Last year was Spain's hottest and driest on record. Elevated growth temperatures associated with climate change impact growth and flowering time and affect crop productivity by increasing yield losses. Oilseed rape (Brassica napus) is a premium oilseed crop also threatened by growing ambient temperatures. Breeding of this crop gave rise to winter and spring varieties, implying different optimal flowering induction and temperature ranges.

To shed light on the oilseed rape vegetative to reproductive phase transition under warm temperatures, in this work, we took advantage of the diverse flowering responses of oilseed rape spring cultivated varieties at different temperature regimes. We have characterized at the molecular level how flowering induction proceeds in this crop species and performed transcriptomic analyses of oilseed rape cultivars representing different responses at optimal and high ambient temperatures. Besides, we have deepened on FT homologs gene expression profiles in different temperature conditions in the same varieties and identified BnaFTA2 as a key candidate in temperature-dependent flowering control.

The current model for flowering response to warm temperature in Arabidopsis hypothesizes that high-temperature induction of FT expression is caused by changes in chromatin accessibility after the eviction of H2A.Z-containing nucleosomes, although the exact mechanism is not well understood. A working mechanism behind H2A.Z control of temperature-dependent flowering repression in Brassica rapa proposes that BraFT is repressed through H2A.Z accumulation under noninductive high ambient temperature. To better understand the role of H2A.Z in this regulation in oilseed rape, we have assessed the presence of this histone variant in the BnaFTA2 locus by ChIP approaches in the different varieties assayed, and in B. napus ARP6 RNAi silenced plants (arp6i), impaired in a critical component of the SWR1-C involved in H2A.Z deposition. Remarkably, Wesway, the variety displaying the earliest flowering time, showed consistent differences in H2A.Z deposition in BnaFTA2 between 21ºC and 28ºC, in line with previous results in Arabidopsis and the observed flowering phenotype. However, we found no clear correlation between H2A.Z presence and BnaFTA2 repression in high-temperature flowering in late accessions, including the commonly used RV31 variety. Furthermore, ChIP analysis in plants with low H2A.Z levels corroborates that BnaFTA2 expression is only slightly affected by the presence of this non-canonical histone. With all these data, we propose that high ambient temperature impacts oilseed rape flowering time through both H2A.Z-dependent and independent mechanisms. Interestingly, we reveal a positive correlation between H3K4me3, an active mark closely coupled with H2A.Z eviction at high ambient temperature, and gene expression levels of BnaFTA2, and found that this histone modification is significantly more abundant at 21ºC than 28ºC in this locus.

Our analyses will help to harness crop response mechanisms for improving adaptation under the current climatic scenario.