Allelopathic Enhancement and Differential Gene Expression in Rice under Low Nitrogen Treatment

The allelopathy-competition separation (ACS) based approach was used to explore the biointerference relationship between rice accessions and barnyardgrass exposed to different nitrogen (N) supplies in hydroponics. Rice accession PI312777 exhibited high allelopathic potential to suppress the growth of accompanying weeds, especially when the culture solution had low N content. The non-allelopathic rice Lemont showed an opposite result. Additionally, subtractive hybridization suppression (SSH) was used to construct a forward SSH-cDNA library of PI312777 to investigate gene expression profiles under low N treatment. A total of 35 positive clones from the SSH-cDNA library were sequenced and annotated. According to the function category, 24 genes were classified into five groups related to primary metabolism, phenolic allelochemical synthesis, plant growth/cell cycle regulation, stress response/signal transduction, and protein synthesis/degradation. Among them, two up-regulated genes that encode PAL and cytochrome P450 were selected. Their transcript abundance at low N level was compared further between the allelopathic rice and its counterpart by utilizing real-time quantitative polymerase chain reaction (qRT-PCR). The transcription levels of the two genes increased in both rice accessions when exposed to low N supply, but PI312777 at a higher magnitude than Lemont. At 1, 3, and 7 days of the treatments, the corresponding relative expression levels of PAL were 11.38, 4.83, and 3.57 fold higher in PI312777 root, but there were 1.15, 2.74, and 2.94 fold increases for Lemont, compared with the control plants fed with regular nutrient. The same trend was found for cytochrome P450. These findings suggest that the stronger ability of PI312777 to suppress target weeds, especially in low N nutrient conditions, might be attributed to the stronger activation of the genes that function in de novo synthesis of allelochemicals.