The growth and development of plants are governed by the induction, suppression or transmission of phytohormones such as Indole-3-acetic acid (IAA), sometimes known as auxin. The sessile nature of plants demands some kind of chemical response to all manners of external stress as opposed to the physical response which animals typically make. Those chemical responses, whether responding to living organisms (biotic stress) such as insects, bacteria, fungi, etc. or to environmental conditions (abiotic stress) such as drought, salinity or cold, must occur repeatedly over the plant’s entire lifetime and help determine its ultimate survival or death.
Since this is such a critical part of the plant’s existence, it stands to reason that any sensible plant would harbor a multitude of redundant pathways, and also a variety of phytohormones, in the event that any one specific pathway or hormone fails to work adequately. I have found this redundancy to be one of the most amazing aspects of plant physiology, and definitely one of the most frustrating as well. Discovering that your model plant is fully capable of working around every one of your knockout mutations, convoluting your hypotheses on molecular pathways and mechanisms, helps explain why there is never any beer left over after an academic plant symposium.
Even though IAA was first discovered in 1937, plant scientists are still befuddled to many of the ways in which IAA functions in the plant and to what extent and in relation to which types of stress. We know how it moves and where it moves. We know that IAA is unique in that it is the only phytohormone that moves both long distances through the phloem of the plant as well as short, cellular distances utilizing polar auxin transport (which requires the use of specific auxin efflux carriers on the plasma membrane). We know that a little bit goes a long way, and that too much is a very bad thing (See 2,4-D). We even know a lot about what IAA does in the plant specifically; driving root tip initiation, apical dominance, flower initiation and fruit growth or even the suspension of fruit senescence. But from a physiological standpoint, that barely scratches the surface of what is actually going on.
Environmental stress is one of the most debilitating effects on cereal crops from an economic perspective. Early freezes or summer droughts run roughshod over grains and fruit/vegetable farms, dealing hundreds of millions of dollars in lost revenue every year. So there’s a strong financial incentive to understanding what’s really going on behind the scenes of abiotic stress on some of the world’s biggest cash crops. Perhaps not so unexpectedly, the results are complicated.
Hao Du et al. published a paper last month, in the online journal Frontiers of Plant Science, studying auxin levels during abiotic stress in rice. Their results showed that the regulation of IAA changes in response to different types of abiotic stress. In drought conditions, the level of IAA was reduced 18% after 3 days, but exposure to either heat stress or cold stress caused the level of IAA to increase up to 1.6 fold after 3 days. They also saw that root tip gravitropism could be inhibited by cold stress nearly to the same effect as the auxin transport inhibitor TIBA (2,3,5-triiodobenzoic acid). Additionally, they looked very closely at a number of the genes (such as OsYUCCA and OsIAA genes) encoding these molecules and their overexpression or suppression during abiotic stress conditions.
So what does all that mean, exactly? First, it confirms that the homeostasis of auxin levels is tightly related to tolerance of these three stress conditions, at least in rice (and maybe other monocots as well). More importantly, it starts pointing a putative finger toward the genes that need to be regulated in order to develop plants which are tolerant to these specific stress conditions. Of course, that’s no guarantee of success…any plant molecular biologist knows that. But it is a start, and just like when dealing with any clump of plant roots, the only way you can begin to see the entire root system is to tease the root tendrils out one by one.
Du, H., Liu, H., & Xiong, L. (2013). Endogenous auxin and jasmonic acid levels are differentially modulated by abiotic stresses in rice. Frontiers in plant science, 4. doi: 10.3389/fpls.2013.00397
Shibasaki, K., Uemura, M., Tsurumi, S., & Rahman, A. (2009). Auxin response in Arabidopsis under cold stress: underlying molecular mechanisms. The Plant Cell Online, 21(12), 3823-3838.
Rahman, A. (2013). Auxin: a regulator of cold stress response. Physiologia Plantarum, 147(1), 28-35.
Category Code: 88221 79101