Morphoanatomical and biochemical factors associated with rice resistance to the South American rice water weevil, Oryzophagus oryzae (Coleoptera: Curculionidae).

The rice water weevil, Oryzophagus oryzae (Coleoptera: Curculionidae), is an economically important pest of flooded rice paddies throughout South America, and species with similar life histories are present in many rice-producing regions globally (collectively referred to here as RWWs). Plant resistance is a key strategy for management of RWWs; however, the mechanisms responsible for rice resistance to RWWs are poorly understood. We investigated morphoanatomical and biochemical plant traits potentially involved in rice resistance to O. oryzae. Resistance-associated traits were characterized in two cultivars, 'Dawn' (resistant) and 'BRS Pampa CL' ('Pamp' = susceptible), which were selected from among six cultivars on 2-year field screenings. Anatomical and morphological traits of leaf tissues from 'Pamp' and 'Dawn' were similar, which perhaps explains the lack of antixenosis during host plant selection. However, significant antibiosis effects were found. The activities of antioxidant enzymes involved in plant defense, as well the content of hydroxycinnamic and hydroxybenzoic acids derivatives and lignin, were higher in roots of 'Dawn' than in 'Pamp', over the period of larval infestation in the field. Additionally, 'Dawn' exhibited a root sclerenchyma arranged in three layers of lignified cells, which differed from the arrangement of cells in 'Pamp', regardless of larval infestation. Our results provide the first evidence for specific resistance-related traits associated with mortality and malnutrition of RWWs in rice.

A fungal endophyte helps plants to tolerate root herbivory through changes in gibberellin and jasmonate signaling.

Plant-microbe mutualisms can improve plant defense, but the impact of root endophytes on below-ground herbivore interactions remains unknown. We investigated the effects of the root endophyte Piriformospora indica on interactions between rice (Oryza sativa) plants and its root herbivore rice water weevil (RWW; Lissorhoptrus oryzophilus), and how plant jasmonic acid (JA) and GA regulate this tripartite interaction. Glasshouse experiments with wild-type rice and coi1-18 and Eui1-OX mutants combined with nutrient, jasmonate and gene expression analyses were used to test: whether RWW adult herbivory above ground influences subsequent damage caused by larval herbivory below ground; whether P. indica protects plants against RWW; and whether GA and JA signaling mediate these interactions. The endophyte induced plant tolerance to root herbivory. RWW adults and larvae acted synergistically via JA signaling to reduce root growth, while endophyte-elicited GA biosynthesis suppressed the herbivore-induced JA in roots and recovered plant growth. Our study shows for the first time the impact of a root endophyte on plant defense against below-ground herbivores, adds to growing evidence that induced tolerance may be an important root defense, and implicates GA as a signal component of inducible plant tolerance against biotic stress.

Factors Influencing Expression of Antixenosis in Soybean to Anticarsia gemmatalis and Spodoptera frugiperda (Lepidoptera: Noctuidae).

This study aimed to evaluate some factors that influence the expression of antixenosis in soybean genotypes against Anticarsia gemmatalis Hübner and Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae). Free-choice and no-choice feeding assays were performed with the resistant soybean genotype IAC 100 and the susceptible genotype BRSGO 8360 using A. gemmatalis and S. frugiperda larvae. The following factors that may affect expression of resistance were evaluated: one larva versus two larvae per leaf disc; use of larvae without prior feeding suspension versus larvae starved for 3 h prior to the assay; leaf discs versus entire leaflets; upper part versus lower part of the plant; and, vegetative versus reproductive growth stages. The level of resistance exhibited by the genotype IAC 100 was high enough to not be obscured by the effects of all factors assayed in the present study upon the feeding preference of A. gemmatalis and S. frugiperda larvae. However, our results demonstrate the importance of knowing the optimal conditions for conducting an assay for evaluating resistance of genotypes for specialist and generalist insect species. Utilization of two larvae of A. gemmatalis per leaf disc, not starved before the assays, with leaf discs from the upper part of plants at the reproductive growth stage provided better discrimination of differences in antixenosis expression in soybean genotypes. For S. frugiperda, use of one larva per leaf disc, not starved before the assays, with leaf discs from the lower part of plants at the reproductive growth stage gave more satisfactory results for feeding preference tests.

Induced jasmonate signaling leads to contrasting effects on root damage and herbivore performance.

Induced defenses play a key role in plant resistance against leaf feeders. However, very little is known about the signals that are involved in defending plants against root feeders and how they are influenced by abiotic factors. We investigated these aspects for the interaction between rice (Oryza sativa) and two root-feeding insects: the generalist cucumber beetle (Diabrotica balteata) and the more specialized rice water weevil (Lissorhoptrus oryzophilus). Rice plants responded to root attack by increasing the production of jasmonic acid (JA) and abscisic acid, whereas in contrast to in herbivore-attacked leaves, salicylic acid and ethylene levels remained unchanged. The JA response was decoupled from flooding and remained constant over different soil moisture levels. Exogenous application of methyl JA to the roots markedly decreased the performance of both root herbivores, whereas abscisic acid and the ethylene precursor 1-aminocyclopropane-1-carboxylic acid did not have any effect. JA-deficient antisense 13-lipoxygenase (asLOX) and mutant allene oxide cyclase hebiba plants lost more root biomass under attack from both root herbivores. Surprisingly, herbivore weight gain was decreased markedly in asLOX but not hebiba mutant plants, despite the higher root biomass removal. This effect was correlated with a herbivore-induced reduction of sucrose pools in asLOX roots. Taken together, our experiments show that jasmonates are induced signals that protect rice roots from herbivores under varying abiotic conditions and that boosting jasmonate responses can strongly enhance rice resistance against root pests. Furthermore, we show that a rice 13-lipoxygenase regulates root primary metabolites and specifically improves root herbivore growth.