and model species for plant research) genome.

The levels of NAC016 were measured in a variety of abiotic stressors in wild type (aka normal) Arabidopsis by researchers in Dr. Nam-Chon Paek lab at Seoul National University. They found that these levels increased dramatically only during dehydration, not under cold or heat stress which suggested that this transcription factor plays a role in drought tolerance. To fully characterize NAC016, the researchers used two different mutants, a knockout (nac016) which has a non-functional NAC016 and an overexpressor (NAC016-OX) which expresses NAC016 at a very high level regardless of external conditions. After dehydration for 2 weeks, all 3 lines were compared and only the nac016 knockout had a high survival and recovery rate. Additionally, they looked at seedlings moved to dry filter paper and after 12 hours the nac016 again were fine while the other two lines were wilted. All of this means that the loss of NAC016 results in more drought tolerance!

Arabidopsis wild type and nac016 mutant after drought stress. Photo credit: Nam-Chon Paek, Copyright ASPB.

How does this work? The loss of NAC016 resulting in an increase of drought tolerance means that NAC016 is a negative regulator. It depresses the genes required for successful drought survival. The researchers examined several different genes known to be important for drought tolerance and indeed they were increased when NAC016 was knocked out and decreased when NAC016 was increased. One important pathway that NAC016 impacted was that of the hormone abscisic acid, which controls water retention via stomata (little pores on the leaves) opening/closing. Preventing water loss out of the stomata is an important defense against dehydration.

So... why care? Right now, this has only been shown in Arabidopsis. Arabidopsis is the model plant species because it is small, has a quick reproduction time, and has a fully sequenced genome. Most plant research begins in Arabidopsis and then is applied to other species. If NAC016 is shown to be a negative regulator of drought tolerance in agriculturally important species, then that's a target to remove. This removal can be done either by transgenic knockout, RNA interference, or traditional breeding techniques. But none of these avenues can be taken until the targets are established and this paper is a great first step towards identifying these targets.

Sakuraba 2015:
ASPB Press release w/ photo

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