Self-fer­til­is­ing maize

In south­ern Mex­i­co, researchers have dis­cov­ered giant maize with aston­ish­ing prop­er­ties. Oth­er vari­eties can ben­e­fit from this.

Get­ting cere­al crops to fix atmos­pher­ic nitro­gen is a long-stand­ing goal for plant researchers and breed­ers. Now, in the case of maize, this goal has unex­pect­ed­ly been brought clos­er, with the dis­cov­ery of an old Mex­i­can vari­ety able to cov­er between 30 and 80% of its nitro­gen require­ments through bac­te­r­i­al sym­bio­sis. Actu­al­ly, the vari­ety has been known by Native Amer­i­cans for a long time. “Sub­sis­tence farm­ers in the Oax­a­ca region have been farm­ing it for hun­dreds of years as a food crop,” explains Dr Jean-Michel Ané, who stud­ies the fea­tures of the plant at the Uni­ver­si­ty of Wis­con­sin in Madi­son, USA. “It is tra­di­tion­al­ly sown in ‘clus­ters’ of five to six seeds and grown with­out fer­tilis­er, reach­ing a height of up to five metres. This brought us to the ques­tion: How does this giant maize plant sat­is­fy its high nitro­gen require­ments under con­di­tions like these?”

Eight years of test­ing have uncov­ered a trait which was pre­vi­ous­ly unprece­dent­ed among the grass fam­i­ly. The Oax­a­ca maize’s aer­i­al roots secrete a mucus-like gel which is colonised by Azospir­il­lum and Herbaspir­il­lum bac­te­ria. These gen­era of bac­te­ria oth­er­wise live on the soil sur­face and are able to absorb nitro­gen from the atmos­phere.

Nitro­gen-absorb­ing maize: A min­i­mal water sup­ply is required to kick­start the secre­tion of the mucilage and sub­se­quent­ly to ini­ti­ate sym­bio­sis. Hybrid vari­eties will require suf­fi­cient rain and pos­si­bly irri­ga­tion.

Reduc­ing fer­tilis­er needs

The project, in which the Uni­ver­si­ty of Cal­i­for­nia, Davis, and Mars Inc. are also involved, has now proved that it is pos­si­ble to trans­fer this trait to com­mon vari­eties through hybridi­s­a­tion. “The goal is to sig­nif­i­cant­ly reduce the use of fer­tilis­ers, with­out affect­ing yields,” says Dr Ané. “Fer­til­i­sa­tion before or dur­ing sow­ing won’t change, but it is the sub­se­quent appli­ca­tions that we will be able to cut back on.” Sev­er­al breed­ing com­pa­nies are show­ing inter­est in this dis­cov­ery, says the researcher, who expects a hybrid vari­ety with­in the next 10 years. “Being able to pro­vide 10% of nitro­gen require­ments via sym­bio­sis would be a major suc­cess. We are hop­ing to reach 20-30%.”

Impor­tant traits to main­tain are the high num­ber of stem nodes and aer­i­al roots, as well as the amount of gel pro­duced by each aer­i­al root. Anoth­er area of research is the selec­tion of the most effec­tive bac­te­ria for devel­op­ing an inoc­u­lant. 

An (almost) unique prop­er­ty

The trop­i­cal Oax­a­ca maize vari­ety pro­duces a sug­ary mucus which is home to nitro­gen-absorb­ing bac­te­ria. Each aer­i­al root secretes 1.5-2ml of this gel after a half-hour peri­od of rain. The con­tained bac­te­ria stays vital for up to three days with­out fur­ther water sup­ply. The phe­nom­e­non occurs for four months of the year. Pre­vi­ous­ly, only one fur­ther exam­ple of this mech­a­nism was known about, in the Brazil­ian giant rhubarb (Gun­nera man­i­ca­ta).