The use of arbuscular mycorrhizal fungi to improve root function and nutrient-use efficiency

Arbuscular mycorrhizal fungi (AMF) form endosymbiosis with over 70 % of land plants, including most crops including cereals. These symbioses facilitate resource exchange between partners and can significantly increase plant nutrient uptake and growth, among other benefits. AMF ubiquity in agricultural soils, in addition to the many roles they are known to play in soil health, demands we consider them when discussing crop function. We discuss how AMF are capable of increasing crop acquisition of macro- and micronutrients. We examine further impacts that AMF have on root system architecture, and how this relates to nutrient acquisition. We highlight reasons why potential benefits of the symbiosis are often not realised and how this influences current perspectives on the utility of AMF. We also discuss aspects of modern agronomy practice which are deleterious to mycorrhizal functioning. Strategies are suggested by which mycorrhizas might be exploited in future highlighting future research priorities.

Understanding and exploiting the genetics of plant root traits

This chapter illustrates how genomics and other -omics approaches coupled with new-generation sequencing (NGS) platforms have been deployed to dissect the genetic make-up of RSA traits and better understand their functions, particularly under environmentally constrained conditions that commonly occur in most farmed soils. The major emphasis is devoted to studies during the past two decades in crops and only occasional reference is provided to the vast literature from RSA studies conducted in Arabidopsis and other model plants. The chapter also provides examples on how, in some cases, this knowledge is already benefiting farmers and how it can help in reducing the environmental impact of agriculture worldwide.

Advances in understanding plant root response to nematode attack

Plant parasitic nematodes are damaging pests on all crops grown across the world. They exploit plants using a range of strategies, ranging from simple browsing ectoparasitism to highly complex biotrophic endoparasites. Some nematodes induce the formation of complex feeding structures in the roots of their hosts that require extensive reprogramming of host gene expression. These changes include changes in fundamentally important plant processes, including the cell cycle. Natural resistance can be used to control plant nematodes, and great progress has been made in mapping and identifying resistance genes against nematodes. Recent work has shown that the dependence of nematodes on a feeding structure has allowed plants to evolve new mechanisms of resistance that target this structure with a toxic response.

Advances in understanding plant root responses to root-feeding insects

This chapter presents an overview of the interactions between plant roots and root-feeding insect herbivores, focussing on changes in growth and physiology and crucially how roots are defended against insect attack. Several reviews have covered the ecology and management of insect root herbivores, together with their interactions with the abiotic and biotic soil environment. Therefore, the chapter focuses particularly on advances in our understanding of how plant mutualistic fungi may affect root-herbivores. This is an emerging area of research, with many attendant knowledge gaps, but we argue that this is an important component of how plants resist attack by belowground insect herbivores.

Advances in understanding plant root hairs in relation to nutrient acquisition and crop root function

Root hairs are found on most terrestrial flowering plant species. They form from epidermal cells at a predetermined distance behind the growing root tip in three main patterns. Their presence, pattern, length, density and function are genetically controlled and numerous genes are expressed solely in root hairs. Their growth and proliferation are attenuated by the environment and root hairs growing in soil are generally shorter and less dense than those in laboratory studies. Root hairs have a number of functions including anchorage, root soil contact and bracing to enable roots to penetrate hard soils. However, their primary function is acquisition of nutrients and water, in particular phosphate. They are the site of transporters, exudation of active compounds and infection point of symbiotic microbial interactions. They have a profound effect on rhizosphere characteristics and are a potentially useful target for breeding crops for future agricultural sustainability.

Advances in root architectural modeling

Root architectural (RSA) models have become important tools in root research and plant phenotyping for studying root traits, processes, and interactions with the environment. The models have been used to simulate how various root traits and processes influence water and nutrient uptake. At a more technical level, they have been used to develop phenotyping technology, particularly for testing algorithms for segmenting roots. To compute these quantitative estimates regarding plant nutrition and root functioning, much development occurred in the last decade increasing the complexity of the models. This chapter describes first the application of the models to questions in plant biology, breeding, and agronomy, and second the development of the models. It concludes with a small outlook suggesting that models need benchmarking and validation and that new developments are likely to include better descriptions of root plasticity responses and focus on biological interactions among (soil) organisms, including mycorrhizal fungi.

Delivering improved phosphorus acquisition by root systems in pasture and arable crops

Improving low efficiency of phosphorus (P) use in agriculture is an imperative because P is one of the key nutrients underpinning sustainable intensification of food production and the rock-phosphate reserves, from which P fertilisers are made, are finite. This paper describes key soil, root and microbial processes that influence P acquisition with a focus on factors that can be managed to ensure optimal use of fertiliser, and development of root systems for improved P acquisition. A case study describes grasslands in southern Australia where the P-balance efficiency of production is very low, mainly because soils are P deficient and moderately to highly P-sorbing. Use of soluble P fertiliser, P-banding and soil testing to guide soil P management ensures effective use of P fertiliser. Progress towards improved P efficiency using pasture legumes with high P-acquisition efficiency is outlined. Development of a ‘whole-of-system’ understanding for effective P acquisition by roots is highlighted.

Advances in understanding plant root uptake of phosphorus

At a global scale, phosphorus (P) deficiency comprises a large area of cropland, while P has also been used in excess of crop requirements in many other regions. Improved crop P-acquisition efficiency would allow lower target critical soil P values and provide savings in P-fertiliser use. At the same time, it would reduce P lost through erosion, leaching and/or soil sorption. This chapter summarises the progress in research on root traits associated with P acquisition, including root morphology, architecture, biochemistry, colonisation by arbuscular mycorrhizal fungi, and fine root endophytes, and the trade-offs among all these traits. Farming-management practices to improve P acquisition under current intensive agricultural systems are also discussed. The chapter summarises breeding progress in improving P-acquisition efficiency. In the face of soil P deficiency or legacy P globally, the chapter suggests future directions to improve P acquisition in five key areas.

Advances in the understanding of nitrogen (N) uptake by plant roots

Efficient use of nitrogen (N) by plants and particularly crops, is of global importance. In agriculture, high crop yields and protein content depend upon extensive N-inputs, however fertilizer N is costly to the farmer, and overuse can be damaging to the environment. A critical component of optimised usage is efficient capture by crop root systems. This chapter focusses on principal mechanisms of uptake and factors influencing efficiency. Genetic variation in root architecture and in an array of transporters known to be involved in nitrogen capture is detailed. The impacts of abiotic stress factors such as soil structure are described. Finally prospects and opportunities for crop improvement are discussed.

Understanding plant-root interactions with rhizobacteria to improve biological nitrogen fixation in crops

Plant roots have evolved with the presence of rhizobacteria that can colonise the surface or interior of the plant. Some of these rhizobacteria are actively recruited by the plant and carry out particular functions, in particular in nutrient acquisition. Nitrogen-fixing bacteria form associations with many plant species, either as external associations or as symbiotic endophytes. The symbiosis between legumes and nitrogen-fixing rhizobia has been studied in most detail and is the most important contributor to nitrogen fixation in agriculture. This chapter highlights our current understanding of the molecular determinants of legume nodulation as well as challenges for improvements of biological nitrogen fixation in legumes and non-legumes. There is a need for connecting out knowledge of the molecular regulation of nodulation with field-based studies that take into account the interaction of nodulation with biotic and abiotic constraints. In addition, current approaches for engineering new symbioses are discussed.