Root Development and Abiotic Stress Adaptation - Abiotic Stress: Plant Responses and Applications in Agriculture - page 137

During the Devonian period (415-360 million years ago) apparition and radiation of embryo‐
phytes with roots caused large changes to the global level. The early land plants with rhizoid-
like filaments that penetrated the top few centimeters of soil, were replaced by plants with deep
RS with complex structures. The apparition of those organs that actively penetrate the rock with
the capacity of uptake and transport mineral nutrients permitted the development of structur‐
ally complex above-ground structures to photosynthesis, which increased the amounts of carbon
fixed on the continent. The increase of primary production of early land plants changed the global
carbon cycle and generates new complex soils which increased the border of land inhabited by
plants. On one hand, the high rates plant production in this period allowed deposition of carbon
on continental area from plant-drive organic matter, organic molecules secreted into the soil; on
the other hand, the increase weathering rate of rocks by root penetration and secretion of organic
compounds permitted the mining of rock-derived inorganic nutrients. Those changes in habitat
turned up to be a part of a stimuli cycle in plant evolution, as themselves allowed the primary
production to rise, which produced changes, and so on. The apparition of RS during Devonian
allows that most of land surface was covered by plants, since Carboniferous forest (300 million
of years ago), through late Cretaceous where basal angiosperms appeared (100-65 million of
years ago) until days [1-3].
2.2. Classification and architecture
The RS consists of all roots that a plant has. It can be classified according to branch structure,
root activity or development. The classification based on development is the more typical and
useful to analyze the RS growth. This approach ontogenetically classified roots into three
categories: primary root (PR), lateral root (LR) and adventitious root (AR; Figure 1 A). This
classification reflects the differences between monocotyledonous and dicotyledonous RS.
During germination PR is the first root to emerge from seed in both monocotyledonous and
dicotyledonous, and is derived from embryonic root. In most of dicotyledonous LR are formed
post-embryonically from pericycle cells (Figure 1 B-C) generating a branching system called
primary root system. Depending on the length of LR relative to the primary axis (PR), the
morphology of the RS will vary between tap rooted (Figure 1 A) and diffuse [3, 6, 9, 10]. Many
monocotyledonous form PR and LR in a manner alike to dicotyledonous, in addition form
nodal roots (AR) to generate a 'fibrous' adventitious roots system [6, 10, 11]. The morphology
of the RS itself is very consistent, depends on the species, however, the spatial configuration
of the RS (number, position and growth position of PR, LR and AR) called root system
architecture (RSA) is highly variable, even among genetically identical plants. RSA is gener‐
ated during post-embyonic root development and is guided by a plastic genetic program which
is modulated by environmental cues [4, 9].
3. Root system development
Root development can be divided in two main stages: a) embryonic development (ED) and b)
post-embryonic development (PED). During the ED, through a suite of highly regulated and
reproducible stages, the fertilized egg cell rises into an embryo. In the embryo, the primary