Reduced Strigolactone Exudation as a Key Resistance Mechanism in Wild Carrots against Phelipanche aegyptiaca.

Phelipanche aegyptiaca is a root parasitic plant that causes significant yield losses in many crops, including carrots (Daucus carota). This study investigates the resistance mechanisms of two wild carrot accessions, PI 21793 (Daucus glaber) and PI 341902 (Daucus littoralis), against Podalirius aegyptiaca compared to a cultivated carrot (P0114; D. carota). Wild carrots induced lower germination rates of P. aegyptiaca seeds and fewer successful tubercles, indicating both pre-attachment and partial post-attachment resistance mechanisms. Strigolactone analysis revealed significant quantitative differences between cultivated and wild carrots. While cultivated carrots exuded high levels of two strigolactones, one of which was putatively identified as the non-canonical strigolactone, 4-oxo-methyl-carlalactone (4-oxo-MeCLA), wild carrots released lower amounts of these compounds. Supplementation with the artificial strigolactone analog GR24 increased germination in P. aegyptiaca inoculated on wild carrots, suggesting that strigolactone deficiency and possibly altered composition are key pre-attachment resistance mechanisms. However, higher germination resulted in no significant improvement in tubercle development on wild carrots. Parasite seedlings showed necrosis-like symptoms at their attachment sites on wild carrot roots, indicating an additional post-attachment resistance mechanism. These findings provide new insights into strigolactone-mediated host resistance and highlight the potential of wild carrot accessions to contribute to the development of resistant cultivars against parasitic plants.