Abstract
Recent research suggests that systemic signalling and communication between roots and leaves plays an important role in plant defence against herbivores. In the present study, we show that the oviposition of the two-spotted spider mite Tetranychus urticae in the systemic leaves of citrus rootstock Citrus aurantium (sour orange) was reduced by 50% when a lower leaf was previously infested with conspecifics. Metabolomic and gene expression analysis of the root efflux revealed a strong accumulation of glutamic acid (Glu) that triggered the expression of the citrus putative glutamate receptor (GRL) in the shoots. Additionally, uninfested sour orange systemic leaves showed increased expression of glutamate receptors and higher amounts of jasmonic acid (JA) and 12-oxo-phytodienoic acid in plants that were previously infested. Glu perception in the shoots induced the JA pathway, which primed LOX-2 gene expression when citrus plants were exposed to a second infestation. The spider mite-susceptible citrus rootstock Cleopatra mandarin (C. unshiu) also expressed systemic resistance, although the resistance was less effective than the resistance in sour orange. Surprisingly, the mobile signal in Cleopatra mandarin was not Glu, which suggests a strong genotype-dependency for systemic signalling in citrus. When the cultivar Clemenules (C. clementina) was grafted onto sour orange, there was a reduction in symptomatic leaves and T. urticae populations compared to the same cultivar grafted onto Cleopatra mandarin. Thus, systemic resistance is transmitted from the roots to the shoots in citrus and is dependent on rootstock resistance.
Highlights
Plants react to a herbivore attack by locally activating physical or chemical defensive barriers around injured tissues
We showed that Cleopatra mandarin (Citrus unshiu) is highly susceptible to this mite and, in contrast, sour orange (C. aurantium) showed elevated levels of resistance associated with the oxylipin pathway (Agut et al, 2014)
systemic resistance (SR) in sour orange resulted in a 50% reduction in the mite population, whereas the reduction was approximately 30% in Cleopatra mandarin
Summary
Plants react to a herbivore attack by locally activating physical or chemical defensive barriers around injured tissues. A subsequent defence mechanism consists of the activation and synthesis of toxic compounds that hamper herbivore development or reproduction in distal undamaged plant parts. Though, some compounds have been identified as mobile signals that could be responsible for the systemic induction of salicylic acid (SA) in distal plant parts, including methyl salicylate, azelaic acid, dehydroabietinal, glycerol-3-phosphate, and pipecolic acid (Park et al, 2007; Vlot et al, 2009: Jung et al, 2009; Liu et al, 2010; Manosalva et al, 2010; Chanda et al, 2011; Chaturvedi et al, 2012; Návarová et al, 2012) In herbivorous arthropods, these direct defence mechanisms can interfere with their reproduction and/or behaviour. Higher levels of [Ca2+] cyt are induced in adjacent leaves with direct vascular connections, which indicates that [Ca2+]cyt is increased irrespective of the vascular connectivity
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