Patrick

The Asian Citrus Psyllid and Citrus Trees

= = = **Introduction** =

The Asian Citrus Psyllid (Diaphorina citiri) is a hemipteran bug that is a major pest of citrus trees of all types. The bug lives in association with these trees by feeding on the sap of the phloem contained in new shoots, as they feed, the psyllid releases a toxin in their saliva that causes these shoots to die back or become contorted which prevents proper leaf opening. However, the more dangerous problem related to the Asian Citrus Psyllid (ACP) association with citrus trees is that the psyllid is one of only two vectors for the bacteria Huanglongbing (HLB) which causes the devastating Citrus Greening Disease. This terrible disease has no cure and results in the swift decline and death of effected plants – commercial citrus ranches throughout Asia, Africa the Middle East, South and Central America, Mexico, the Caribbean have all been afflicted by this fast spreading disease which has resulted in the loss of millions of dollars worth of fruit that is produced by the citrus trees. In 2005 the ACP carrying HLB was first discovered in the United states in Florida, and since then, there have been massive eradication efforts by the USDA to prevent the pest from spreading further. Despite these efforts, the ACP was found in backyard citrus plants in 2008 in California, and now California ranchers are working frantically to eradicate the bug wherever it is found to prevent it from establishing itself in Southern California’s valuable citrus ranches and potentially spreading introducing HLB.

= **Organisms involved** =

Citrus trees are their own genus of flowering plants in the Rutaceae family and are most commonly known for their fruits such as oranges (Citrus reticulata), lemons (Citrus limon), grapefruits (Citrus paradisi) and limes (Citrus aurantifolia). They are believed to have originated in Southeastern Asia near the border that India’s northeaster border that is shares with China and Myanmar. Now the citrus tree has been cultivated commercially for its fruit all over the world in places such as Southern China, the Mediterranean Basin, South Africa, Australia, the Southernmost United States (Florida, California, Arizona, and Texas) and parts of South America (1-Save Our Citrus). This widespread commercial success of citrus has made the eradication of the Asian Citrus Psyllid and the bacteria it carries (HLB) a top priority for governments everywhere. The Asian Citrus Psyllid (Diaphorina citiri) is a hemipteran sap sucking bug from the family Psyllidae. The bug can make its habitat anywhere where there are citrus trees and is a widespread pest throughout Asia, Africa the Middle East, South and Central America, Mexico, the Caribbean and now increasingly in the United States (1-Save Our Citrus).

= **Biological Interaction** =

The Asian Citrus Psyllid makes its home in citrus trees and feed by tapping into the plants phloem and sucking the sap from it. Females lay up to 800 eggs at a time on the tips of growing shoots between the unfolding leaves. When the eggs hatch, the nymphs feed on the phloem of these new shoots and produce a toxin that is in their saliva that causes shoot tips to die back and prevents the leaves from expanding normally. It is through this method of feeding (for both nymphs and adults) that the HLB bacteria is introduced to the phloem where it multiplies and becomes virulent (1-Save Our Citrus). The Huanglongbing bacteria (Liberibacter asiaticus) is a gram negative bacteria from the Rhizobiaceae family. It is very challenging to study this bacteria because it cannot be grown in culture and only lives in the phloem of the plants it diseases. It is because of this problem that the pathogenesis of the bacteria is still not understood and there has been no cure found for it. Once a plant is infected with HLB by the Asian Citrus Psyllid it will experience a quick decline and subsequent death. The diseases progression can be noted by a number of symptoms: first there will be a yellowing of the veings and tissues surrounding the infection and eventually the whole leaf, next there is a premature defoliation of the tree and a dieback of twigs and lateral roots, and finally there will be a decline in plant vigor and death of the tree. The affected trees will have inhibited growth, off-season flowers that fall of the tree, and will produce small abnormal green fruit that has a bitter taste to it (2-Grafton) = **Impact/Importance** =

When a citrus ranches are infected with HLB by the Asian Citrus Psyllid it is necessary to implement 3-6 pesticide treatments per year which disrupt other pest management programs and also implement programs to find and remove infected trees, both of these effects are very costly to ranchers (1-Save Our Citrus). The United States is the 3rd leading producer of citrus fruit in the world and the spread of ACP through our commercial citrus ranches is something that has a major impact on us by these costs that are incurred to the industry that are then passed down to the buyer through higher citrus prices. In addition to this, diseased ranches results in a significantly lower amount of citrus available for export.

=Current Research=

Most of the current research on Asian Citrus Psyllid/Citrus Tree interactions is cenetered around the characterization of the Liberibacter asiaticus bacteria that causes Huanglongbing. This research is of extreme importance because attempts to control the spread of HLB by eradicating the Asian Citrus Psyllid has been widely unsuccessful, and any HLB treatment or cure could make ACP control unnecessary. At the University of Florida, Dr. Gmitter Wang and his group are attempting to characterize Liberibacter movement in the phloem, characterize virulence factors it secretes, identify genes turned on or off by infection of the bacterium, and determine how the bacterium causes the symptoms. Their main hypothesis is that Liberibacter asiaticus causes the disease by blocking nutrient transport in the phloem and controlling the host response to its presence (3-Trivedi). A project run the Brazilian Embrapa Institute by principal researchers Girardi and Stuchi is working on research that aims at finding HLB resistance via biotechnology and finding other horticultural practices to mitigate HLB effects (i.e. finding repellent and attactive plants, use of resistance elicitors, etc.) (4-Girardi). Another research team consisting of members from the USDA, University of Florida and Virginia Bioinformatics Institute has worked on characterizing Liberobacter asiaticus by using metagenomics to sequence the entire genome of the bacteria. Initial results show that there are high levels of genes for both cell motility and active transport, both of which may contribute to virulence. In addition to this, genome analysis shows that the bacteria has the Type 1 secretion system necessary for multidrug efflux and toxin effector secretion. Another major finding made by this research shows that the bacteria has limited abilities for aerobic respiration and also is likely auxotrophic for 5 different amino acids which explains why it is so hard to grow in culture (5-Yongping).

=References=

 1. // Save Our Citrus. United States Department of Agriculture: Animal and Plant Health Inspection Service, 25 Apr. 2011. Web. []. //

 2.Grafton-Cardwell, Elizabeth. "Citrus Greening." //Center for Invasive Species Research. University of California at Riverside. Web. 25 Apr. 2011. .//  3.Trivedi, P., Duan, Y., and Wang, N. 2010. Huanglongbing, a systemic disease, restructures the bacterial community associated with citrus roots. Applied and Environmental Microbiology 76:3427.

 4.Girardi, Eduardo A., and Eduardo S. Stuchi. "Use of Horticultural Practices in Citriculture to Survive Huanglongbing." Embrapa Casava and Fruits Institute, 2010. Web. 25 Apr. 2011. .

5.Yongping, Duong. "Complete Genome Sequence of Citrus Huanglongbing Bacterium, ‘Candidatus Liberibacter Asiaticus’ Obtained Through Metagenomics." American Physical Society, 2009. Web. 25 Apr. 2011. .

=
 Page authored by Patrick Harkins, student of Dr. Michelle Lum at Loyola Marymount University, Los Angeles. =====