Effect of precipitation change on the photosynthetic performance of Phragmites australis under elevated temperature conditions.

Background: As a fundamental metabolism, leaf photosynthesis not only provides necessary energy for plant survival and growth but also plays an important role in global carbon fixation. However, photosynthesis is highly susceptible to environmental stresses and can be significantly influenced by future climate change. Methods: In this study, we examined the photosynthetic responses of Phragmites australis (P. australis) to three precipitation treatments (control, decreased 30%, and increased 30%) under two thermal regimes (ambient temperature and +4 °C) in environment-controlled chambers. Results: Our results showed that the net CO2 assimilation rate (P n), maximal rate of Rubisco (V cmax), maximal rate of ribulose-bisphosphate (RuBP) regeneration (J max) and chlorophyll (Chl) content were enhanced under increased precipitation condition, but were declined drastically under the condition of water deficit. The increased precipitation had no significant effect on malondialdehyde (MDA) content (p > 0.05), but water deficit drastically enhanced the MDA content by 10.1%. Meanwhile, a high temperature inhibited the positive effects of increased precipitation, aggravated the adverse effects of drought. The combination of high temperature and water deficit had more detrimental effect on P. australis than a single factor. Moreover, non-stomatal limitation caused by precipitation change played a major role in determining carbon assimilation rate. Under ambient temperature, Chl content had close relationship with P n (R2 = 0.86, p < 0.01). Under high temperature, P n was ralated to MDA content (R2 = 0.81, p < 0.01). High temperature disrupted the balance between V cmax and J max (the ratio of J max to V cmax decreased from 1.88 to 1.12) which resulted in a negative effect on the photosynthesis of P. australis. Furthermore, by the analysis of Chl fluorescence, we found that the xanthophyll cycle-mediated thermal dissipation played a major role in PSII photoprotection, resulting in no significant change on actual PSII quantum yield (Φ PSII) under both changing precipitation and high temperature conditions. Conclusions: Our results highlight the significant role of precipitation change in regulating the photosynthetic performance of P. australis under elevated temperature conditions, which may exacerbate the drought-induced primary productivity reduction of P. australis under future climate scenarios.

Independent Evolution of the MYB Family in Brown Algae.

Myeloblastosis (MYB) proteins represent one of the largest families of eukaryotic transcription factors and regulate important processes in growth and development. Studies on MYBs have mainly focused on animals and plants; however, comprehensive analysis across other supergroups such as SAR (stramenopiles, alveolates, and rhizarians) is lacking. This study characterized the structure, evolution, and expression of MYBs in four brown algae, which comprise the biggest multicellular lineage of SAR. Subfamily 1R-MYB comprised heterogeneous proteins, with fewer conserved motifs found outside the MYB domain. Unlike the SHAQKY subgroup of plant 1R-MYB, THAQKY comprised the largest subgroup of brown algal 1R-MYBs. Unlike the expansion of 2R-MYBs in plants, brown algae harbored more 3R-MYBs than 2R-MYBs. At least ten 2R-MYBs, fifteen 3R-MYBs, and one 6R-MYB orthologs existed in the common ancestor of brown algae. Phylogenetic analysis showed that brown algal MYBs had ancient origins and a diverged evolution. They showed strong affinity with stramenopile species, while not with red algae, green algae, or animals, suggesting that brown algal MYBs did not come from the secondary endosymbiosis of red and green plastids. Sequence comparison among all repeats of the three types of MYB subfamilies revealed that the repeat of 1R-MYBs showed higher sequence identity with the R3 of 2R-MYBs and 3R-MYBs, which supports the idea that 1R-MYB was derived from loss of the first and second repeats of the ancestor MYB. Compared with other species of SAR, brown algal MYB proteins exhibited a higher proportion of intrinsic disordered regions, which might contribute to multicellular evolution. Expression analysis showed that many MYB genes are responsive to different stress conditions and developmental stages. The evolution and expression analyses provided a comprehensive analysis of the phylogeny and functions of MYBs in brown algae.