Brassica napus (oilseed rape) is sensitive to boron (B) deficiency and exhibits young leaf curling in response to low-B stress at the seedling stage, which leads to reduced photosynthesis and plant growth. So far, no gene has been identified to be involved in B deficiency induced leaf curling. Our previous results showed the transcription factor BnaA1.WRKY53 might be involved in B-deficiency tolerance. However, altered BnaA1.WRKY53 expression does not influence B concentration in shoot, root and leaf cell walls, which suggests BnaA1.WRKY53 might be involved in other biological processes. Indeed, phenotypic and anatomical analyses revealed that BnaA1.WRKY53 negatively regulated the leaf curling induced by leaf epinasty by suppressing the overexpansion of palisade cells under B deficiency. Further transcriptome enrichment analysis of differentially expressed genes (DEGs) between wild-type and BnaA1.WRKY53 overexpression line showed auxin response pathway was enriched. In addition, Arabidopsis DR5::GFP auxin reporter line showed B deficiency caused predominant auxin signal accumulation in the adaxial side and concomitant adaxial cell expansion, which indicated that B deficiency may induce leaf curling by altering auxin distribution. Phytohormone quantification and gene expression analysis demonstrated that BnaA1.WRKY53 prevent auxin overaccumulation in leaves by suppressing auxin biosynthetic genes under B deficiency. Furthermore, exogenous 1-naphthlcetic acid (NAA) treatment experiments revealed that high auxin could induce leaf curling and BnaA1.WRKY53 expression. Overall, these findings demonstrate that auxin and the transcription factor BnaA1.WRKY53 synergistically regulate leaf curling to maintain an optimal leaf area under B deficiency, and provide novel insights into the resistance mechanisms against B-deficiency-induced leaf curling in oilseed rape.

Rapeseed (Brassica napus L.) is a major oil crop worldwide that is vigorously promoted for cultivation in China. Boron (B) is an essential micronutrient for plant growth and development. However, the agricultural soils in rapeseed planting areas often show either B deficiency or severe B deficiency. Increasing the resistance to B deficiency is a pivotal goal in the breeding of rapeseed, yet the genetic basis for variations in B efficiency-related traits remains unclear. In this study, a natural population with 391 rapeseed accessions and a nutrient solution system were used to investigate B efficiency-related traits, including relative root length (RRL), shoot dry weight (SDW), root dry weight (RDW), and B efficiency coefficient (BEC), all of which exhibited extensive phenotypic variations under B deficiency. Through a genome-wide association study (GWAS) of B efficiency-related traits using high-density SNP markers obtained from whole-genome resequencing, 106 significantly associated SNPs were identified by employing both the general linear model and the mixed linear model. Among these SNP loci, two prominent SNP clusters were detected on chrA03: 14,087,835–14,764,672 and chrC03: 20,110,319–22,135,492 at low B levels across three repeated experiments of multiple traits. Integrating those results with a transcriptome analysis, four genes exhibiting higher differentially expressed fold-change along with favorable haplotypes within the promoter or coding region, BnaA03g29020D, BnaA03g29440D, BnaC03g33010D, and BnaC03g34490D, were identified as candidate genes that could potentially be involved in efficient B utilization, and their favorable haplotypes were found to improve seedling growth and productivity under B deficiency. Considering the lack of B mineral resources in China, the rapid and accurate identification of more B-efficient alleles and studying the genetic mechanism underlying crop responses to B deficiency have important theoretical and practical significance for cultivating B-efficient varieties and maintaining green, sustainable agriculture.