In Brachypodium a complex signaling is actuated to protect cells from proteotoxic stress and facilitate seed filling

Abstract

A conserved UPR machinery is required for Brachypodium ER stress resistance and grain filling. Human and livestock diets depend on the accumulation of cereal storage proteins and carbohydrates, including mixed-linkage glucan (MLG), in the endosperm during seed development. Storage proteins and proteins responsible for the production of carbohydrates are synthesized in the endoplasmic reticulum (ER). Unfavorable conditions during growth that hamper the ER biosynthetic capacity, such as heat, can cause a potentially lethal condition known as ER stress, which activates the unfolded protein response (UPR), a signaling response designed to mitigate ER stress. The UPR relies primarily on a conserved ER-associated kinase and ribonuclease, IRE1, which splices the mRNA of a transcription factor (TF), such as bZIP60 in plants, to produce an active TF that controls the expression of ER resident chaperones. Here, we investigated activation of the UPR in Brachypodium, as a model to study the UPR in seeds of a monocotyledon species, as well as the consequences of heat stress on MLG deposition in seeds. We identified a Brachypodium bZIP60 orthologue and determined a positive correlation between bZIP60 splicing and ER stress induced by chemicals and heat. Each stress condition led to transcriptional modulation of several BiP genes, supporting the existence of condition-specific BiP regulation. Finally, we found that the UPR is elevated at the early stage of seed development and that MLG production is negatively affected by heat stress via modulation of MLG synthase accumulation. We propose that successful accomplishment of seed filling is strongly correlated with the ability of the plant to sustain ER stress via the UPR.

In the grass species Brachypodium distachyon, the production of mixed‐linkage (1,3;1,4)‐β‐glucan (MLG) occurs in the Golgi apparatus

“…Leaves from 8‐week‐old plants were ground in liquid nitrogen using a mortar and pestle. Protein preparation and detection of protein were followed as described previously (Kim et al ., ). Anti‐GFP serum (Abcam, http://www.abcam.com) was used at a 1:3000 dilution.…”

StresSeed: The Unfolded Protein Response During Seed Development

“…Thus, these studies indicated that, in general, the synthesis of storage proteins during normal seed development may challenge the protein folding capacity of the ER. Direct correlation with UPR signaling was indicated by the increase in both bZIP60 expression and unconventional splicing when synthesis of storage proteins is active in the endosperm of maize ( Gayral et al, 2017 ) and of Brachypodium distachyon , a grass evolutionarily close to wheat and barley ( Kim et al, 2017 ).…”

“…Increased temperatures beyond physiological levels is perhaps the most serious abiotic insult encountered by crops. In Brachypodium distachyon , measurements in developing endosperm showed increases of one BiP and one PDI transcript and both spliced and unspliced bZIP60 at early developmental stages, when storage protein synthesis is particularly high, compared to other plant tissues ( Kim et al, 2017 ). Mild heat stress (30°C) did not further increase BiP and PDI transcripts that are induced during seed development, and actually decreased those of the two chaperones ( Kim et al, 2017 ).…”

“…In Brachypodium distachyon , measurements in developing endosperm showed increases of one BiP and one PDI transcript and both spliced and unspliced bZIP60 at early developmental stages, when storage protein synthesis is particularly high, compared to other plant tissues ( Kim et al, 2017 ). Mild heat stress (30°C) did not further increase BiP and PDI transcripts that are induced during seed development, and actually decreased those of the two chaperones ( Kim et al, 2017 ). The authors concluded that early endosperm development specifically activates UPR but this is actually inhibited under mild heat stress, possibly because temperature increase caused a general reduction of grain filling.…”

IRE1-mediated cytoplasmic splicing and regulated IRE1-dependent decay of mRNA in the liverwort <i>Marchantia polymorpha</i>

“…One arm consists of the most conserved ER stress sensor IRE1, which mediates cytoplasmic splicing of bZIP60 mRNA and RIDD (Deng et al 2011;Mishiba et al 2013;Nagashima et al 2011). Cytoplasmic splicing in plants was first reported in Arabidopsis bZIP60, and was experimentally verified also in other flowering plants such as rice, maize, and Brachypodium (Hayashi et al 2012;Kim et al 2017;Li et al 2012). bZIP60 mRNA encodes a bZIP domain followed by a transmembrane domain, which anchors bZIP60 proteins on the ER membrane.…”