344 G. McCOLLUM AND E. BALDWIN
scion selections withPoncirusin their pedigree, it remains to be seen
if such hybrids are in fact less susceptible to HLB than are currently
available varieties.
The variation that has been seen among cultivars in their response
to CLas infection suggests that it will be possible to generate new cul-
tivars that are less susceptible to HLB than are current cultivars. There
are highly active citrus breeding programs being conducted in Florida
(Grosser and Gmitter 2012; Stover et al. 2015b) and California (Ramdugu
and Roose, personal communication) to develop new citrus varieties
with reduced susceptibility to HLB. Evaluation of susceptibility to HLB
in a meaningful way requires multiple years of commercial scale trials,
and thus far, only very limited data are available regarding meaningful
differences among new hybrids.
In addition to conventional breeding, there is considerable interest in
transgenic strategies to reduce susceptibility to HLB in industry stan-
dard cultivars. Felipe et al. (2013) reported on the response of trans-
genic “Hamlin,” “Natal,” “Pera,” and “Valˆ encia” sweet orange cultivarsˆ
expressing theattacinA gene to CLas infection. CLas and HLB symp-
toms were present in nontransgenic and transgenic plants expressing
theattacinA gene of the four sweet orange cultivars. However, five
transgenic lines (transformation events) of “Pera” sweet orange express-ˆ
ing theattacinA gene had significantly lower CLas titers in compari-
son with nontransgenic plants of the same cultivar. In addition, use of
spinach defensive proteins, patent application by Texas A&M Univer-
sity (Mirkov and Gonzalez-Ramos 2014) for control of HLB as well as
other diseases, was used to transform orange trees. At first, the canker
bacteria was targeted to control citrus canker disease, but later delivery
of the antimicrobial proteins to the phloem to target HLB was accom-
plished, and is now being tested by a large citrus grower-processor in
South Florida (Voosen 2009; Texas A&M AgriLife 2015; Giles 2015).
Stover et al. (2015c) described several strategies being explored to
create practical transgenic citrus with HLB-resistance. Sweet orange
cultivars “Hamlin” and “Valencia” transformed with anArabidopsis
thalianaNPR1 gene under the control of either a constitutive CaMV
35S promoter or a phloem-specific Arabidopsis SUC2 (AtSUC2) pro-
moter have shown less severe symptoms of HLB, and some lines tested
CLas-negative 36 months after planting amongst other citrus trees in
an orchard with high incidence and severity of HLB (Dutt et al. 2015).
Expression of the NPR1 gene induced expression of several native genes
involved in the plant defense signaling pathways. Van Vuuren and Man-
icom (2009) rescued embryos from seeds in healthy chimera sections
of HLB-symptomatic fruit. Plants derived from two clones remained
negative after challenge with CLaf. The authors acknowledge that the