BvCaM7 Participates in Sugar Beet ( Beta vulgaris L.) Resistance to CLS (Cercospora Leaf Spot) by Activating BvPAL via BvMYB108 and Is Accompanied by Calcium Dynamics

CLS (Cercospora leaf spot) is a major constraint in sugar beet (Beta vulgaris L.) cultivation, yet the functions of CaM/CML family members in CLS resistance are unclear. This study identified the CaM/CML family in sugar beet and focused on BvCaM7, which was significantly upregulated under CLS. Calcium measurements revealed differential calcium dynamics between resistant and susceptible varieties at late stages. Lignin staining revealed enhanced vascular lignification in resistant lines. Molecular interaction assays confirmed that BvCaM7 interacts with BvCNGC14 and BvMYB108. LUC assays confirmed that BvMYB108 activates the PAL promoter, and coexpression with BvCaM7 enhanced this effect. Transient overexpression of BvCaM7 or BvMYB108 in tobacco was associated with reduced lesion development following Cercospora beticola (C. beticola.) inoculation, although defence gene induction kinetics (PR2/3/4, PAL, COL, PDF1.2) differed: BvMYB108 peaked at 12 h, whereas BvCaM7 sustained expression until 24 h. Both overexpressions increased superoxide dismutase (SOD) and peroxidase (POD) activities, but decreased catalase (CAT) activity. In summary, BvCaM7 activates the PAL pathway via BvMYB108, enhancing lignin-related defence. Furthermore, the BvCaM7-BvCNGC14 interaction and concomitant calcium changes provide clues for the potential involvement of calcium signalling. However, its specific role requires further validation. This study provides candidate genes for breeding CLS-resistant sugar beet.