Determinant effect of PAR on role of FR photons; FR light as a resource or signal for stress

Abstract

Rice is one of the most important crops for agricultural purposes. The space required for farming (i.e., rice) will likely increase to feed a growing world population. Research on plant growth responses, may contribute to understanding factors limiting potential yield. Plants absorb mostly red and blue light for photosynthesis and reflect green and far-red light. Therefore, neighbouring plants, especially in high density growth conditions, lower ambient R:FR ratios. This triggers a range of responses referred to as the shade avoidance syndrome (SAS), in shade-avoiding plants. The SAS is important as it may negatively impact yield and limit the density of crops grown in agriculture. In this report, we discuss an observed response of rice to low R:FR light conditions that is atypical of what generally is observed in the SAS. The research question was: why is the observed response of rice under low R:FR light conditions so distinctly different from the typical SAS? We have seen increased or decreased shoot biomass and number of tillers (branches) and leaves in rice grown in FR light in autumn or winter, respectively. As an extension to existing literature, our results suggest that FR light and light that can be used for photosynthesis together, determine the growth response of rice under low R:FR light conditions. Additionally, we found that, even though it has not been described in other literature, rice also shows a (typical) SAS. We concluded this after a light treatment that removes the effect of FR light on photosynthesis. Then, we also investigated how different genes of interest were expressed in rice that was grown under low R:FR light conditions. We grew rice and exposed it to FR light 5 days after sowing, and harvested three tissues (meristem, culm and leaves) separately at three different timepoints (19, 24 and 29 hours after FR light exposure). We used these samples to determine different expression patterns in low R:FR grown rice compared to control grown rice in a quantitative real-time polymerase chain reaction (qPCR) assay. Our genes of interest likely play a role in plant hormone biosynthesis, cell division, the SAS pathway, photosynthesis, and flowering. We have found a potential role of plant hormones (auxin and ethylene) in mediating the response to low R:FR light conditions. Increased biomass or number of leaves and tillers may be due to increased cell division, based on CYCB1;1 expression. Two genes encoding photosynthetic components (PSBR and PSB1B2) also showed a response in their expression, though the differential expression is not uniform. FPFL3 is a gene that was most strongly differentially expressed, though its function is unknown. Lastly PIL16, an upstream component of the SAS pathway, was upregulated. This may indicate that even in rice that responds atypically to low R:FR light conditions, responses of the SAS are still present, and the pathway is still active, but it is probably attenuated or inhibited more downstream in the pathway.