Intrinsically Disordered Proteins as Important Players during Desiccation Stress of Soybean Radicles

Intrinsically disordered
proteins (IDPs) play a variety of important
physiological roles in all living organisms. However, there is no
comprehensive analysis of the abundance of IDPs associated with environmental
stress in plants. Here, we show that a set of heat-stable proteins
(i.e., proteins that do not denature after boiling at 100 °C
for 10 min) was present in R0mm and R15mm radicles
(i.e., before radicle emergence and 15 mm long radicles) of soybean
(Glycine max) seeds. This set of 795 iTRAQ-quantified
heat-stable proteins contained a high proportion of wholly or highly
disordered proteins (15%), which was significantly higher than that
estimated for the whole soybean proteome containing 55,787 proteins
(9%). The heat-stable proteome of soybean radicles that contain many
IDPs could protect lactate dehydrogenase (LDH) during freeze–thaw
cycles. Comparison of the 795 heat-stable proteins in the R0mm and R15mm soybean radicles revealed that many of these
proteins changed abundance during seedling growth with 170 and 89
proteins being more abundant in R0mm and R15mm, respectively. KEGG analysis identified 18 proteins from the cysteine
and methionine metabolism pathways and nine proteins from the phenylpropanoid
biosynthesis pathway. As an important type of IDP related to stress,
30 late embryogenesis abundant proteins were also found. Ten selected
proteins with high levels of predicted intrinsic disorder were able
to efficiently protect LDH from the freeze–thaw-induced inactivation,
but the protective ability was not correlated with the disorder content
of these proteins. These observations suggest that protection of the
enzymes and other proteins in a stressed cell can be one of the biological
functions of plant IDPs.