symptoms of nutrient deficiencies and toxicity are visible on above ground parts,
namely the leaves and overall plant architecture. Chlorosis, necrosis, stunting, and
purple reddish coloring are some of the key diagnostic symptoms (McCauley et al.
2009). However, even before the symptoms are visible on above ground parts,
altered growth and developmental patterns in roots are manifested. This is because
roots are the primary organs that are involved in initial sensing, assimilation, and
uptake. The present chapter, therefore, focuses on few aspects of mineral nutrients,
including phenotypic indicators, and molecular regulators of mineral nutrient uptake
and assimilation with special reference on roots. In order to retain focus and because
of paucity of space, the present chapter does not cover many important aspects on
mineral nutrition, most notably role of phytohormones in uptake, transport, and
assimilation, and in shaping root system architecture (RSA); and role of root and soil
microbiome except in a few passing references and mentions.
5.2
Effect of Nutrient Stress on Root System Architecture
Plant roots have been classified based on several criteria. A developmental origin
based nomenclature groups them as primary, secondary, and tertiary; whereas a
classification based on morphology and architecture groups them as taproot, fibrous
root, lateral root, and shoot-borne (adventitious) roots (Zobel and Waisel 2010).
Taproot and fibrous roots are often encountered in dicots and monocots, respectively. The 3-D spatial and temporal arrangement of plant root axes present underground is referred to as root system architecture (RSA; Lynch 1995). This
architecture is influenced by complex interaction among myriad host and non-host
factors such as environment including geotropism, light, water, soil temperature,
moisture, mineral nutrients; biotic factors such as microbes, soil fungi, roots, and
root exudates of other plants; endogenous genetic and molecular regulators, and
stage of plant growth and development (Morris et al. 2017; Van Gelderen et al.
2018). Variations in RSA and morphology are commonly encountered both at
between- and within-species level exhibiting developmental and phenotypic plasticity; developmental and phenotypic plasticity are thus considered an integral part of
RSA. (Malamy 2005; Phung et al. 2016; Sanchez et al. 2018; Van Gelderen et al.
2018). As with all other phenotypes, RSA is influenced by multiple overlapping
factors and it is extremely difficult to dissect out the effect of a specific factor
(Malamy 2005; Van Gelderen et al. 2018). Among all factors, RSA is most dramatically influenced by mineral nutrition and is a definite indicator of stress (Malamy
2005; Gruber et al. 2013).
5.2.1 Altered RSA Under Mineral Nutrient Stress in Various Plants
As mentioned previously, nutrient deficiency is caused when the available mineral
nutrients fall below the sufficiency range which is highly variable across plants. For
instance, for the three major nutrients—N, P, and K, sufficiency ranges are 3.00%,
5 Plant Roots and Mineral Nutrition: An Overview of Molecular Basis of Uptake and. . . 139
namely the leaves and overall plant architecture. Chlorosis, necrosis, stunting, and
purple reddish coloring are some of the key diagnostic symptoms (McCauley et al.
2009). However, even before the symptoms are visible on above ground parts,
altered growth and developmental patterns in roots are manifested. This is because
roots are the primary organs that are involved in initial sensing, assimilation, and
uptake. The present chapter, therefore, focuses on few aspects of mineral nutrients,
including phenotypic indicators, and molecular regulators of mineral nutrient uptake
and assimilation with special reference on roots. In order to retain focus and because
of paucity of space, the present chapter does not cover many important aspects on
mineral nutrition, most notably role of phytohormones in uptake, transport, and
assimilation, and in shaping root system architecture (RSA); and role of root and soil
microbiome except in a few passing references and mentions.
5.2
Effect of Nutrient Stress on Root System Architecture
Plant roots have been classified based on several criteria. A developmental origin
based nomenclature groups them as primary, secondary, and tertiary; whereas a
classification based on morphology and architecture groups them as taproot, fibrous
root, lateral root, and shoot-borne (adventitious) roots (Zobel and Waisel 2010).
Taproot and fibrous roots are often encountered in dicots and monocots, respectively. The 3-D spatial and temporal arrangement of plant root axes present underground is referred to as root system architecture (RSA; Lynch 1995). This
architecture is influenced by complex interaction among myriad host and non-host
factors such as environment including geotropism, light, water, soil temperature,
moisture, mineral nutrients; biotic factors such as microbes, soil fungi, roots, and
root exudates of other plants; endogenous genetic and molecular regulators, and
stage of plant growth and development (Morris et al. 2017; Van Gelderen et al.
2018). Variations in RSA and morphology are commonly encountered both at
between- and within-species level exhibiting developmental and phenotypic plasticity; developmental and phenotypic plasticity are thus considered an integral part of
RSA. (Malamy 2005; Phung et al. 2016; Sanchez et al. 2018; Van Gelderen et al.
2018). As with all other phenotypes, RSA is influenced by multiple overlapping
factors and it is extremely difficult to dissect out the effect of a specific factor
(Malamy 2005; Van Gelderen et al. 2018). Among all factors, RSA is most dramatically influenced by mineral nutrition and is a definite indicator of stress (Malamy
2005; Gruber et al. 2013).
5.2.1 Altered RSA Under Mineral Nutrient Stress in Various Plants
As mentioned previously, nutrient deficiency is caused when the available mineral
nutrients fall below the sufficiency range which is highly variable across plants. For
instance, for the three major nutrients—N, P, and K, sufficiency ranges are 3.00%,
5 Plant Roots and Mineral Nutrition: An Overview of Molecular Basis of Uptake and. . . 139