The sunflower HaHB4 gene confers enhanced tolerance to heat stress at the pre-anthesis stage in three different wheat cultivars.

The sunflower gene HaHB4 was previously described as conferring drought tolerance to Cadenza wheat plants, as assessed in 37 field trials. HaHB4-wheat is the first transgenic wheat trait approved for global markets. In this work, we show that transgenic plants also exhibit heat tolerance in the pre-anthesis stage. During 2023 and 2024, the experiments were conducted in open-air conditions, using the modern cultivars Algarrobo and Feroz by introgression of the original Cadenza HaHB4. The newly obtained transgenic wheat plants, particularly Algarrobo, as well as the original Cadenza HaHB4, maintained stabilized grain yields under high-temperature stress applied at the pre-anthesis stage, whereas wild-type genotypes suffered severe productivity losses. This yield advantage was primarily driven by a higher grain number rather than grain weight. Interestingly, heat stress applied after anthesis showed no significant difference impact in yield, identifying pre-anthesis as the vital window for HaHB4 efficacy. HaHB4 plants exhibited a higher harvest index and a greater number of fertile florets and spikelets per spike, suggesting that the transgene protects the reproductive potential during early development. At the molecular level, HaHB4 wheat appears constitutively prepared for stress. Transcript levels analysis showed the differential regulation of heat shock proteins and stress-responsive transcription factors even before the onset of heat. Furthermore, the stability of transgenic plants was notably higher, showing significantly lower coefficients of variation across yield components compared to wild-type counterparts. This increased stability suggests that HaHB4 acts as a buffer against environmental variability, ensuring more predictable harvests under fluctuating thermal conditions.