AUTHOR=Martini Xavier , Coy Monique , Kuhns Emily , Stelinski Lukasz L. 

TITLE=Temporal Decline in Pathogen-Mediated Release of Methyl Salicylate Associated With Decreasing Vector Preference for Infected Over Uninfected Plants

JOURNAL=Frontiers in Ecology and Evolution

VOLUME=6

YEAR=2018

URL=https://www.frontiersin.org/journals/ecology-and-evolution/articles/10.3389/fevo.2018.00185

DOI=10.3389/fevo.2018.00185

ISSN=2296-701X

ABSTRACT=<p>The Asian citrus psyllid, <italic>Diaphorina citri</italic> Kuwayama, is the vector of the phytopathogenic bacterium, <italic>Candidatus</italic> Liberibacter asiaticus (<italic>C</italic>Las), that causes citrus greening. <italic>Diaphorina citri</italic> is attracted to methyl salicylate (MeSA) emitted by <italic>C</italic>Las-infected citrus. MeSA is catalyzed from salicylic acid (SA) by salicylic acid methyl transferase (SAMT). In addition, salicylate hydroxylase, an enzyme that degrades SA and suppresses host plant defense, is unregulated in <italic>C</italic>Las-infected as compared with uninfected citrus. Based on these findings, we hypothesized that <italic>C</italic>Las-induced plant volatile emissions and SAMT expression may decrease over time following <italic>C</italic>Las infection. We first identified the putative <italic>Citrus sinensis</italic> SAMT (<italic>CsSAMT</italic>) that methylates SA to MeSA. Thereafter we compared changes in SAMT expression and volatile expression between uninfected citrus, citrus recently (&lt; 6 months) infected with <italic>C</italic>Las, and citrus infected with <italic>C</italic>Las for a long (&gt;1 year) duration. Emission of MeSA increased in citrus recently infected with <italic>C</italic>Las, whereas both MeSA emissions and SAMT expression decreased in long-infected citrus as compared with uninfected controls. Also, there was a significant decrease in release of limonene from citrus infected &gt;1 year, and a concomitant increase in release of β-caryophyllene in both recent and long-infected citrus as compared with controls. In behavioral assays, <italic>D. citri</italic> preferred settling on asymptomatic citrus recently infected with <italic>C</italic>Las over uninfected or long-infected citrus trees. The odor of citrus recently infected with <italic>C</italic>Las was more attractive to <italic>D. citri</italic> than the odor of uninfected or long-infected citrus. There was an association between the preference for citrus odor of recently infected plants and an increase of MeSA release following <italic>C</italic>Las infection. Overall, our study demonstrated pulsed changes in citrus odor release following <italic>C</italic>Las infection. We hypothesize that allocation of resources to plant defense in citrus decreases as a function of huanglongbing disease symptom progression. These changes affected the behavior of the <italic>C</italic>Las vector. Our study confirms previous reports demonstrating that plant defenses boosted following phytopatogen infection tend to decrease over time. Implications on the <italic>C</italic>Las epidemic are discussed.</p>