Abstract:
Mealybugs (Hemiptera: Pseudococcidae) are soft bodied, sexually dimorphic,
hemimetabolous insects with sucking mouthparts. There are a number of pest mealybug
species in Pakistan, but citrus mealybug, Planococcus citri and cotton mealybug,
Phenacoccus solenopsis are relatively more important as they cause significant damage
to fruit gardens and crop plants, respectively. P. citri is a phloem feeder and has a very
broad plant host range. Loss of plant vigor and stunting are characteristic symptoms, but
P. citri also reduces fruit quality and causes fruit drop leading to significant yield
reductions. P. citri secretes honey dew and wax onto plant surfaces allowing for sooty
mold development, and infestations can result in rejecting plants shipped for trade.
Similarly, P. solenopsis is considered a highly invasive pest of agricultural and
horticultural crops feeding on a wide variety of plants from an estimated 246 plant
families. Cotton mealybug emerged as a serious pest in Pakistan during 2005-2006 when
it caused significant losses to cotton crop. P. solenopsis feeds on all parts of plants
especially on tender shoots, leaves joining the stem or along the leaf veins. Its massive
reproduction on plants causes distortion, weakening, defoliation, die back and even death
of susceptible plants. P. solenopsis also secretes honeydew that causes growth of sooty
molds.
Although genetically modified (GM) crops have proved successful in controlling
chewing insects, they have generally failed to control sucking insects. One reason of this
failure is considered the mode of feeding of sap sucking insects. Success of GM crops
against chewing insects has highlighted the importance of biotechnology options to
control sucking insects. RNA interference (RNAi) is an emerging tool for functional
genomics studies and is being investigated as a practical tool for highly targeted insect
control. RNAi acts at mRNA level thus impeding translation to proteins, and if vital
genes are targeted, insect development can be hampered and mortality can be achieved.
Here I investigated whether RNAi effects can be induced in P. citri and P. solenopsis and
whether candidate genes could be identified as possible targets for RNAi-based mealybug
control. RNAi effects were induced in P. citri, as demonstrated by specific target
reductions of P. citri β-actin, chitin synthase 1 and V-ATPase mRNAs after injection of
the corresponding specific double-stranded RNA inducers. Tobacco mosaic virus (TMV)
was used as a vector to express these RNAi effectors in N. benthamiana plants. It was
found that P. citri exposed to recombinant TMV-infected plants showed lower fecundity
and pronounced nymphal mortality. Further, these phenotypic results were confirmed by
target mRNA reduction through qRT-PCR.
Similarly, Potato virus X (PVX) was used as a vector to express RNAi effectors
in Nicotiana tabacum against P. solenopsis chitin synthase 1 (PsCHS1), V-ATPase (PsV-
ATPase) and bursicon (PsBur) as target genes. It was found that RNAi effects can be
induced in P. solenopsis through feeding on N. tabacum inoculated with recombinant
PVX vector with all three genes (PsCHS1, PsBur and PsV-ATPase) as was revealed by
reduction in mRNA levels and phenotypic effects like physical deformities, mortality and
reduced fecundity.
Taken together, the data suggests that β-actin, chitin synthase 1, V-ATPase and
bursicon are potential targets for RNAi against P. citri and P. solenopsis, and that
recombinant TMV and PVX are effective tools for evaluating candidate RNAi effectors
in plants against phloem feeders.