Objective This study aimed to investigate the regulatory effects of α-naphthaleneacetic acid (NAA) priming on lateral root growth of soybean during the emergence stage under drought stress and to elucidate the associated physiological mechanisms involving sugar and respiratory metabolism in lateral roots, thereby providing a theoretical basis and technical approach for drought-resistant soybean cultivation.

Method Using the stable soybean line 'Xu 9302' as the material, soil relative water content (RWC) was controlled by the pot-weighing method. Four treatments were established: non-primed + non-stress (NP+CK, RWC 75%), non-primed + drought stress (NP+D, RWC 50%), hydro-primed + drought stress (HP+D, RWC 50%), and 5 μmol/L NAA-primed + drought stress (NAA+D, RWC 50%). This study investigated the effects of different priming treatments on soybean emergence rate, hypocotyl growth and lateral root development at the emergence stage under drought stress. Additionally, it analyzed the regulatory effects of NAA priming on the contents of starch, sucrose, and soluble sugars, the respiratory rate and the activities of key enzymes in related metabolic pathways in lateral roots.

Result Compared with the NP+CK treatment, drought stress significantly inhibited the growth of both primary and lateral roots in soybean. The NAA+D treatment significantly alleviated the reduction in dry mass and length of lateral roots, which increased by 16.3%-55.6% and 32.8%-88.0%, respectively, compared to the NP+D treatment. The NAA+D treatment also promoted hypocotyl growth and seedling establishment, with hypocotyl length, RWC, and seedling emergence rate increasing by 28.9%, 12.9, and 19.9 percentage points, respectively, relative to the NP+D treatment. There were no significant differences between the HP+D and NP+D treatments in lateral root dry mass, lateral root length, hypocotyl RWC, hypocotyl length, or emergence rate. Mechanistically, NAA priming promoted the accumulation of starch, soluble sugars, and sucrose by enhancing the activities of ADP-glucose pyrophosphorylase (AGPase), β-amylase, sucrose phosphate synthase (SPS), sucrose synthase (SuSy), and acid invertase (AI) in lateral roots. Under the NAA+D treatment, the levels of these three compounds increased significantly by 15.8%-25.0%, 13.5%-18.6%, and 17.8%-32.9%, respectively, compared to the NP+D treatment. Furthermore, NAA priming effectively alleviated the inhibitory effects of drought stress on the respiration rate and the activities of phosphofructokinase (PFK) and malate dehydrogenase (MDH) in lateral roots of soybean seedlings. Under the NAA+D treatment, the respiration rate, PFK activity, and MDH activity were significantly increased by 27.0%-35.2%, 21.8%-46.9%, and 25.7%-45.9%, respectively, relative to the NP+D treatment.

Conclusion Under drought stress, NAA priming coordinately enhanced carbon reserve and respiratory metabolism in lateral roots, thereby directing carbon flux toward osmoregulatory substances and its efficient conversion into growth energy, which ultimately promoted lateral root development and seedling establishment in soybean.