Sound the alarm! Roots and systemic plant defence

05 Dec 2016 - By: Jonathan Ingram

Sound the alarm! Roots and systemic plant defence

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Sour orange. Photo: Pixabay, CC0 Public Domain

By Jonathan Ingram, Journal of Experimental Botany

When we’re threatened we tend to alert everyone around us, and it’s the same at the cellular level. The recent Journal of Experimental Botany paper from Agut et al. takes the specific case of two-spotted spider mite (Tetranychus urticae) attack on citrus, and in so doing brings new insights to the idea of a ‘shoot–root–shoot’ (SRS) loop in defending a multitude of plants against a similarly large variety of pests and pathogens. See also the separate Insight article from Simon Groen for an independent perspective on this vitally important area of research.

Systemic resistance in citrus to Tetranychus urticae induced by conspecifics is transmitted by grafting and mediated by mobile amino acids

Signalling in systemic plant defence – roots put in hard graft

The experiments presented by Agut et al. are particularly intriguing, building directly from work published in 1984 and using that most traditional of horticultural techniques, grafting. The 1984 work by Karban and Carey showed that when cotton seedlings had previously been infected with two-spotted spider mite, they were less susceptible to infestation. Move forward more than 30 years and we know a good deal more about this type of systemic resistance in plants, but our understanding of the part played by roots, as so often, lags behind what we know is going on aboveground.

The grafting comes in because the authors from Victor Flors’ group (Universitat Jaume I de Castelló) have two contrasting rootstocks at their disposal, differing in susceptibility: sour orange (Citrus aurantium) is the bigger problem for the spider mites, with increased resistance, while the Cleopatra mandarin (C. unshiu) is much more susceptible to infestation. By grafting clementine scions onto these rootstocks it was possible to show that the systemic resistance was graft-transmissible and that root-to-shoot signalling was an important component of the overall defence response.

Economics and science

The findings are important because citrus is an economically significant crop, but also for their more general relevance to our understanding of systemic resistance in plants. This is explained further by Simon Groen, putting the work into context in his Insight article in the same issue of the journal. Metabolomic studies revealed considerable detail, including the involvement of myo-inositol, citric acid, two fatty acids and glutamic acid (Glu) from infected leaves, and increased transport of Glu from the root to the shoot with the more resistant sour orange rootstock. In the sour orange, the expression of glutamate receptor-like protein (GRL) increased, activating the jasmonic acid pathway and reducing infestation.

Simon Groen: ‘The findings contribute to a growing body of evidence for the existence of an integrated SRS loop that regulates systemic defences after detection of an initial attack … The elegant series of studies by Agut et al. … have done much to “close the SRS loop” and pave the way for future functional studies that will further enrich our understanding of the plant defence system.’

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Journal of Experimental Botany publishes an exciting mix of research, review and comment on fundamental questions of broad interest in plant science. Regular special issues highlight key areas.




Author: Jonathan Ingram
Category: Plant Biology
Jonathan Ingram - Author Profile

Jonathan Ingram

Jonathan Ingram is Senior Commissioning Editor/ Science Writer for Journal of Experimental Botany. Jonathan moved from lab research into publishing and communications with the launch of Trends in Plant Science in 1995, then going on to New Phytologist and, in the third sector, Age UK and Mind.