water can be taken up by roots through an osmotic gradient (Steudle 2000) and has
high resistance because water goes through cells, traveling in the membrane continuum (endoplasmic reticulum and plasmodesmata) using membrane transporters
(aquaporins—AQPs) (Steudle and Peterson 1998; Steudle 2000) highlighting a
possible role of AQPs to alter the hydraulic properties of the roots (Tyerman et al.
2002; Maurel et al. 2009). AQPs are integral membrane proteins that increase
membrane permeability to water and other small molecules (Kaldenhoff and Fischer
2006). Root water uptake in soybean can be enhanced or reduced by the overexpression or loss of one or more PIP genes, respectively (Javot 2003; Zhou et al.
2014b). In addition to water transport in roots, a variety of AQPs are expressed in the
coats of developing seeds (Schuurmans et al. 2003). Nutrient and water transport
across plasma membranes (PMs) in seed coats is highly coordinated by regulatory
mechanisms and integrates the activities of many nutrient transporters and
facilitators. Thus, it is expected that plasma membrane intrinsic proteins (PIPs)
that are specifically expressed in native PMs of seed coats are important for seed
filling (Zhou et al. 2007). A soybean GmPIP2 subfamily member, GmPIP2;9, was
found predominately expressed in roots and developing seeds (Lu et al. 2018). The
soybean genome contains a total of 22 PIP genes (Sakurai et al. 2005). A recent
study showed that altered plant transpiration led to rapid changes in root expression
of soybean PIP1;6 (GmPIP1;6) that correlated with changes in root hydraulic
conductance (Vandeleur et al. 2014). Notwithstanding evidence for importance of
root traits in drought tolerance (Garay and Wilhelm 1982; Chen et al. 2007b;
Manavalan et al. 2010; Sinclair et al. 2010; Fenta et al. 2014; Fried et al. 2018),
little work has been done in breeding for drought-tolerant soybean varieties.
4.3
Breeding Approaches for Drought Tolerance in Soybean
Drought-tolerant traits, introduced through breeding approaches, resulted in soybean
transpiration rates that plateau at VPD levels above 1.4–2.1 kPa (Mourtzinis et al
2019). Developing high productivity genotypes under water-limited scenario by
introgressing traits explaining plant water relations and hydraulic processes into a
single genetic background either through breeding and/or genomic approaches is a
way forward in realizing genetic combinations supported by plant genetic resource
activities identifying candidate drought-tolerant parental lines and genomic
resources (Satpute et al. 2020). Advance phenotyping-based breeding approaches
are pre-requisite and being adopted systematically by developing early generation
biparental, backcross, or multi-parent intercross populations (Shivakumar et al.
2018) using identified candidate drought-tolerant exotic and/or indigenous parental
lines and wider-adaptability high yielding variety(-ies). The populations are
advanced through F 2 generation by mass selection where bulks are chemically
desiccated with potassium iodide 0.2% (Blum et al. 1991; Bhatia et al. 2014;
Satpute et al. 2019b) followed by selections made divergently for seeds size by
mechanical sieving. Mass selection for large seeds under chemical desiccation
significantly improved seed weight and grain yield under chemical desiccation
4 Breeding and Molecular Approaches for Evolving Drought-Tolerant Soybeans
89
high resistance because water goes through cells, traveling in the membrane continuum (endoplasmic reticulum and plasmodesmata) using membrane transporters
(aquaporins—AQPs) (Steudle and Peterson 1998; Steudle 2000) highlighting a
possible role of AQPs to alter the hydraulic properties of the roots (Tyerman et al.
2002; Maurel et al. 2009). AQPs are integral membrane proteins that increase
membrane permeability to water and other small molecules (Kaldenhoff and Fischer
2006). Root water uptake in soybean can be enhanced or reduced by the overexpression or loss of one or more PIP genes, respectively (Javot 2003; Zhou et al.
2014b). In addition to water transport in roots, a variety of AQPs are expressed in the
coats of developing seeds (Schuurmans et al. 2003). Nutrient and water transport
across plasma membranes (PMs) in seed coats is highly coordinated by regulatory
mechanisms and integrates the activities of many nutrient transporters and
facilitators. Thus, it is expected that plasma membrane intrinsic proteins (PIPs)
that are specifically expressed in native PMs of seed coats are important for seed
filling (Zhou et al. 2007). A soybean GmPIP2 subfamily member, GmPIP2;9, was
found predominately expressed in roots and developing seeds (Lu et al. 2018). The
soybean genome contains a total of 22 PIP genes (Sakurai et al. 2005). A recent
study showed that altered plant transpiration led to rapid changes in root expression
of soybean PIP1;6 (GmPIP1;6) that correlated with changes in root hydraulic
conductance (Vandeleur et al. 2014). Notwithstanding evidence for importance of
root traits in drought tolerance (Garay and Wilhelm 1982; Chen et al. 2007b;
Manavalan et al. 2010; Sinclair et al. 2010; Fenta et al. 2014; Fried et al. 2018),
little work has been done in breeding for drought-tolerant soybean varieties.
4.3
Breeding Approaches for Drought Tolerance in Soybean
Drought-tolerant traits, introduced through breeding approaches, resulted in soybean
transpiration rates that plateau at VPD levels above 1.4–2.1 kPa (Mourtzinis et al
2019). Developing high productivity genotypes under water-limited scenario by
introgressing traits explaining plant water relations and hydraulic processes into a
single genetic background either through breeding and/or genomic approaches is a
way forward in realizing genetic combinations supported by plant genetic resource
activities identifying candidate drought-tolerant parental lines and genomic
resources (Satpute et al. 2020). Advance phenotyping-based breeding approaches
are pre-requisite and being adopted systematically by developing early generation
biparental, backcross, or multi-parent intercross populations (Shivakumar et al.
2018) using identified candidate drought-tolerant exotic and/or indigenous parental
lines and wider-adaptability high yielding variety(-ies). The populations are
advanced through F 2 generation by mass selection where bulks are chemically
desiccated with potassium iodide 0.2% (Blum et al. 1991; Bhatia et al. 2014;
Satpute et al. 2019b) followed by selections made divergently for seeds size by
mechanical sieving. Mass selection for large seeds under chemical desiccation
significantly improved seed weight and grain yield under chemical desiccation
4 Breeding and Molecular Approaches for Evolving Drought-Tolerant Soybeans
89