Pest-resistant transgenic poplars maintain efficiency under water stress

Plastid-mediated RNA interference has emerged as a promising and effective approach for managing agricultural pests. The technique is based on the expression of high levels of double-stranded RNAs (dsRNAs) in plant plastids targeting essential pest genes. Studies have demonstrated its effectiveness in controlling herbivorous beetles and mites. However, it is still unclear whether abiotic stress, such as drought, influences the accumulation of dsRNAs produced in plastids and their effectiveness in pest control.

A recent study aimed to investigate the effects of water stress on the expression of dsACT in transplastomic poplar plants and its control efficiency against the willow leaf beetle. The results revealed that drought stress did not significantly affect dsRNA contents in plants, but led to greater mortality of insect larvae. This increased mortality may be attributed to elevated levels of jasmonic acid and cysteine ​​proteinase inhibitors induced by water deficit.

The willow leaf beetle, Plagiodera versicolora (Coleoptera: Chrysomelidae), is a significant forest pest worldwide, feeding mainly on the leaves of plants in the Salicaceae family. Traditionally, this pest is managed using chemical insecticides, which pose a significant threat to the environment and human health. With advances in genetic modification technologies, transgenic poplar plants that express poplar toxins have been developed. Bacillus thuringiensis (Bt) and double-stranded RNAs (dsRNAs) for the control of P. versicolora. Due to the high copy number of the plastid genome and the absence of an endogenous RNA interference pathway in plastids, high levels of dsRNA can accumulate in aspen plastids, resulting in significant mortality of P. versicolora.

In addition to insect attacks, plants are also exposed to various types of abiotic stresses, such as drought and high temperatures. Drought, in particular, is recognized as an environmental factor that limits aspen growth and productivity. When faced with water deficit conditions, poplars undergo acclimatization processes that involve physiological, morphological, biochemical and molecular changes. These changes can impact plant-insect relationships, altering the nutritional quality of leaves, palatability and host location.

According to the scientists, although water stress reduces the concentration of Bt toxins in transgenic cotton, it does not appear to affect the effectiveness in controlling Bt. Helicoverpa armigera. In the current study, researchers observed that drought stress did not lead to a decrease in dsRNA production in transplastomic lines. Furthermore, there was greater mortality of larvae P. versicolora.

To explore possible reasons for the increased mortality of larvae under drought stress, we analyzed the contents of phytohormones and proteinase inhibitors (PIs). Jasmonic acid (JA), involved in herbivorous and defense responses against various abiotic stresses, including drought, has been found at increased levels under water deficit conditions. Furthermore, transcripts of two JA pathway marker genes, COI1 and JAZ, were found at elevated levels in poplar leaves when fed by P. versicolora.

Plant proteinase inhibitors (PIs) are classified into three main families based on the reactive amino acid residue in their active sites: CPIs, aspartyl and metallocarboxypeptidase inhibitors. Serine inhibitors and CPIs are the most studied regarding their effectiveness against insect pests. In our study, we observed that poplar CPI-6 expression was upregulated under drought stress. These findings suggest that the increased larval mortality observed under drought stress conditions may be attributed to the upregulation of CPI-6 expression.

In summary, the scientists say, when drought stress is present, it is highly likely that increased levels of JA and CPIs will amplify the insecticidal properties of the dsRNAs produced in Pdb-dsACT. Consequently, this leads to a more potent insecticidal effect of dsRNAs, improving their overall action against insects.

Article written by the scientists detailing the study can be read at doi.org/10.1016/j.pestbp.2024.106037