Plant-Derived, Nodule-Specific Cysteine-Rich Peptides as a Novel Source of Biopesticides for Controlling Citrus Greening Disease.

Nodule-specific cysteine-rich (NCR) peptides, encoded in the genome of the Mediterranean legume Medicago truncatula (barrelclover), are known to regulate plant-microbe interactions. A subset of computationally derived 20-mer peptide fragments from 182 NCR peptides was synthesized to identify those with activity against the unculturable vascular pathogen associated with citrus greening disease, 'Candidatus Liberibacter asiaticus' (CLas). Grounded in a design of experiments framework, we evaluated the peptides in a screening pipeline involving three distinct assays: a bacterial culture assay with Liberibacter crescens, a CLas-infected excised citrus leaf assay, and an assay to evaluate effects on bacterial acquisition by the nymphal stage of hemipteran vector Diaphorina citri. A subset of the 20-mer NCR peptide fragments inhibits both CLas growth in citrus leaves and CLas acquisition by D. citri. Two peptides induced higher levels of D. citri mortality. These findings reveal 20-mer NCR peptides as a new class of plant-derived biopesticide molecules to control citrus greening disease.

Efficiency of Biodegradable and pH-Responsive Polysuccinimide Nanoparticles (PSI-NPs) as Smart Nanodelivery Systems in Grapefruit: In Vitro Cellular Investigation.

Biodegradable pH-responsive polysuccinimide nanoparticles (PSI-NPs) are synthesized for directly delivering agrochemicals to plant phloem to improve their efficacy. The PSI-NPs have an average size of 20.6 nm with negative charge on the surface. The desired responsiveness to changes in pH is demonstrated by release efficiency of the model molecule (Coumarin 6), which increases with increasing pH over 24 h. The internalization of PSI-NPs into grapefruit cells occurs in 10 min, and into nucleus in 2 h, with most of the PSI-NPs being distributed in cytoplasm and nucleus. The proportion of PSI-NPs in plant cells significantly increases with time, from 19.1% at 10 min to 55.5% at 2 h of administering. The PSI-NPs do not show significant inhibitory effects on soil microbial growth and activity. These results indicate that this smart nanodelivery system has potential of application in agriculture for mitigating phloem-limited diseases, such as citrus huanglongbing.

Mixture-amount design and response surface modeling to assess the effects of flavonoids and phenolic acids on developmental performance of Anastrepha ludens.

Host plant resistance to insect attack and expansion of insect pests to novel hosts may to be modulated by phenolic compounds in host plants. Many studies have evaluated the role of phenolics in host plant resistance and the effect of phenolics on herbivore performance, but few studies have tested the joint effect of several compounds. Here, we used mixture-amount experimental design and response surface modeling to study the effects of a variety of phenolic compounds on the development and survival of Mexican fruit fly (Anastrepha ludens [Loew]), a notorious polyphagous pest of fruit crops that is likely to expand its distribution range under climate change scenarios. (+)- Catechin, phloridzin, rutin, chlorogenic acid, and p-coumaric acid were added individually or in mixtures at different concentrations to a laboratory diet used to rear individuals of A. ludens. No effect was observed with any mixture or concentration on percent pupation, pupal weight, adult emergence, or survival from neonate larvae to adults. Larval weight, larval and pupal developmental time, and the prevalence of adult deformities were affected by particular mixtures and concentrations of the compounds tested. We suggest that some combinations/concentrations of phenolic compounds could contribute to the management of A. ludens. We also highlight the importance of testing mixtures of plant secondary compounds when exploring their effects upon insect herbivore performance, and we show that mixture-amount design is a useful tool for this type of experiments.

Insect diets as mixtures: optimization for a polyphagous weevil.

Development or improvement of artificial insect diets can be tedious, convoluted and often under-appreciated. Using n-dimensional mixture designs, we identified a set of response-optimized meridic diets that contain fewer ingredients than the current commercial diet for Diaprepes abbreviatus, a polyphagous weevil pest of the Caribbean and southern U.S. A diet blend optimized to produce maximum adult weight was predicted to produce adult D. abbreviatus that weigh 28% more compared with adults reared on the standard commercial diet. Diet blends that produced greater individual adult weights resulted in lower survival compared with those blends that yielded adults of more modest proportions. In contrast, a simplified high cottonseed meal blend produced smaller adults more similar to field-collected individuals, and produced the greatest number of adults and the greatest biomass at relatively low cost compared with diets that yielded adult weevils of greater weight. We think that many insect-rearing programs would benefit from application of mixture design methods to situations where diet optimization is desired for researcher-selected criteria. This approach is broadly applicable to any problem that can be conceptualized as a mixture problem.

Putative protease inhibitor gene discovery and transcript profiling during fruit development and leaf damage in grapefruit (Citrus paradisi Macf.).

Seven putative protease inhibitor (PPI) cDNAs, representing four protein families, were isolated from a grapefruit (Citrus paradisi Macf. Cv. Marsh) immature fruit flavedo cDNA library. Cloned open reading frames encoded proteins with similarity to, and protein signatures for: legume Kuntiz inhibitors (lkiL-1, lkiL-2, lkiL-3), potato trypsin inhibitor I (ptiIL-1), serpins (serpL-1), cystatins (cystL-1), and gamma thionins (gthL-1). Response of transcript abundance to fruit development and leaf wounding was determined for all but lkiL-1 using real-time RT-PCR. Immature leaves had the highest transcript levels for all PPIs. The gthL-1 transcript in immature leaves was the most abundant transcript but was absent from healthy mature leaves. In fruit flavedo, transcripts for all PPIs were most abundant in youngest fruit (<15 mm dia. fruit), and declined during development, but displayed different patterns of developmental change. Mechanical or Diaprepes root weevil (DRW) feeding damage to leaves caused a <10-fold reduction or had no effect on transcript level with the exception of gthL-1 which, as a result of damage, increased >50-fold in mature leaves and decreased >1400-fold in immature leaves. This developmental control of transcript response to wounding in a woody perennial is opposite of what has been observed for defensive proteinase inhibitors (PIs) in other plants (typically herbaceous and/or annual plants), where younger leaves typically invoke a higher defensive proteinase inhibitor transcript accumulation than older tissues. Except for gthL-1, the PPI transcripts were minimally responsive or unresponsive to wounding. Changes in PPI transcript levels suggest diverse roles for the products of these genes in citrus, with only gthL-1 responding in a defense-like manner.