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Precise responses to changes in light quality are crucial for plant growth and development. For example, hypocotyls of shade‐avoiding plants typically elongate under shade conditions. Although this typical shade‐avoidance response (TSR) has been studied in Arabidopsis (Arabidopsis thaliana), the molecular mechanisms underlying shade tolerance are poorly understood. Here we report that B. napus (Brassica napus) seedlings exhibit dual shade responses. In addition to the TSR, B. napus seedlings also display an atypical shade response (ASR), with shorter hypocotyls upon perception of early‐shade cues. Genome‐wide selective sweep analysis indicated that ASR is associated with light and auxin signaling. Moreover, genetic studies demonstrated that phytochrome A (BnphyA) promotes ASR, whereas BnphyB inhibits it. During ASR, YUCCA8 expression is activated by early‐shade cues, leading to increased auxin biosynthesis. This inhibits hypocotyl elongation, as young B. napus seedlings are highly sensitive to auxin. Notably, two non‐canonical AUXIN/INDOLE‐3‐ACETIC ACID (Aux/IAA) repressor genes, BnIAA32 and BnIAA34, are expressed during this early stage. BnIAA32 and BnIAA34 inhibit hypocotyl elongation under shade conditions, and mutations in BnIAA32 and BnIAA34 suppress ASR. Collectively, our study demonstrates that the temporal expression of BnIAA32 and BnIAA34 determines the behavior of B. napus seedlings following shade‐induced auxin biosynthesis. [ABSTRACT FROM AUTHOR]

SUMMARY: Petals in rapeseed (Brassica napus) serve multiple functions, including protection of reproductive organs, nutrient acquisition, and attraction of pollinators. However, they also cluster densely at the top, forming a thick layer that absorbs and reflects a considerable amount of photosynthetically active radiation. Breeding genotypes with large, small, or even petal‐less varieties, requires knowledge of primary genes for allelic selection and manipulation. However, our current understanding of petal‐size regulation is limited, and the lack of markers and pre‐breeding materials hinders targeted petal‐size breeding. Here, we conducted a genome‐wide association study on petal size using 295 diverse accessions. We identified 20 significant single nucleotide polymorphisms and 236 genes associated with petal‐size variation. Through a cross‐analysis of genomic and transcriptomic data, we focused on 14 specific genes, from which molecular markers for diverging petal‐size features can be developed. Leveraging CRISPR‐Cas9 technology, we successfully generated a quadruple mutant of Far‐Red Elongated Hypocotyl 3 (q‐bnfhy3), which exhibited smaller petals compared to the wild type. Our study provides insights into the genetic basis of petal‐size regulation in rapeseed and offers abundant potential molecular markers for breeding. The q‐bnfhy3 mutant unveiled a novel role of FHY3 orthologues in regulating petal size in addition to previously reported functions. Significance Statement: Using a comprehensive analysis of genomic and transcriptomic data, this study proposes a set of 14 genes that can serve to develop molecular markers for diverging petal‐size features. These genes can also be potential targets for genetic manipulation aimed at reducing petal size. A q‐bnfhy3 mutant with reduced petal size was created, allowing for the validation of the previously unrecognized role of FHY3 orthologues in Brassica napus in the regulation of petal‐size formation. [ABSTRACT FROM AUTHOR]