Programmed cell death (PCD) is required for many aspects of plant biology, including stress responses, immunity, and plant development including root and flower development. Our understanding of PCD regulation is incomplete, especially concerning regulators involved in multiple divergent processes. The botrytis-suscetible1 (bos1) mutant is one of the genotypes most susceptible to Botrytis cinerea (Botrytis) and has revealed the role of BOS1 in cell death propagation during plant responses to wounding. The bos1-1 allele harbours a T-DNA located in the 5'UTR upstream from the start codon that results in elevated BOS1 transcript levels. Here, we resequenced the bos1-1 genome and found a MAS promoter at the ends of the T-DNAs. Expression of the BOS1 gene under control of the MAS promoter conferred the characteristic bos1-1 Botrytis-sensitivity and wounding phenotypes in wildtype plants. We used Crispr-Cas9 to create new bos1 alleles that disrupt exons. These lines lacked the typical bos1-1 wounding and Botrytis phenotypes, but exhibited reduced fertility, as previously observed in other bos1 T-DNA alleles. With multiple overexpression lines of BOS1, we demonstrate that BOS1 is involved in regulation of cell death propagation in a dosage dependent manner. Our data support that bos1-1 is a gain-of-function mutant and that BOS1 acts as a positive regulator of wounding and Botrytis-induced PCD. Taken together these finding suggest that BOS1 function in both fertility and Botrytis response could be unified under cell death control.

Programmed cell death is integral to plant life and required for stress responses, immunity, and development. Our understanding of the regulation of programmed cell death is incomplete, especially concerning regulators involved in multiple divergent processes. The botrytis-susceptible (bos1) mutant of Arabidopsis is highly susceptible to fungal infection by Botrytis cinerea (Botrytis). BOS1 (also known as MYB108) regulates cell death propagation during plant responses to wounding. The bos1-1 allele contains a T-DNA insertion in the 5’ untranslated region upstream of the start codon. This insertion results in elevated expression of BOS1/MYB108. We used Crispr/Cas9 to create new bos1 alleles with disrupted exons, and found that these lines lacked the typical bos1-1 wounding and Botrytis phenotypes. They did exhibit reduced fertility, as was previously observed in other bos1 alleles. Resequencing of the bos1-1 genome confirmed the presence of a MAS promoter at the T-DNA left border. Expression of the BOS1 gene under control of the MAS promoter in wild-type plants conferred the characteristic phenotypes of bos1-1: Botrytis-sensitivity and response to wounding. Multiple overexpression lines demonstrated that BOS1 was involved in regulation of cell death propagation in a dosage dependent manner. Our data indicate that bos1-1 is a gain-of-function mutant and that BOS1 function in regulation of fertility and Botrytis response can both be understood as misregulated cell death.

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