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Background Duckweed is considered a promising feedstock for bioethanol production due to its high biomass and starch production. The starch content can be promoted by plant growth regulators after the vegetative reproduction being inhibited. Maleic hydrazide (MH) has been reported to inhibit plant growth, meantime to increase biomass and starch content in some plants. However, the molecular explanation on the mechanism of MH action is still unclear. Results To know the effect and action mode of MH on the growth and starch accumulation in Spirodela polyrrhiza 7498, the plants were treated with different concentrations of MH. Our results showed a substantial inhibition of the growth in both fronds and roots, and increase in starch contents of plants after MH treatment. And with 75 µg/mL MH treatment and on the 8th day of the experiment, starch content was the highest, about 40 mg/g fresh weight, which is about 20-fold higher than the control. The I2-KI staining and TEM results confirmed that 75 µg/mL MH-treated fronds possessed more starch and big starch granules than that of the control. No significant difference for both in the photosynthetic pigment content and the chlorophyll fluorescence parameters of PII was found. Differentially expressed transcripts were analyzed in S. polyrrhiza 7498 after 75 µg/mL MH treatment. The results showed that the expression of some genes related to auxin response reaction was down-regulated; while, expression of some genes involved in carbon fixation, C4 pathway of photosynthesis, starch biosynthesis and ABA signal transduction pathway was up-regulated. Conclusion The results provide novel insights into the underlying mechanisms of growth inhibition and starch accumulation by MH treatment, and provide a selective way for the improvement of starch production in duckweed.

Pythium myriotylum is a destructive soil-borne pathogen, which causes severe yield losses in many crops. The pathogen is a major economic issue for the production of ginger. Due to environmental and regulatory concerns, it is necessary to find biological alternatives to chemical pesticide for P. myriotylum control. In the present study, trans-cinnamic acid (TCA) and the fermentation broth of symbiotic bacteria from eight species of entomopathogenic nematodes alone and the combination of TCA with the fermentation broth of each symbiotic bacterium, were tested for their effects on mycelial growth and zoospore germination of P. myriotylum. Results showed that TCA significantly inhibited mycelial growth. Fermentation broths from seven of the eight strains of symbiotic bacteria directly inhibited mycelial growth, especially symbiotic bacteria isolated from Steinernema feltiae (SN strain) and S. riobrave (7-12 strain). Moreover, adding TCA significantly increased the extent of the inhibitory effect of mycelial growth by fermentation broths of seven tested bacteria except that associated with S. rarum. All bacteria fermentation broths showed inhibitory effects on zoospore germination. However, TCA alone did not show an apparent inhibitory effect on zoospore germination as it did on mycelial growth. Antimicrobial effects on mycelial growth and zoospore germination were proportional to the concentration of symbiotic bacteria isolated from S. feltiae (SN strain). This research provides new options for biorational control of P. myriotylum using TCA and symbiotic bacteria of entomopathogenic nematodes and will facilitate the sustainable production of ginger and other crops affected by this oomycete pathogens disease.

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