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 a pot experiment involving defoliation treatments and an isolated culture experiment to determine whether the invasive species Mikania micrantha exhibits greater plasticity in the stems than do three native 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 photosynthesis 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.

The rapid stem elongation of the invasive weed Mikania micrantha in the forest understory is of vital significance for its successful invasion. To understand the physiological and molecular mechanisms for this process, here we comparatively investigated the physiological characteristics and transcriptome patterns of M. micrantha stem under low light (30%) and full light (100%) conditions. The results showed that M. micrantha stem had photosynthetic capacity, which was highly plastic to light intensities, constituting of an indispensable part of the plastic response of M. micrantha to shading. M. micrantha had longer internodes, epidermal cells, and consequently longer stems under low light than full light conditions, which could be attributed to the reduced photoprotective substances (flavonoid and anthocyanin) and increased synthesis of phytohormones (gibberellin, GA and Auxin) as observed under shading treatment. The transcriptome sequencing and qPCR verified the results from physiological investigation, and showed that under low light condition the expression levels of genes involving in photosynthesis (e.g. MmPsaA, MmPsbO1 and MmFd3) were generally down-regulated in comparison to full light condition, so were the genes related to the photoprotective substances synthesis (e.g. MmCHS, and MmF3H1) and the negative regulators of phytohormone (e.g. MmAUX1, MmRR1 and MmGAI). It was concluded that the regulation of phytohormones and photoprotective substances are the important material basis for the rapid elongation of M. micrantha stems with high plasticity, which really matters to the vine to have high invasiveness in the forest understory.