Yangping Li

Many studies have shown that individuals from invasive populations of many different plant species grow larger than individuals from native populations and that this difference has a genetic basis. This increased vigor in invasive populations is thought to be due to life history tradeoffs, in which selection favors the loss of costly defense traits, thereby freeing resources that can be devoted to increased growth or fecundity. Despite the theoretical importance of such allocation shifts for invasions, there have been no efforts to understand apparent evolutionary shifts in defense-growth allocation mechanistically. Reallocation of nitrogen (N) to photosynthesis is likely to play a crucial role in any growth increase; however, no study has been conducted to explore potential evolutionary changes in N allocation of introduced plants. Here, we show that introduced Ageratina adenophora, a noxious invasive plant throughout the subtropics, appears to have evolved increased N allocation to photosynthesis (growth) and reduced allocation to cell walls, resulting in poorer structural defenses. Our results provide a potential mechanism behind the commonly observed and genetically based increase in plant growth and vigor when they are introduced to new ranges.

Sucrose is not only the carbon source for starch synthesis, but also a signal molecule. Alone or in coordination with ABA, it can regulate the expression of genes involved in starch synthesis. To investigate the molecular mechanisms underlying this effect, maize endosperms were collected from Zea mays L. B73 inbred line 10 d after pollination and treated with sucrose, ABA, or sucrose plus ABA at 28 °C in the dark for 24 h. RNA-sequence analysis of the maize endosperm transcriptome revealed 47 candidate transcription factors among the differentially expressed genes. We therefore speculate that starch synthetic gene expression is regulated by transcription factors induced by the combination of sucrose and ABA. ZmEREB156, a candidate transcription factor, is induced by sucrose plus ABA and is involved in starch biosynthesis. The ZmEREB156-GFP-fused protein was localized in the nuclei of onion epidermal cells, and ZmEREB156 protein possessed strong transcriptional activation activity. Promoter activity of the starch-related genes Zmsh2 and ZmSSIIIa increased after overexpression of ZmEREB156 in maize endosperm. ZmEREB156 could bind to the ZmSSIIIa promoter but not the Zmsh2 promoter in a yeast one-hybrid system. Thus, ZmEREB156 positively modulates starch biosynthetic gene ZmSSIIIa via the synergistic effect of sucrose and ABA.

1. Many studies testing the evolution of increased competitive ability hypothesis have focused on whether plants from invasive populations of a species show reduced allocation to defence and increased allocation to growth than plants from native populations. But few have attempted to understand ecophysiological mechanisms by which decreased allocation to defence may increase growth. 2. Previously, we found that invasive Ageratina adenophora plants increase nitrogen allocation to photosynthesis and reduce allocation to cell walls compared with native Ageratina plants, suggesting a shift from defence to growth in invasive populations. Here, carrying this work forward, we measured construction costs and benefits associated with photosynthesis at light saturation to leaves. We hypothesized that invasive Ageratina populations might employ a quicker return energy-use strategy by increasing light-saturated photosynthetic rates and photosynthetic energy-use efficiency (PEUE) and by decreasing leaf construction costs. 3. Faster-growing plants from invasive populations (China and India) had significantly higher leaf nitrogen concentrations and specific leaf areas than plants from native populations (Mexico). Inconsistent with our prediction, leaf construction costs were not significantly different between plants from invasive and native populations, but higher light-saturated photosynthetic rates and in turn higher PEUE resulted in a significantly shorter payback time of construction costs, which allowed plants from invasive populations to grow faster. 4. Synthesis. Our results indicate that Ageratina plants from populations in non-native ranges have a distinct quick return energy-use strategy, a high PEUE and a short payback time but not lower construction costs, which might provide a mechanistic explanation for the commonly observed increase in growth when plants are introduced to new parts of the world. To our knowledge, this is the first study to compare energy-use strategy for plants from invasive and native populations of a noxious invasive species. We cannot exclude some alternative hypotheses for these patterns, such as founder effects, but these ecophysiological differences might provide mechanistic insight for how the evolution of decreased allocation to defence may increase growth and competitive ability.