Identification of Indicator Proteins Associated with Flooding Injury in Soybean Seedlings Using Label-free Quantitative Proteomics

Flooding injury is one of the abiotic
constraints on soybean growth. An experimental system established
for evaluating flooding injury in soybean seedlings indicated that
the degree of injury is dependent on seedling density in floodwater.
Dissolved oxygen levels in the floodwater were decreased by the seedlings
and correlated with the degree of injury. To understand the molecular
mechanism responsible for the injury, proteomic alterations in soybean
seedlings that correlated with severity of stress were analyzed using
label-free quantitative proteomics. The analysis showed that the abundance
of proteins involved in cell wall modification, such as polygalacturonase
inhibitor-like and expansin-like B1-like proteins, which may be associated
with the defense system, increased dependence on stress at both the
protein and mRNA levels in all organs during flooding. The manner
of alteration in abundance of these proteins was distinct from those
of other responsive proteins. Furthermore, proteins also showing specific
changes in abundance in the root tip included protein phosphatase
2A subunit-like proteins, which are possibly involved in flooding-induced
root tip cell death. Additionally, decreases in abundance of cell
wall synthesis-related proteins, such as cinnamyl-alcohol dehydrogenase
and cellulose synthase-interactive protein-like proteins, were identified
in hypocotyls of seedlings grown for 3 days after flooding, and these
proteins may be associated with suppression of growth after flooding.
These flooding injury-associated proteins can be defined as indicator
proteins for severity of flooding stress in soybean.