Strong Response of Stem Photosynthesis to Defoliation in Mikania micrantha Highlights the Contribution of Phenotypic Plasticity to Plant Invasiveness.

Phenotypic plasticity affords invasive plant species the ability to colonize a wide range of habitats, but physiological plasticity of their stems is seldom recognized. Investigation of the stem plasticity of invasive plant species could lead to a better understanding of their invasiveness. We performed pot experiments involving defoliation treatments and isolated culture experiments to determine whether the invasive species Mikania micrantha exhibits greater plasticity in the stems than do three non-invasive species that co-occur in southern China and then explored the mechanism underlying the modification of its stem photosynthesis. Our results showed that the stems of M. micrantha exhibited higher plasticity in terms of either net or gross photosynthetic rate in response to the defoliation treatment. These effects were positively related to an increased stem elongation rate. The enhancement of stem photosynthesis in M. micrantha resulted from the comprehensive action involving increases in the Chl a/b ratio, D1 protein and stomatal aperture, changes in chloroplast morphology and a decrease in anthocyanins. Increased plasticity of stem photosynthesis may improve the survival of M. micrantha under harsh conditions and allow it to rapidly recover from defoliation injuries. Our results highlight that phenotypic plasticity promotes the invasion success of alien plant invaders.

Interpretation of the difference in shade tolerance of two subtropical forest tree species of different successional stages at the transcriptome and physiological levels.

Differences in plant shade tolerance constitute a major mechanism driving the succession of forest communities in subtropical forests. However, the indirect effects of differences in light requirements on the growth of mid- and late-successional tree species are unclear, and this potential growth effect has not been explained at the transcriptome level. Here, a typical mid-successional dominant tree species, Schima superba Gardn. et Champ, and a typical late-successional dominant tree species, Cryptocarya concinna Hance were used as materials and planted under 100% full light (FL) and 30% FL (low light, LL) to explore the responses of tree species in different successional stages of subtropical forests to different light environments. Transcriptome sequencing was used to analyze the expression changes in genes related to growth and photoprotection under different light environments. The young leaves of S. superba accumulated more malondialdehyde (MDA) and superoxide radicals (${\mathrm{O}}_2^{{{}^{\bullet}}^{-}}$) under LL. A lower hormone content (auxin, cytokinin, gibberellin) in the young leaves, a weaker photosynthetic capacity in the mature leaves and significant downregulation of related gene expression were also found under LL, which resulted in the total biomass of S. superba under LL being lower than that under FL. The young leaves of C. concinna had less MDA and ${\mathrm{O}}_2^{{{}^{\bullet}}^{-}}$, and a higher hormone contents under LL than those under FL. There was no significant difference in photosynthetic capacity between mature leaves in contrasting light environments. Although the biomass of C. concinna under LL was less than that under FL, the height of C. concinna under LL was higher than that under FL, indicating that C. concinna could grow well under the two light environments. Our results describing the acclimatization of light at the physiological, molecular and transcriptome levels are important for a complete understanding of successional mechanisms.

The major photoprotective role of anthocyanins in leaves of Arabidopsis thaliana under long-term high light treatment: antioxidant or light attenuator?

Anthocyanins are water-soluble pigments in plants known for their photoprotective role against photoinhibitory and photooxidative damage under high light (HL). However, it remains unclear whether light-shielding or antioxidant activity plays a major role in the photoprotection exerted by anthocyanins under HL stress. To shed light on this question, we analyzed the physiological and biochemical responses to HL of three Arabidopsis thaliana lines (Col, chi, ans) with different light absorption and antioxidant characteristics. Under HL, ans had the highest antioxidant capacity, followed by Col, and finally chi; Col had the strongest light attenuation capacity, followed by chi, and finally ans. The line ans had weaker physiological activity of chloroplasts and more severe oxidative damage than chi after HL treatment. Col with highest photoprotection of light absorption capacity had highest resistance to HL among the three lines. The line ans with high antioxidant capacity could not compensate for its disadvantages in HL caused by the absence of the light-shielding function of anthocyanins. In addition, the expression level of the Anthocyanin Synthase (ANS) gene was most upregulated after HL treatment, suggesting that the conversion of colorless into colored anthocyanin precursors was necessary under HL. The contribution of anthocyanins to flavonoids, phenols, and antioxidant capacity increased in the late period of HL, suggesting that plants prefer to synthesize red anthocyanins (a group of colored antioxidants) over other colorless antioxidants to cope with HL. These experimental observations indicate that the light attenuation role of anthocyanins is more important than their antioxidant role in photoprotection.

The relationship between anthocyanin accumulation and photoprotection in young leaves of two dominant tree species in subtropical forests in different seasons.

Increasing amounts of experimental evidence show that anthocyanins provide physiological protection to plants under stress. However, the difference in photoprotection mediated by anthocyanins and other photoprotective substances in different seasons is still uncertain. To determine the relationship between anthocyanin accumulation and the photoprotective effects in different seasons, Castanopsis chinensis and Acmena acuminatissima, whose anthocyanin accumulation patterns differ in different seasons, were used as materials to explain how plants adapt to different seasons; as such, their physiological and biochemical responses were analyzed. Young leaves of C. chinensis and A. acuminatissima presented different colors in the different seasons. In summer, the young leaves of C. chinensis were purplish red, while those of A. acuminatissima were light green. In winter, the young leaves of C. chinensis were light green, while those of A. acuminatissima were red. Compared with the young red leaves, the young light green leaves that did not accumulate anthocyanins had higher flavonoid and phenolics contents, total antioxidant capacity, non-photochemical quenching (NPQ), and relative membrane leakage, and a slower recovery rate in the maximum photochemical efficiency (Fv/Fm) after high-light treatment. In addition, the net photosynthesis rate (Pn), transpiration rate (Tr), stomatal conductance (gs), and the effective quantum yield of PSII (ΦPSII) of the young leaves in winter were significantly lower than those in summer, while the activities of catalase (CAT, EC 1.11.1.6), peroxidase (POD, EC 1.11.1.7), and superoxide dismutase (SOD, EC 1.15.1.1) were significantly higher than those in summer. These data indicate that to adapt to seasonal changes anthocyanins, other antioxidative substances and antioxidative enzymes, as well as components involved in the safe dissipation of excitation energy as heat need to cooperate with one another.

Photosynthetic compensation of non-leaf organ stems of the invasive species Sphagneticola trilobata (L.) Pruski at low temperature.

Biological invasion is a hot topic in ecological research. Most studies on the physiological mechanisms of plants focus on leaves, but few studies focus on stems. To study the tolerance of invasive plant (Sphagneticola trilobata L.) to low temperature, relevant physiological indicators (including anthocyanin and chlorophyll) in different organs (leaves and stems) were analyzed, using a native species (Sphagneticola calendulacea L.) as the control. The results showed that, upon exposure to low temperature for 15 days, the stems of two Sphagneticola species were markedly reddened, their anthocyanin content increased, chlorophyll and chlorophyll fluorescence parameters decreased, and the accumulation of reactive oxygen species in the stem increased. The percentage increases of antioxidants and total antioxidant capacities in stems were significantly higher in S. trilobata than in S. calendulacea. This showed that S. trilobata had higher cold tolerance in stems while leaves were opposite. To further verify the higher cold tolerance of the stem of S. trilobata, a defoliation experiment was designed. We found that the defoliated stem of S. trilobata reduced anthocyanin accumulation and increased chlorophyll content, while alleviating membrane lipid damage and electrical conductivity, and the defoliated stem still showed an increase in stem diameter and biomass under low temperature. The discovery of the physiological and adaptive mechanisms of the stem of S. trilobata to low temperature will provide a theoretical basis for explaining how S. trilobata maintains its annual growth in South China. This is of great significance for predicting the future spread of cloned and propagated invasive plants.

Comparative physiological and transcriptomic analyses of photosynthesis in Sphagneticola calendulacea (L.) Pruski and Sphagneticola trilobata (L.) Pruski.

Sphagneticola trilobata (L.) Pruski is one of the fast-growing malignant weeds in South China. It has severely influenced local biodiversity and native plant habitat. Photosynthesis is the material basis of plant growth and development. However, there are few reports on the photosynthetic transcriptome of S. trilobata. In this study, S. trilobata had a relatively large leaf area and biomass. The gas exchange parameters per unit area of leaves, including net photosynthetic capacity (Pn), intercellular CO2 (Ci), stomatal conductance (Gs), transpiration rate (Tr), water use efficiency (WUE), photosynthetic pigment and Rubisco protein content were higher than those of the native plant Sphagneticola calendulacea (L.) Pruski. On this basis, the differences in photosynthesis pathways between the two Sphagneticola species were analyzed by using the Illumina HiSeq platform. The sequencing results for S. trilobata and S. calendulacea revealed 159,366 and 177,069 unigenes, respectively. Functional annotation revealed 119,350 and 150,846 non-redundant protein database annotations (Nr), 96,637 and 115,711 Swiss-Prot annotations, 49,159 and 60,116 Kyoto Encyclopedia of Genes and Genomes annotations (KEGG), and 83,712 and 97,957 Gene Ontology annotations (GO) in S. trilobata and S. calendulacea, respectively. Additionally, our analysis showed that the expression of key protease genes involved in the photosynthesis pathway, particularly CP43, CP47, PsbA and PetC, had high expression levels in leaves of S. trilobata in comparison to native species. Physiological and transcriptomic analyses suggest the high expression of photosynthetic genes ensures the high photosynthetic capacity of leaves, which is one of the inherent advantages underlying the successful invasion by S. trilobata.

ANS-deficient Arabidopsis is sensitive to high light due to impaired anthocyanin photoprotection.

Light attenuation and antioxidation are the main mechanisms of photoprotection by anthocyanin under high light (HL) stress. Anthocyanin synthase (ANS) is the key enzyme in the downstream portion of anthocyanin synthetic pathways. To explore the role of ANS in photoprotection by anthocyanin under HL stress, homozygous ANS-deficient Arabidopsis mutants were screened from SALK_073183 and SALK_028793. Here, we obtained two deficient mutants, ans-1 and ans-2, which had ANS gene expression levels equal to 5.9 and 32.9% of that of Col respectively. By analysing their physiological and biochemical responses to HL stress, we found that there were positive correlations among ANS expression level, anthocyanin content and resistance to HL. The line with the lowest ANS expression level, ans-1, was also the most sensitive to HL, showing the lowest anthocyanin content, chlorophyll content, Fv/Fm ratio, and Rubisco content and the highest O2•- accumulation and membrane leakage rate, although it also had the highest antioxidant capacity. Experimental evidence suggests that ANS mainly regulated the light-attenuating function of anthocyanin in photoprotection under HL. Blocking excess light is an important function of anthocyanin that protects plants from HL stress, and a high antioxidant capacity cannot compensate for the absence of the light-shielding function of anthocyanin.