When subjected to LL stress, plants ultilize two different adaptation strategies: shade avoidance and shade tolerance [21, 22]. In this study, LL tolerant cultivar Arid 3 displayed better performance under LL stress than LL sensitive cultivar Airlie. The most notable features are that Arid 3 had relatively higher biomass and produced more tillers than Airlie under LL stress. Tiller is one of the important characteristics of turfgrass, which directly affects trampling resistance and the turf quality. Moreover, more tillers mean higher stem density and biomass content of fescue [23]. Chlorophyll has a significant response to LL stress; it traps light energy and drives electron transfer process, playing a central role in plant photosynthesis [24]. The content of Chl b in Arid 3 was observably higher than in Airlie under LL stress (Fig. 1). Under LL stress, the content of chlorophyll in cells decreases significantly, especially chlorophyll b of Scenedesmus dimorphus [25]. However, some plant species produce high levels of Chl b to improve light-catching ability under LL stress, as well as increase the content of light-catching pigment complexes and the activity of photophosphorylation [26]. The synthesis of chlorophyll is regulated by many factors, including the stimulation of photoreceptors by external light [27]. The transformation from glutamic acid (Glu) to chlorophyll is catalyzed by many enzymes, such as GluTR, uroporphyrinogen III synthase (UROS), uroporphyrinogen III decarboxylase (UROD), chlorophyll specific synthase (MgCh), DVR, POR, CAO, and CHLG. The expression levels of genes encoding these key enzymes differed between Arid 3 and Airlie (Fig. 5). GluTR is the key enzyme in the biosynthesis of porphyrin compounds, regulated by biological rhythm, light, and end products [28]. As one of the three known light-dependent enzymes in the chlorophyll biosynthesis pathway, POR catalyses the reduction of photosensitizer and substrate protochlorophyllide to form the pigment chlorophyllide. The active site of POR promotes the light-driven reduction of protochlorophyllide through the transfer of local hydride and long-distance proton [29]. The different expression levels of genes encoding for these key enzymes in Arid 3 and Airlie under LL stress may explain the varied chlorophyll contents between the two cultivars. The gene encoding for chlorophyll degradation enzyme pheophorbide, a monooxygenase (PAO), was down-regulated after LL treatment. The down-regulation may be a self-protection mechanism of plants when subjected to LL stress. The expression level of FaCHLI, the gene related to chlorophyll biosynthesis, was higher in Arid 3 than Airlie under LL stress, which is an important evidence that Arid 3 produced more Chl than Airlie under LL stress (Fig. 7).
Photosynthesis and light are two closely related, primary processes in all plants [30]. LL stress decreases photosynthetic and transpiration rates, as reported in seashore paspalum and bermudagrass [31]. In this study, GO and KEGG enrichment analysis of Arid 3 and Airlie suggested obvious changes in photosynthesis in the two cultivars under LL stress (Fig. 6). Under the LL stress, the values of Fv/Fm and ΦPSII remarkably decreased in Airlie, but the decrease in Arid 3 was not substantial. Consistent with other stresses, the degree of opening of reaction centers of PSⅡ also declined under LL stress [32] Compared to control, tall fescue under LL stress broke the maintenance of photochemical health due to lower ΦPSII activity. Due to LL stress, several proteins of PSⅡ, including the PsbP ,PsbQ, PsbR, PsbW, Psb27, and Psb28, were down-regulated in Arid 3, more than in Airlie. The different effects of LL on PSII proteins of Arid 3 and Airlie imply a stronger photosynthetic capacity of Arid 3 (Fig. 6). Chlorophyll fluorescence provides valuable information for the study of light in the PSⅡ photochemical reaction. The results of this study showed that LL stress inhibited Fv/Fm and ΦPSII in tall fescue, suggesting that electronic produced from PSⅡ shift from chlorophyll was limited by PQ [33]. The Fv/Fm, ΦPSII, qp, and ETR indicators were higher in Arid 3 than in Airlie under LL stress, suggesting that Arid 3 had high PSⅡ maximal quantum yield and electron transfer efficiency.
Interestingly, the value of Fv/Fm and ΦPSII in Arid 3 only changed slightly under LL stress compared with control, while in Airlie, these two indicators were substantially reduced. The qP value represents the open ratio of PSII reaction centers. A rise in qP value accounts for the increased capacity for electron transport, but qP value decreases under abiotic stress [34, 35]. This study showed that qP significantly decreased under LL stress, although the qP value in Arid 3 maintained a remarkably higher level than in Airlie (Fig. 1), demonstrating that Arid 3 mitigates the damage of the PSII reaction center and maintains a higher ETR between electron acceptors and providers compared with Airlie. LL stress also decreased the expression of genes encoding for PetF, PetH, and PetJ, which are related to photosynthetic electron transport. Moreover, the decreased levels of these genes in Arid 3 were lower than in Airlie. These results could explain why Arid 3 maintained a higher ETR than Airlie under LL.
Phytochrome is a widely studied family of photoreceptors that respond to red and far-red light in Arabidopsis thaliana [36, 37], which involves in many physiological processes, such as seed germination, seedling development, photosynthesis, and shading reaction [38]. LL stress, defined as the reduced photosynthetically active radiation (PAR) and changed red/far-red light ratio, involves phytochrome A and phytochrome B [39, 40]. Photochrome A (PhyA) is the only photoreceptor capable of sensing far-red light and responding to this signal in plants [41]. In this study, the expression levels of FaPhyA were significantly up-regulated in both cultivars, and at higher levels in Arid 3 than in Airlie (Fig. 7). Studies have shown that PhyA plays an important role under LL stress, inhibiting the negative regulatory factor phytochrome-interacting factors (PIFs) in the photomorphogenesis process [40]. PIFs, a branch of the superfamily of basic helic-loop helix transcription factors, are key inhibitors of photomorphogenesis [42]. PIF4 plays a role in the phytochrome signaling pathway [43]. PIFs interact with abscisic acid (ABA) receptors to regulate ABA signal transduction under LL and inhibits plant photomorphogenesis [44]. FaPIF5 was up-regulated in two cultivars under LL stress. However, the expression level in Airlie was higher than that in Arid 3, indicating that Arid 3 inclined to photomorphogenesis, while Airlie was inclined to skotomorphogenesis under LL stress. PhyB functions as red-light receptors and accumulates in large amounts under the induction of red light [45].
Phytochrome A (PhyB) degrades PIF5 through phosphorylation and then promotes photomorphogenesis; On the one hand, MYB30 inhibits this degradation process, on the other hand, it directly induces the expression of PIF5 [46]. This study showed that the expression of FaPhyB was significantly decreased in Arid 3, but up-regulated in Airlie. The differential expression of the positive regulatory factor FaPhyB in photomorphogenesis may explain shade avoidance and shade tolerance strategies utilized by the two cultivars. Constitutively photomorphogenic 1 (COP1), an E3 ubiquitin ligase, is a protein complex that acts as a central inhibitor in photomorphogenesis [47]. COP1 is believed to be a central switch of global light-responsive gene expression in Arabidopsis thaliana by disrupting HY5 and other transcriptional regulators critical to light morphogenesis in the dark [48, 49]. It was found that (Fig. 7) the expression of FaCOP1 decreased remarkably in Arid 3 under LL stress compared to control, but little change was observed in Airlie. Under red light, red receptor PhyB enters the nucleus, where it inhibits the activity of COP1 and promotes its degration. PhyB also promotes the degradation of PIFs, thus promoting photomorphogenesis [50]. Synthesis of COP1 is induced by far-red light and inhibited by PhyA, indicating the positive regulatory role of COP1 in photomorphogenesis [51].
Except for FaPIF5 and FaCOP1, both FaPhyA and FaPhyB played positive roles in regulating photomorphogenesis of tall fescue cultivars Arid 3 and Airlie under LL stress and the control. FaPIF5, FaCOP1, FaPhyA, and FaPhyB inter-regulated with each other to jointly promote the photomorphogenesis of Arid 3 and the skotomorphogenesis of Airlie, which explains the better performance of Arid 3 under LL stress. However, the functional mechanisms of these transcription factors in tall fescue remains to be further explored.