Regulation of plant root system architecture: implications for crop advancement.

Roots are crucial for nutrient and water acquisition and can be targeted to enhance plant productivity under a broad range of growing conditions. A current challenge for plant breeding is the limited ability to phenotype and select for desirable root characteristics due to their underground location. Plant breeding efforts aimed at modifying root traits can result in novel, more stress-tolerant crops and increased yield by enhancing the capacity of the plant for soil exploration and, thus, water and nutrient acquisition. Available approaches for root phenotyping in laboratory, greenhouse and field encompass simple agar plates to labor-intensive root digging (i.e., shovelomics) and soil boring methods, the construction of underground root observation stations and sophisticated computer-assisted root imaging. Here, we summarize root architectural traits relevant to crop productivity, survey root phenotyping strategies and describe their advantages, limitations and practical value for crop and forage breeding programs.

Root Traits and Phenotyping Strategies for Plant Improvement

Prioritization is indispensable for the management of biological invasions, as recognized by the Convention on Biological Diversity, its current strategic plan, and specifically Aichi Target 9 that concerns invasive alien species. Here we provide an overview of the process, approaches and the data needs for prioritization for invasion policy and management, with the intention of informing and guiding efforts to address this target. Many prioritization schemes quantify impact and risk, from the pragmatic and action-focused to the data-demanding and science-based. Effective prioritization must consider not only invasive species and pathways (as mentioned in Aichi Target 9), but also which sites are most sensitive and susceptible to invasion (not made explicit in Aichi Target 9). Integrated prioritization across these foci may lead to future efficiencies in resource allocation for invasion management. Many countries face the challenge of prioritizing with little capacity and poor baseline data. We recommend a consultative, science-based process for prioritizing impacts based on species, pathways and sites, and outline the information needed by countries to achieve this. This should be integrated into a national process that incorporates a broad suite of social and economic criteria. Such a process is likely to be feasible for most countries.

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Prioritizing species, pathways, and sites to achieve conservation targets for biological invasion

The number of alien plants escaping from cultivation into native ecosystems is increasing steadily. We provide an overview of the historical, contemporary and potential future roles of ornamental horticulture in plant invasions. We show that currently at least 75% and 93% of the global naturalised alien flora is grown in domestic and botanical gardens, respectively. Species grown in gardens also have a larger naturalised range than those that are not. After the Middle Ages, particularly in the 18th and 19th centuries, a global trade network in plants emerged. Since then, cultivated alien species also started to appear in the wild more frequently than non-cultivated aliens globally, particularly during the 19th century. Horticulture still plays a prominent role in current plant introduction, and the monetary value of live-plant imports in different parts of the world is steadily increasing. Historically, botanical gardens – an important component of horticulture – played a major role in displaying, cultivating and distributing new plant discoveries. While the role of botanical gardens in the horticultural supply chain has declined, they are still a significant link, with one-third of institutions involved in retail-plant sales and horticultural research. However, botanical gardens have also become more dependent on commercial nurseries as plant sources, particularly in North America. Plants selected for ornamental purposes are not a random selection of the global flora, and some of the plant characteristics promoted through horticulture, such as fast growth, also promote invasion. Efforts to breed non-invasive plant cultivars are still rare. Socio-economical, technological, and environmental changes will lead to novel patterns of plant introductions and invasion opportunities for the species that are already cultivated. We describe the role that horticulture could play in mediating these changes. We identify current research challenges, and call for more research efforts on the past and current role of horticulture in plant invasions. This is required to develop science-based regulatory frameworks to prevent further plant invasions.

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The changing role of ornamental horticulture in alien plant invasions

Many non-native plants in the US have become problematic invaders of native and managed ecosystems, but a new generation of invasive species may be at our doorstep. Here, we review trends in the horticultural trade and invasion patterns of previously introduced species and show that novel species introductions from emerging horticultural trade partners are likely to rapidly increase invasion risk. At the same time, climate change and water restrictions are increasing demand for new types of species adapted to warm and dry environments. This confluence of forces could expose the US to a range of new invasive species, including many from tropical and semiarid Africa as well as the Middle East. Risk assessment strategies have proven successful elsewhere at identifying and preventing invasions, although some modifications are needed to address emerging threats. Now is the time to implement horticulture import screening measures to prevent this new wave of plant invasions.

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Global change, global trade, and the next wave of plant invasions

Root plasticity, a trait that can respond to selective pressure, may help plants forage for nutrients in heterogeneous soils. Agricultural breeding programs have artificially selected for increased yield under comparatively homogeneous soil conditions, potentially decreasing the capacity for plasticity in crop plants like barley (Hordeum vulgare). However, the effects of domestication on the evolution of root plasticity are essentially unknown. Using a split container approach, we examined the differences in root plasticity among three domestication levels of barley germplasm (wild, landrace, and cultivar) grown under different concentrations and distribution patterns of soil nutrients. Domestication level, nutrient concentration, and nutrient distribution interactively affected average root diameter; differential root allocation (within-plant plasticity) was greatest in wild barley (Hordeum spontaneum), especially under low nutrient levels. Correlations of within-plant root plasticity and plant size were most pronounced in modern cultivars under low-nutrient conditions. Barley plants invested more resources to root systems when grown in low-nutrient soils and allocated more roots to higher-nutrient locations. Root plasticity in barley is scale dependent and varies with domestication level. Although wild barley harbors a greater capacity for within-plant root plasticity than domesticated barley, cultivars exhibited the greatest capacity to translate within-plant plasticity into increased plant size.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1752-4571.2012.00263.x

Evolution of root plasticity responses to variation in soil nutrient distribution and concentration

Although prevention is the most cost- effective way to avoid the enormous expenses associated with plant invasions, invasive plants continue to be imported as trade commodities for horticultural use. With very little government regulation of horticultural imports of invasive plants, efforts have turned toward fostering volun- tary initiatives to encourage self-regulation by the horticulture trade. Our study takes the first step toward evaluating the potential success of these voluntary initiatives. We conducted a survey of nursery professionals to gauge their perceptions of invasive species, the role of the horticulture trade in invasive plant introductions, and their participa- tion—potential and actual—in preventive mea- sures outlined in the St Louis Voluntary Codes of Conduct for nursery professionals. We found nurs- ery professionals to be highly aware of invasive plants and to accept responsibility as a trade for horticultural introductions. Although only 7% of respondents had heard of the St Louis Voluntary Codes of Conduct, the majority (57%) reported having participated in at least two of seven preventive measures, and most (78%) reported willingness to engage in the majority of preventive measures. We found that several factors signifi- cantly predict increased participation in preventive measures, particularly awareness of invasive plants and involvement in trade associations. We also identified incentives and obstacles to participating in preventive behaviors, including ''concern for the environment'' and ''lack of information,'' respec- tively. Our results suggest that participation in voluntary initiatives will improve through increased outreach, and we provide specific recommenda- tions for improving participation in voluntary programs in the horticulture trade.

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Preventing horticultural introductions of invasive plants: potential efficacy of voluntary initiatives

The aquarium trade is an important and rapidly growing vector for introduced species in the United States. We examined this vector by surveying pet stores in the San Francisco Bay-Delta region to compile a list of aquarium fish species commonly stocked. We identified which of these species might be able to survive in the Bay-Delta, and investigated store representatives' knowledge and attitudes about bio- logical invasions. A restrictive analysis using conservative estimates of fish temperature tolerances and environmental conditions found that the local aquarium trade includes 5 fish species that can survive in a temperate system such as the Bay-Delta. Under more inclusive parameters, up to 27 fish species met the criteria for survival in the Bay-Delta. We further explored these results by comparing potential invader incidence between different types of stores. In the more restrictive analysis, three national retail chains stocked significantly more potentially invasive species than independent aquarium stores, but there was no differ- ence in the more inclusive analysis. A significantly higher percentage of fish taxa were easily identifiable and well-labeled in chain stores than in independent stores. Most aquarium store representatives indicated willingness to take action to reduce the threat of trade- related introductions, although chain store employees were more willing to assign responsibility for reducing this threat to the aquarium industry than were inde- pendent store employees. Management efforts for this vector should focus on (a) improving labeling and identification of fish species in stores, (b) expanding the often spotty data on fish physiological tolerances, especially for saltwater species, (c) educating custom- ers and store employees about the risks posed by pet release, and (d) providing better options for responsible disposal of unwanted fish.

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Tackling aquatic invasions: risks and opportunities for the aquarium fish industry

Elevated atmospheric CO2 has been shown to rapidly alter plant physiology and ecosystem productivity, but contemporary evolutionary responses to increased CO2 have yet to be demonstrated in the field. At a Mojave Desert FACE (free-air CO2 enrichment) facility, we tested whether an annual grass weed (Bromus madritensis ssp. rubens) has evolved in response to elevated atmospheric CO2. Within 7 years, field populations exposed to elevated CO2 evolved lower rates of leaf stomatal conductance; a physiological adaptation known to conserve water in other desert or water-limited ecosystems. Evolution of lower conductance was accompanied by reduced plasticity in upregulating conductance when CO2 was more limiting; this reduction in conductance plasticity suggests that genetic assimilation may be ongoing. Reproductive fitness costs associated with this reduction in phenotypic plasticity were demonstrated under ambient levels of CO2. Our findings suggest that contemporary evolution may facilitate this invasive species' spread in this desert ecosystem.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/ele.12274

Contemporary evolution of an invasive grass in response to elevated atmospheric CO2 at a Mojave Desert FACE site

Nicrophorus beetles reproduce by burying small vertebrate carcasses underground in a brood chamber. All stages of the phoretic mite Poecilochirus (Mesostigmata: Parasitidae) associate closely with Nicrophorus beetles by clinging to them for transportation and by reproducing in the beetles’ brood chamber. Presumably, the fitness of phoretic mites is affected by the ability of their beetle host to successfully locate and bury a carcass. Previous studies have shown that thermoregulation during flight, competitive ability, burying speed, and burial depth are all positively correlated with adult beetle body size. This study, conducted in the Rocky Mountains of southwestern Colorado, addresses whether deuteronymph mites discriminate among male Nicrophorus investigator Zetterstedt (Coleoptera: Silphidae) hosts based on beetle body size, a proxy for beetle condition. We found that the number of phoretic mites on wild-caught male N. investigator beetles was positively correlated with beetle body weight an...

Judah D. Grossman

Papers

Evolution of root plasticity responses to variation in soil nutrient distribution and concentration

Preventing horticultural introductions of invasive plants: potential efficacy of voluntary initiatives

Tackling aquatic invasions: risks and opportunities for the aquarium fish industry

Contemporary evolution of an invasive grass in response to elevated atmospheric CO2 at a Mojave Desert FACE site

Phoretic Mite Discrimination Among Male Burying Beetle (Nicrophorus investigator) Hosts