Sphagneticola trilobata is an invasive plant in South China. A hybrid between S. trilobata and Sphagneticola calendulacea (a native related species) has also been found in South China. The drought resistance of S. calendulacea, S. trilobata and their hybrid was studied in this paper. Under drought stress, the leaves of S. trilobata synthesized more abscisic acid (ABA) than those of the other species to reduce stomatal opening and water loss. The activities of antioxidant enzymes were the highest in S. trilobata and the lowest in S. calendulacea. The leaves of S. calendulacea suffered the most serious damage, and their maximum photochemical efficiency was the lowest. RNA-sequencing ware used to analyze the expression levels of genes in ABA, antioxidant enzyme, sugar and proline synthesis and photosynthesis pathways. Further real-time PCR detection verified the RNA-sequence results, and the results were in accordance with the physiological data. The results showed that S. trilobata was the most drought tolerant, and the drought tolerance of the hybrid did not show heterosis but was higher than S. calendulacea. Therefore, compared with S. trilobata and the hybrid, the population number and distribution of S. calendulacea may be less in arid areas.

Mikania micrantha, recognized as one of the world’s top 10 pernicious weeds, is a rapidly spreading tropical vine that has invaded the coastal areas of South China, causing serious economic losses and environmental damage. Rapid stem growth is an important feature of M. micrantha which may be related to its greater number of genes involved in auxin signaling and transport pathways and its ability to synthesize more auxin under adverse conditions to promote or maintain stem growth. Plant growth and development is closely connected to the regulation of endogenous hormones, especially the polar transport and asymmetric distribution of auxin. The PIN-FORMED (PIN) auxin efflux carrier gene family plays a key role in the polar transport of auxin and then regulates the growth of different plant tissues, which could indicate that the rapid growth of M. micrantha is closely related to this PIN-dependent auxin regulation. In this study, 11 PIN genes were identified and the phylogenetic relationship and structural compositions of the gene family in M. micrantha were analyzed by employing multiple bioinformatic methods. The phylogenetic analysis indicated that the PIN proteins could be divided into five distinct clades. The structural analysis revealed that three putative types of PIN (canonical, noncanonical and semi-canonical) exist among the proteins according to the length and the composition of the hydrophilic domain. The majority of the PINs were involved in the process of axillary bud differentiation and stem response under abiotic stress, indicating that M. micrantha may regulate its growth, development and stress response by regulating PIN expression in the axillary bud and stem, which may help explain its strong growth ability and environmental adaptability. Our study emphasized the structural features and stress response patterns of the PIN gene family and provided useful insights for further study into the molecular mechanism of auxin-regulated growth and control in M. micrantha.

In stressful environments, invasive plants acclimate more efficiently than native plants and hybridization mainly contributes to this process. We examined changes in the morphological characteristics, photosynthetic characteristics, and antioxidant capacity of Sphagneticola trilobata and its hybrids in a low-light environment to explore their invasiveness, with Sphagneticola calendulacea serving as the control. The morphological plasticity of S. trilobata was not dominant, the maximal photochemical efficiency of PSII, actual quantum yield of PSII, and electron transport rate of PSⅡ increased and nonphotochemical quenching decreased, while S. calendulacea and the hybrid produced opposite results. S. trilobata showed fewer spots stained for reactive oxygen species in tissues, with an increase in superoxide dismutase activity. Although S. trilobata is a heliophilous plant, we found that the shade tolerance of S. trilobata and the hybrid were stronger than that of S. calendulacea, which may be one important mechanism of invasion. Highlights• Shading promoted the plant height but decreased the stem length of Sphagneticola trilobata • S. calendulacea has the strong photosynthetic capacity and antioxidant activity in the shade • Shade tolerance of the hybrid is more efficient than that of S. calendulacea

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