Like us, plants have an inbuilt immune system that can be activated in response to disease threats. When a problem is detected, the plant immune system mounts a chemical attack against the invading organisms as well as reinforcing its own defences.
The plant defence system can be primed in advance by various compounds that sensitise it to disease, or to other environmental stresses. One such priming compound is beta-aminobutyric acid (BABA), which has been used by researchers for years in experiments designed to test plant stress responses. BABA is a non-protein amino acid that primes the defence system to protect against a wide range of stresses, including various microbial pathogens, drought and salt stress.
BABA is one of three aminobutyric acid molecule isomers, along with alpha- and gamma-aminobutyric acid (AABA and GABA, respectively). Unlike AABA and GABA, BABA was considered to be xenobiotic, meaning it is not produced in plants, but the recent discovery of a BABA-specific receptor led a Swiss team of researchers to wonder why plants would have evolved a receptor for a xenobiotic compound. In their new paper, published in New Phytologist, Damien Thevenet and colleagues describe how they developed a new BABA detection system to investigate potential BABA production in plants.
Using a complex analytical chemistry approach involving ‘ultra-high pressure liquid chromatography tandem mass spectrometry’, Thevenet et al. were able to differentiate between the three isomers, AABA, BABA and GABA. Excitingly, when they turned their attention to plant samples, Thevenet and his colleagues showed that BABA is in fact produced by a range of species, including Arabidopsis, maize (Zea mays), wheat (Triticum aestivum), Chinese cabbage (Brassica rapa) and the moss Physcomitrella patens, albeit in much lower amounts than AABA and GABA (~60 and 1000 times less, respectively).
BABA primes plant defence to a range of stresses, so Thevenet and colleagues tested the impact of stress on its production in plants. When exposed to infection by various pathogens, including the fungus Plectosphaerella cucumerina, the bacterium Pseudomonas syringae, and the oomycete Hyaloperonospora arabidopsidis, Arabidopsis began to produce more BABA, reaching levels around 3- to 15-fold higher than in disease-free plants. Salt stress and submergence in water led to BABA increases of up to 10- and 7-fold, respectively. Just like in the experiments with external BABA treatments, it appears that plant-produced BABA helps to defend against many different stresses.
A new look at plant defence priming
BABA is a fascinating compound. As Thevenet and colleagues note, its ability to prime so many different plants (including more basal species such as moss) against such a wide range of stresses has intrigued researchers since its discovery. BABA appears to promote stress resilience over multiple generations, protecting the offspring of BABA-treated plants against infection.
The presence of BABA production in plants opens up new avenues for research into its roles in plant priming and other biological functions. Thevenet et al. are now trying to identify the genes involved in the synthesis of BABA, which could result in new less-damaging ways to protect crop plants in the field.
Read the paper: Thevenet, D., Pastor, V., Baccelli, I., Balmer, A., Vallat, A., Neier, R., Glauser, G. and Mauch-Mani, B. (2016), The priming molecule β-aminobutyric acid is naturally present in plants and is induced by stress. New Phytologist. doi: 10.1111/nph.14298