Abstract
In full sunlight, plants often experience a light intensity exceeding their photosynthetic capacity and causing the activation of a set of photoprotective mechanisms. Numerous reports have explained, on the molecular level, how plants cope with light stress locally in photosynthesizing leaves; however, the response of below-ground organs to above-ground perceived light stress is still largely unknown. Since small RNAs are potent integrators of multiple processes including stress responses, here, we focus on changes in the expression of root miRNAs upon high-intensity-light (HL) stress. To achieve this, we used Arabidopsis thaliana plants growing in hydroponic conditions. The expression of several genes that are known as markers of redox changes was examined over time, with the results showing that typical HL stress signals spread to the below-ground organs. Additionally, micro-transcriptomic analysis of systemically stressed roots revealed a relatively limited reaction, with only 17 up-regulated and five down-regulated miRNAs. The differential expression of candidates was confirmed by RT-qPCR. Interestingly, the detected differences in miRNA abundance disappeared when the roots were separated from the shoots before HL treatment. Thus, our results show that the light stress signal is induced in rosettes and travels through the plant to affect root miRNA levels. Although the mechanism of this regulation is unknown, the engagement of miRNA may create a regulatory platform orchestrating adaptive responses to various simultaneous stresses. Consequently, further research on systemically HL-regulated miRNAs and their respective targets has the potential to identify attractive sequences for engineering stress tolerance in plants.
Highlights
The simplistic model of plant roots taking up water and nutrients that are essential for plant growth and receiving from shoots sugars and auxins which drive root development is much more complicated than an availability–growth relationship
To study the high light intensity (HL) response in A. thaliana roots, we used plants growing in a hydroponic system, which enables the continuous growth of roots in the dark and minimizes mechanical damage and stress
Plants growing in low-light conditions (LL; 100–120 μmol photons m−2 s−1) and a short-day photoperiod (Figure 1A) were subjected to two hours of HL stress at an intensity of 1500 μmol photons m−2 s−1, and the expression of several HL-response-related genes was checked immediately following the stress and after 4 h of recovery, to confirm whether the stress signal spread to dark-grown roots
Summary
The simplistic model of plant roots taking up water and nutrients that are essential for plant growth and receiving from shoots sugars and auxins which drive root development is much more complicated than an availability–growth relationship. Knowledge about the role of roots as a component of the plant signaling network integrating environmental cues has greatly expanded, revealing roots’ central role in optimizing plant nutrient demand in response to shoot-derived stress signals and changes in photosynthesis capacity [1,2]. In terms of light intensity, two extreme situations can occur: (1) light deficiency and (2) excess light (EL) caused by high light intensity (HL) Because such HL incidents may lead to photoinhibition, photoprotective mechanisms are triggered to avoid or dissipate the excess of light energy. These mechanisms include ultrastructural adaptations (e.g., chloroplast movement and thylakoid proteins arrangement), physical energy dissipation (e.g., by heat and chlorophyll fluorescence), and a number of biochemical processes such as photochemical and non-photochemical quenching, chlororespiration, photorespiration, production of antioxidant enzymes (e.g., APX, SOD) as well as carotenoids, tocopherols, or small antioxidant molecules (e.g., ascorbate and glutathione) [3,4,5,6,7,8]
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