"Long before it's in the papers"
February 01, 2013

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Plants help their own, too, study finds

Feb. 1, 2013
Courtesy of University of Colorado at Boulder
and World Science staff

Not un­like dol­phins, people and many other ani­mals, some plants help their own kin, a study sug­gests.

Re­search­ers stud­ied corn, in which each fer­ti­lized ker­nel ac­tu­ally con­tains two small or­gan­isms: an em­bry­o of a new corn plant, and an­oth­er liv­ing thing called en­do­sperm. The en­do­sperm is bi­o­log­ic­ally the sib­ling em­bryo's sib­ling, but the em­bry­o eats it in or­der to grow. This gen­er­al struc­ture in fact char­ac­ter­izes most flow­er­ing, seed-bearing plants.

Doctoral student Chi-Chih Wu ex­amined tens of thous­ands of ker­nels of multi-col­ored corn (as in inset) to find out whether corn plants have an altru­istic side. (Credit: U. of Co­lor­ado)
In a ti­ny per­cent­age of corn ker­nels, the em­bryo and en­do­sperm are ac­tu­ally half-sib­lings rath­er than full sib­lings: they have the same moth­er but dif­fer­ent fa­thers.

The study found that in these cases, the en­do­sperm “does not hand over as much food—it ap­pears to be act­ing less co­op­er­a­tive­ly,” said Pam­e­la Dig­gle, a bi­ol­o­gist at the Uni­vers­ity of Col­o­rad­o Boul­der. 

“Em­bryos with the same moth­er and fa­ther as the en­do­sperm… weighed sig­nif­i­cantly more than em­bry­os with the same moth­er but a dif­fer­ent fa­ther.”

The study was pub­lished the week of Jan. 21 in the jour­nal Pro­ceed­ings of the Na­t­ional Acad­e­my of Sci­ences.

Dig­gle said it’s fairly clear from pre­vi­ous re­search that plants can pref­er­en­tially with­hold nu­tri­ents from in­fe­ri­or off­spring when re­sources are lim­it­ed. But “our study is the first to spe­cif­ic­ally test the idea of coop­era­t­ion among sib­lings in plants.”

“One of the most fun­da­men­tal laws of na­ture is that if you are go­ing to be an al­tru­ist, give it up to your clos­est rel­a­tives,” said Wil­liam “Ned” Fried­man, a Har­vard Uni­vers­ity re­searcher who worked on the corn study while form­erly at CU-Boul­der. “Al­tru­ism only evolves if the ben­e­fac­tor is a close rel­a­tive of the ben­e­fi­ciary. When the en­do­sperm gives all of its food to the em­bry­o and then dies, it does­n't get more al­tru­is­tic than that.”

Male flow­ers at the top of corn plants spread pol­len grains two at a time to ti­ny cobs, in a pro­cess called dou­ble fer­til­iz­a­tion. In this way pair of pol­len grains pro­duces a seed con­tain­ing an em­bry­o and en­do­sperm.

The sci­en­tists took ad­van­tage of an ex­tremely rare phe­nom­e­non in plants called “hetero-fer­til­iz­a­tion,” in which two dif­fer­ent fa­thers sire in­di­vid­ual corn ker­nels, said Dig­gle, cur­rently a vis­it­ing pro­fes­sor at Har­vard. The ma­nipula­t­ion of corn plant genes that has been go­ing on for mil­len­ni­a—re­sult­ing in mul­ti­col­ored “In­dian corn” cobs of var­i­ous col­ors like red, pur­ple, blue and yel­low—helped the re­search­ers in as­sess­ing the par­ent­age of the ker­nels, she added.

CU-Boul­der doc­tor­al stu­dent Chi-Chih Wu cul­ti­vat­ed corn and har­vested more than 100 ears over three years. He re­moved, mapped and weighed each ker­nel. While most had an en­do­sperm and em­bry­o of the same col­or—an in­dica­t­ion they shared the same moth­er and fa­ther—some had dif­fer­ent col­ors for each, such as a pur­ple out­er ker­nel with yel­low em­bry­o.

Wu was search­ing for such rare ker­nels—far less than one in 100—that had two dif­fer­ent fa­thers. “It was like look­ing for a nee­dle in a hay­stack, or in this case, a ker­nel in a silo,” Fried­man said.

En­do­sperm, in the form of corn, rice, wheat and oth­er crops, is crit­i­cal to hu­mans, pro­vid­ing about 70 per­cent of calo­ries we con­sume an­nu­ally world­wide. “The tis­sue in the seeds of flow­er­ing plants is what feeds the world,” said Fried­man, who al­so di­rects the Ar­nold Ar­bo­re­tum at Har­vard. “If flow­er­ing plants were­n't here, hu­mans would­n't be here.”

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We've all heard examples of animal altruism: Dogs caring for orphaned kittens, chimps sharing food or dolphins nudging injured mates to the surface. Now, a study suggests some plants are altruistic too, at least toward close kin. Researchers studied corn, in which each fertilized kernel actually contains two small organisms: an embryo of a new corn plant, and another living thing called endosperm. The endosperm is biologically the sibling embryo's sibling, but the embryo eats it in order to grow. This general structure in fact characterizes most flowering, seed-bearing plants. In a tiny percentage of corn kernels, the embryo and endosperm are actually half-siblings rather than full siblings: they have the same mother but different fathers. The study “found that endosperm that does not share the same father as the embryo does not hand over as much food—it appears to be acting less cooperatively,“ said Pamela Diggle, a biologist at the University of Colorado Boulder. “Embryos with the same mother and father as the endosperm… weighed significantly more than embryos with the same mother but a different father.“ A paper on the subject was published during the week of Jan. 21 in the Proceedings of the National Academy of Sciences. Diggle said it is fairly clear from previous research that plants can preferentially withhold nutrients from inferior offspring when resources are limited. “Our study is the first to specifically test the idea of cooperation among siblings in plants.“ “One of the most fundamental laws of nature is that if you are going to be an altruist, give it up to your closest relatives,“ said William “Ned“ Friedman, a Harvard University researcher who worked on the corn study while formerly at CU-Boulder. “Altruism only evolves if the benefactor is a close relative of the beneficiary. When the endosperm gives all of its food to the embryo and then dies, it doesn't get more altruistic than that.“ Male flowers at the top of corn plants spread pollen grains two at a time through individual tubes to tiny cobs on the stalks covered by strands known as silks in a process known as double fertilization. When the two pollen grains come in contact with an individual silk, they produce a seed containing an embryo and endosperm. Each embryo results in one kernel of corn. The scientists took advantage of an extremely rare phenomenon in plants called “hetero-fertilization,“ in which two different fathers sire individual corn kernels, said Diggle, currently a visiting professor at Harvard. The manipulation of corn plant genes that has been going on for millennia—resulting in the production of multicolored “Indian corn“ cobs of various colors like red, purple, blue and yellow—helped the researchers in assessing the parentage of the kernels, she added. CU-Boulder doctoral student Chi-Chih Wu cultivated corn and harvested more than 100 ears over a three-year period, then removed, mapped and weighed every kernel. While most kernels had an endosperm and embryo of the same color—an indication they shared the same mother and father—some had different colors for each, such as a purple outer kernel with yellow embryo. Wu was searching for such rare kernels—far less than one in 100—that had two different fathers. “It was like looking for a needle in a haystack, or in this case, a kernel in a silo,“ Friedman said. Endosperm, in the form of corn, rice, wheat and other crops, is critical to humans, providing about 70 percent of calories we consume annually worldwide. “The tissue in the seeds of flowering plants is what feeds the world,“ said Friedman, who also directs the Arnold Arboretum at Harvard. “If flowering plants weren't here, humans wouldn't be here.“