Effect of long-term high temperatures on seed maturation and seed coat integrity in Brassica napus

Cultivation of canola at temperatures above the optimum growth temperature of 21°C for prolonged periods, especially during the flowering stage, resulted in several adverse effects, including rapid vegetative growth, reduced viability of female gametophytes, increased seed abortion rate, accelerated embryo development, and a reduction in seed oil composition (Young et al., 2004; Mácová et al., 2022; Secchi et al., 2023). One of the distinctive phenotypes observed during the seed development of certain canola cultivars subjected to prolonged heat stress affecting the seed yield is the occurrence of the pre-harvest sprouting phenotype (PHS) (Mácová et al., 2022). Misregulation of seed dormancy by abscisic acid and dormancy-related genes is thought to be the primary cause of PHS in many cereal crops (Benech-Arnold & Rodríguez, 2018; Tai et al., 2021). This phenotype is associated with seed coat rupture (SCR), observed in seeds during the early stages of maturation. In this study, we employed a multi-methodological approach to investigate the occurrence of SCR phenotype in seeds of Brassica napus cv. Topas. The results demonstrate that SCR occurs in seeds around 20 days after pollination (20DAP) when the plants are cultivated in elevated temperatures over an extended period. The unrestricted embryonic growth exerts pressure on the seed coat, as evidenced by a reduction in the thickness of the seed coat cell layers. This results in an early alteration to the cell wall composition, with an increased proportion of demethylesterified pectin, which is likely to stiffen the seed coat, thereby rendering it more susceptible to rupture. The precise mechanism by which accelerated embryo development influences heat stress-mediated seed development in canola plants has yet to be elucidated. Overall design: To investigate the impact of high temperature on seed coat rupture, we collected seeds of Brassica napus cv. Topas at 14 days after pollination (DAP), 18 DAP, and 20DAP from plants grown in control conditions (TCT) (21oC day / 18oC night, long-day) and high-temperature conditions (THT) (32oC day / 18 oC night, long-day). Plants were moved to THT at the start of bolting, and kept in THT until ripenning. Samples of 20DAP seeds at THT with SCR phenotype were also collected. We performed gene expression profiling analysis using data obtained from RNA-seq for these 3 samples at TCT and 4 samples at THT at 3 time points, in four biological replicates.