Synthesis of Wheat Storage Proteins Is Not Drastically Affected by High Temperature
MHT treatment (experiment 1) caused variation in the amount of only few individual SP of cv. Arche: two ω-gliadins were increased, whereas one γ-gliadin and one LMW-GS were reduced. No HMW-GS was affected by MHT. However, MHT resulted in a significant decrease in the amount of SP expressed in micrograms of protein per grain, as only 65.7 % and 65.5 % of the total amount of gliadin and glutenin, respectively, were accumulated at maturity, compared to control.
VHT treatment increased (2to 27-folds) the amount of only three α-gliadins in the mature grain of cv. Thésée (Majoul et al. 2003) and HMW-GS and LMW-GS were not modified by the treatment.
HS treatment affected the kinetics of accumulation of 13 SPs. Ten of these proteins still differed significantly at maturity: four α-gliadins, two γ-gliadins, and two LMW-GS were decreased and one β-gliadin and one ω-gliadin were increased compared to control. In an early study, one HMW-GS encoded by Glu-A1 significantly
increased when expressed as a percentage volume (Branlard et al. 2008). Reassessing the analysis of all 2D gel images failed to confirm this higher abundance when expressed in micrograms of protein per grain (Majoul-Haddad et al. 2013), again providing evidence that the accumulation of HMW-GS was not affected by the HS treatments.
In addition to the remarkable stability in abundance exhibited by HMW-GS in response to HT and considering that more than 80 SP spots were detected on 2D gels whatever the cultivar analysed, it should be noted that only few SP were influenced by HT treatments. In all three experiments, the kinetics of individual SP was analysed. During grain filling, the ratio of gliadin to glutenin was different at one (640 °C days after anthesis) out of seven sampling stages for cv. Arche (medium quality wheat) whereas no difference was found for cv. Tamaro (high quality wheat). At maturity, no difference between heat exposed and control grains were detected for the ratio of gliadin to glutenin. Altogether, the amount of individual SP during grain filling demonstrated remarkable stability in response to HT treatments, indicating the existence of regulatory mechanisms, which could differ between cultivars. This underlines the need for a better understanding of the regulatory gene networks involved in SP synthesis when breeding future high quality cultivars.
Glutenin Polymers Are Strongly Impacted by High Temperature
The molecular mass of glutenin polymers was previously shown to be significantly influenced by grain hardness (Lesage et al. 2011). Further investigations (Lesage et al. 2012) of the puroindoline function provided clear evidence that the endoplasmic reticulum (ER) is the site of the unfolded protein response (UPR), a phenomenon first described in animal cells (Schröder and Kaufman 2005). Protein folding is known to be inhibited by several factors including excess proteins in ER, temperature, magnetic field, and electric field. It was thus necessary to investigate the influence of the characteristics of the polymers for cultivars of different genetic origin and grown in different locations. The polymer characteristics (molecular mass Mw2, polydispersity index Mw2/Mn2 and radius of polymers Rw2) of the 240 samples from the multi-location field trial were evaluated using asymmetric flow field flow fractionation (AFFFF; Lemelin et al. 2005). Glutenin alleles and puroindoline alleles, as well as their interactions were shown to significantly influence Mw2, which, unexpectedly, varied from 5 × 106 to 49 × 106 Da (Lesage et al. 2013). In addition, it was shown that the sum of daily mean air temperatures for June and July (i.e. during the grain filling period; SumT-JuJy), differentially affected Mw2 in the three hardness classes (hard, medium and soft). A difference of 110 °C in SumTJuJy was found across the six environments, which was correlated with an increase of Mw2 from 10 × 106 to 20× 106 Da in hard cultivars and from 13 × 106 to 33× 106 Da in soft cultivars. Partial least square regression used to explain Mw2 with several grain characteristics including glutenin and puroindoline alleles and SumT-JuJy, provided clear evidence for the major influence of SumT-JuJy on phenotypic variations in polymer characteristics (Fig. 28.1a). Why does temperature have such a strong effect on the characteristics of glutenin polymers? All the proteomics studies on grain response to HT reported that two to four of the redox enzymes ascorbate dismutase, ascorbate peroxidase, dehydroascorbate reductase, and glutathione reductase were increased by HT. This, together with evidences of UPR, suggests that grains exposed to HT undergo sever oxidative stresses causing higher polymer mass.