Four glycoside hydrolases are differentially modulated by auxins, cytokinins, abscisic acid and gibberellic acid in apple fruit callus cultures

α-l-Arabinofuranosidase, α- and β-d-xylosidase, and β-d-glucosidase activity was detected in the soluble fraction (S-F) extracted with water and in the NaCl-released fraction (NaCl-F) extracted with a high-salt concentration buffer from apple callus cultures. The activity was found to be differentially modulated by the addition of various plant growth regulators (PGRs) to calluses that had lost their requirement for specific PGRs (“habituation” phenomenon). α-l-Arabinofuranosidase activity was 93%, 130%, 126% and 186% higher in the NaCl-F from IAA-, IBA-, ABA- and GA₃-treated callus than in that extracted from untreated callus while S-F α-l-arabinofuranosidase activity was only 71%, 24%, 55% and 66% higher, respectively. α-d-Xylosidase displayed low activity levels in both S-F and NaCl-F but 2iP-treated callus showed higher α-d-xylosidase activity in both fractions than the control. 2,4-D increased α-d-xylosidase activity by 110% in the NaCl-F but decreased it by 40% in the S-F. β-d-Xylosidase activity increased by 99% in S-F from 2iP-treated callus but slightly decreased in the NaCl-F. In GA₃-treated callus, NaCl-F β-d-xylosidase activity increased by 188%. S-F and NaCl-F from Picloram-treated callus showed undetectable or only slightly noticeable α-l-arabinofuranosidase, α-d-xylosidase and β-d-xylosidase activity. Interestingly, β-d-glucosidase activity rose 28-fold in the S-F extracted from Picloram-treated callus. β-d-glucosidase was the only enzyme assayed that greatly increased its NaCl-F activity after 10 subcultures, and the addition of any PGR to the callus culture –except for Picloram and ABA– decreased its activity, suggesting that this enzyme may be associated with certain stress conditions, such as PGR starvation or Picloram addition. This is the first report on glycoside hydrolases from fruit callus as modulated by different PGRs.