Plant Anatomy By B P Pandey.pdf

Plant Anatomy By B P Pandey.pdf

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Suboptimal water and nutrient availability are primary constraints in global agriculture. Root anatomy plays key roles in soil resource acquisition. In this article we summarize evidence that root anatomical phenotypes present opportunities for crop breeding.

Root anatomical phenotypes influence soil resource acquisition by regulating the metabolic cost of soil exploration, exploitation of the rhizosphere, the penetration of hard soil domains, the axial and radial transport of water, and interactions with soil biota including mycorrhizal fungi, pathogens, insects, and the rhizosphere microbiome. For each of these topics we provide examples of anatomical phenotypes which merit attention as selection targets for crop improvement. Several cross-cutting issues are addressed including the importance of phenotypic plasticity, integrated phenotypes, C sequestration, in silico modeling, and novel methods to phenotype root anatomy including image analysis tools.

An array of anatomical phenes have substantial importance for the acquisition of water and nutrients. Substantial phenotypic variation exists in crop germplasm. New tools and methods are making it easier to phenotype root anatomy, determine its genetic control, and understand its utility for plant fitness. Root anatomical phenotypes are underutilized yet attractive breeding targets for the development of the efficient, resilient crops urgently needed in global agriculture.

A better understanding of resource capture by plant roots is important because water and nutrient availability limit plant growth in the majority of terrestrial ecosystems. In natural ecosystems, improved understanding of this topic will expand our knowledge of key factors driving the productivity and function of these systems and will be useful in mitigating the increasingly severe consequences of global climate change and human encroachment. In managed ecosystems, such insight would create opportunities to sustain productivity despite environmental degradation and increasing population pressure. This is most clearly evident in the case of crop production. In rich nations, intensive fertilization and irrigation of crops is costly, damages the environment, depletes limited resources, and is unsustainable (Lynch 2007, 2019). In developing nations, low crop yields caused by drought and low soil fertility are primary constraints to food security, economic development, and political stability (Lynch 2007, 2019). These challenges are intensifying because of population growth, soil degradation, depletion of freshwater resources, and global climate change (Oldeman 1992; St. Clair and Lynch 2010; Mbow et al. 2019). We urgently need better crops and cropping systems that can sustain adequate yields with less demand for fertilizers and irrigation, while sustaining or improving soil fertility (Lynch 2007, 2019). Resource capture by plant roots is closely linked to soil exploration, and therefore carbon sequestration from the atmosphere, which is a promising avenue to mitigate global climate change (Kell 2011; 2012, Lynch and Wojciechowski 2015). Improved understanding of soil resource capture by plant roots is therefore an important component of the grand challenge of the twenty-first century: how to sustain 10B people while reversing environmental degradation.

While improved understanding of soil resource capture has manifold benefits, it would be directly useful in the breeding of more resource-efficient, stress-tolerant crops. The identification of traits improving soil resource capture is needed for the development of ideotypes for specific environments, for the deployment of specific traits in breeding programs (phenotypic selection), and when possible, for the use of marker-assisted or genotypic selection in molecular breeding (Lynch 2019). The large number of root traits affecting soil resource capture, and their genetic and mechanistic complexity, means that it is highly improbable to select optimal phenotypes on the basis of coarse metrics of plant performance such as yield under stress (Lynch 2019). While breeding programs that directly employ root phenotypes for improved water and nutrient capture are rare, they have been successful when attempted (e.g. Burridge et al. 2019).

Anatomy is a primary determinant of the metabolic costs of root construction and maintenance. Some tissues are more metabolically demanding than others. For example, mature xylem vessels and some sclerenchyma cells are dead, in contrast with xylem parenchyma or phloem companion cells which are highly active. Cell walls, cytoplasm, and vacuole have very different construction and maintenance costs. Living cell types have varying proportions of polysaccharides, protein, and nucleic acids. Some cells like cortical parenchyma are relatively expendable while others like phloem cells are critical for root function. Anatomical features like aerenchyma regulate oxygen availability and thereby respiration. By determining the proportion of living and dead cells, highly active vs. less active cells, cell composition and oxygen availability, root anatomy is a key determinant of the metabolic costs of soil exploration.

Root cortical aerenchyma (RCA) is caused by programmed cell death of cortical parenchyma, resulting in air-filled lacunae (Fig. 1). Although RCA has been primarily researched for its role in oxygenation of root tissue under hypoxia (Jackson et al. 1985), constitutive RCA formation is common in grasses. In addition to hypoxia, RCA is induced by a range of abiotic stresses (Jackson et al. 1985), including drought (Zhu et al. 2010a; Chimungu et al. 2015b), heat (Hu et al. 2014), and suboptimal availability of N (Saengwilai et al. 2014), P (Fan et al. 2007; Galindo-Castañeda et al. 2018), and S (Bouranis et al. 2003). RCA formation under edaphic stress could be adaptive by reducing the metabolic costs of soil exploration by converting living cortical parenchyma into air space, thereby reducing root nutrient content and respiration. RCA formation also reduces the radial transport of water and nutrients to the stele (Fan et al. 2007; Hu et al. 2014; Bo et al. 2014). Reduced radial transport of water may not be disadvantageous in dry soils since the majority of water uptake occurs in lateral roots (Schneider and Lynch 2018) and younger root tissue before RCA develops. Reduced radial transport offers potential benefits of parsimonious water capture under drought, which would conserve soil water for future use, and enforce more efficient use of water via reduced leaf growth and stomatal aperture. Likewise, under nutrient stress, reduced radial nutrient transport of mature root axes with RCA may not be detrimental since most nutrient capture occurs by younger root tissues and lateral roots. In silico analysis found substantial fitness benefits for RCA in maize growing in soils with suboptimal availability of N, P and K, via reduced root respiration as well as nutrient reallocation from cortical tissue (Postma and Lynch 2011a). The benefits of RCA for P capture were greater in maize than bean (Postma and Lynch 2011b), showing the importance of a persistent root cortex in monocots, as opposed to dicots, which lose their root cortex through secondary growth (Strock et al. 2018). In silico results were confirmed in empirical studies in the field and in greenhouse mesocosms under suboptimal N and P availability, where RCA formation among contrasting maize phenotypes was associated with reduced root respiration, greater root growth, greater N and P capture, better shoot nutrient status under nutrient stress, and hence greater photosynthesis, growth, and yield (Saengwilai et al. 2014; Galindo-Castañeda et al. 2018). Similar benefits were observed for RCA formation under drought stress, where greater RCA formation among contrasting maize lines was associated with reduced root respiration, greater rooting depth, better shoot water status, leaf photosynthesis, plant growth, and yield (Fig. 2; Zhu et al. 2010a). These results were supported by analysis of maize yields under natural drought environments in Malawi, which found greater yields in high RCA landraces than in low RCA landraces (Chimungu et al. 2015b). A recent study associated greater RCA with drought adaptation in specific phenotypic clusters of diverse maize inbreds (Klein et al. 2020).

Root cortical senescence (RCS) is similar to RCA in being formed via programmed cell death but differs in having more restricted taxonomic distribution, having been reported in the Poaceae, including principal cereal crops such as wheat, barley, rye, and oat, and in causing entire loss of the cortex instead of the formation of discrete lacunae as in RCA (Schneider and Lynch 2018) (Fig. 3). As with RCA, loss of cortical parenchyma by RCS reduces root respiration and nutrient content as well as radial water and nutrient transport (Schneider et al. 2017b). In silico analysis showed that these effects would be beneficial for barley plants growing with suboptimal availability of N, P, and K (Schneider et al. 2017a). As with RCA, reduced radial water transport in older axes caused by RCS may not be detrimental under drought because such root segments are not normally active in water transport, although further research is needed (Schneider and Lynch 2018).

Schematic of RCS and soil resource capture. In edaphic stress, plants with RCS have greater root length, reduced root respiration, and reduced radial water and nutrient uptake. Greater root growth in plants with RCS is driven by savings in metabolic costs of root tissue. Reduced radial water and nutrient transport of axial root tissue after RCS has small effects on total plant nutrient uptake, as lateral roots do not form RCS and perform the majority of root nutrient and water uptake. From Schneider and Lynch (2018) 2b1af7f3a8