Scientists Identify Gene Responsible for Olive Oil’s Aroma

Summary

Spanish researchers have dis­cov­ered how to manip­u­late the expres­sion of the 13-HPL gene in olive trees, affect­ing the aroma of olive oil pro­duced. This dis­cov­ery could allow grow­ers to select spe­cific cul­ti­vars for desired aro­mas or breed new cul­ti­vars with pre-deter­mined aro­mas, poten­tially enhanc­ing the fruity or pun­gent qual­i­ties of the oil.

A team of Spanish researchers has inves­ti­gated a key genetic trait of the olive tree and devised a way to manip­u­late the asso­ci­ated gene’s expres­sion with rel­e­vant con­se­quences on olive oil aroma.

The dis­cov­ery may allow grow­ers to select cul­ti­vars to pro­duce olive oils with spe­cific aro­mas or researchers to breed new cul­ti­vars with pre-deter­mined aro­mas. Genetically mod­i­fied olives could fea­ture a more fruity or pun­gent aroma.

“Plants pro­duce and emit a huge diver­sity of volatile organic com­pounds, which are released from dif­fer­ent tis­sues such as leaves, fruits, flow­ers and roots,” the researchers wrote. ​“From a chem­i­cal point of view, these plant volatiles are organic lipophilic mol­e­cules char­ac­ter­ized by low boil­ing points and high vapor pres­sures at ambi­ent tem­per­a­tures.”

Volatile com­pounds are mol­e­cules that, once released by their source, reach the olfac­tory recep­tors and cre­ate the aroma.

The 13-HPL gene has been inves­ti­gated for decades as it plays a piv­otal role in the syn­the­sis of the main volatile com­pounds that make up the aroma of vir­gin olive oils.

“It was not until approx­i­mately a decade ago that we were able to iso­late and char­ac­ter­ize the 13-HPL gene from the olive tree, as well as to demon­strate the puta­tive [com­monly thought] func­tion­al­ity of the pro­tein it encodes, the 13-HPL enzyme,” Carlos Sanz, the leader of the bio­chem­istry and plant food tech­nol­ogy research group at the Spanish National Research Council’s Fat Institute, told Olive Oil Times.

The 13-HPL is an enzyme that pro­duces the six-car­bon C6 aliphatic alde­hy­des hexa­nal or hex­e­nals from polyun­sat­u­rated fatty acids with a hydroper­ox­ide group at car­bon 13.

These volatile alde­hy­des, and their alco­hol and ester deriv­a­tives, are com­po­nents of the aroma of the fruits of dif­fer­ent plant species. In olives, they are also respon­si­ble for the smell of cut grass that is pro­duced when the plant leaves are crushed.

“The 13-HPL is an enzyme that is part of the lipoxy­ge­nase path­way, which is a highly con­served bio­chem­i­cal path­way in plants that func­tions in dif­fer­ent plant organs,” Sanz said. ​“We have ver­i­fied in pre­vi­ous stud­ies that the pro­file of volatile com­pounds obtained after crush­ing the olive leaf is basi­cally the same as that obtained after crush­ing the olive fruit since, in both organs, the dam­age to the tis­sue trig­gers the func­tion­ing of the lipoxy­ge­nase path­way and con­se­quently the syn­the­sis of volatile com­pounds.”

“This is essen­tially the ori­gin of the aroma of vir­gin olive oil,” he added. ​“This aroma is only pro­duced when the integrity of the olive fruit is destroyed in the milling stage of the vir­gin olive oil extrac­tion process.”

The study also found that there is only a sin­gle 13-HPL gene in the olive, unlike in other meta­bolic steps in syn­the­siz­ing these volatile com­pounds, in which a con­sid­er­able num­ber of genes with the same func­tion may be involved.

“Well, once the gene had been iden­ti­fied and char­ac­ter­ized, the next step was to deter­mine its func­tion­al­ity in vivo, that is, to ver­ify that this gene is expressed in the olive to pro­duce 13-HPL and that this enzyme works as it does in vitro,” Sanz said.

To ver­ify this, researchers grew two sets of olive trees, one in which the gene was silenced and the other in which it was ampli­fied to its high­est pos­si­ble level. From there, the sci­en­tists at the Fat Institute and the University of Malaga ana­lyzed the result­ing volatile com­pounds.

The researchers found that silenc­ing the gene brings unwanted con­se­quences.

“The mod­i­fi­ca­tion of the expres­sion of the 13-HPL gene in olive trees not only pro­duces a large decrease in C6 volatiles… it also causes less plant growth,” Sanz said. ​“This shows that the 13-HPL activ­ity is essen­tial for the nor­mal growth and devel­op­ment of the olive plant.”

One of the rea­sons for this might be the role the 13-HPL gene plays in elim­i­nat­ing polyun­sat­u­rated fatty acid hydroper­ox­ides that are toxic to the plant. An excess of the hydroper­ox­ide deriv­a­tives in the plan­t’s tis­sues prob­a­bly affects its growth.

“On the other hand, we must not for­get that the lipoxy­ge­nase path­way is the ori­gin of dif­fer­ent phys­i­o­log­i­cal reg­u­la­tors, so an increase in the con­tent of these hydroper­ox­ides can lead to an increase in the con­cen­tra­tion and activ­ity of some of these reg­u­la­tors with the observed phe­no­typic effect,” Sanz said.

According to the researchers, the study’s results would make it pos­si­ble to pro­duce plants with a mod­i­fied gene expres­sion. ​“But it would only affect the char­ac­ter­is­tics of the green attribute of the oil’s aroma,” Sanz said.

An increase in the expres­sion of the gene would enhance the green fruity attrib­utes of the aroma. ​“On the con­trary, a con­trol of the expres­sion of this gene would cause… the oils to have green aro­mas but with a less fruity and more pun­gent char­ac­ter, which are appre­ci­ated by some sec­tors of con­sumers,” he said.

“The infor­ma­tion we got con­firms the sus­pi­cions we had about the func­tion­ing of the 13-HPL gene and enzyme and their involve­ment in the syn­the­sis of the main com­pounds respon­si­ble for the aroma of vir­gin olive oil,” Sanz added.

“Although it would be fea­si­ble to obtain trans­genic lines that carry a mod­i­fied expres­sion of this gene, in my opin­ion, the main use of this knowl­edge would be in olive breed­ing pro­grams to develop mol­e­c­u­lar mark­ers for the marker-assisted selec­tion of new olive cul­ti­vars that pro­duce oils with an improved aroma,” he con­cluded.

• Plant Science
• Spanish National Research Council