First new plant tissue discovered in 160 years boosts crop yields

A research group led by Nagoya University in Japan has discovered a new tissue in plants that is essential for seed formation. Their discovery represents the first new plant tissue discovered in 160 years. Their findings open a new field for research and have already demonstrated practical applications, with the team increasing yields of important crops, including rice. The journal Current Biology published the study.  

Since 2005, scientists have known that fertilization is necessary for the seed body, known as the hypocotyl, to receive nutrients from the ‘mother’ parts of the plant. Understanding how plants detect successful fertilization is important for maximizing yields from crop species during breeding. 

The research group led by Ryushiro Kasahara and Michitaka Nodaguchi made the discovery of the new tissue by chance. Kasahara had been staining seeds to track the deposition of callose, a waxy substance commonly studied because of its association with fertilization, to verify findings from a previous study.  

While examining the stained areas, Kasahara noticed something unexpected. “Plants fertilize by the insertion of a pollen tube, so most scientists are only interested in the place where this occurs. However, we found signals on the opposite side too,” he said. “Nobody was looking where I was looking. I remember being surprised, especially when we realized that this signal was particularly strong when fertilization failed.” 

Further analysis revealed a distinctive rabbit-shaped tissue structure that functions as a gateway. This structure, named the ‘Kasahara Gateway’ in honor of its discoverer, represents the first new plant tissue identified since the mid-19th century. 

The signal Kasahara observed resulted from callose deposition, which blocks the flow of nutrients and hormones into unfertilized seeds. Closure of the gateways led to the seeds not receiving nutrients and dying. The researchers termed this the ‘closed state.’  On the other hand, when fertilization occurs, the hypocotyl detects this success and dissolves the callose, allowing nutrients to flow into the seed and enabling growth. The researchers called this the ‘open state’.  

“When the flow of nutrients was compared between successfully fertilized and unsuccessful embryos, it was found that the inflow of nutrients was observed only in the successful embryos, whereas it was completely blocked in the unsuccessful ones,” Kasahara explained. “This limits the amount of resources wasted on unviable seeds.” 

The gateway's ability to switch between open-and-closed states suggested genetic regulation. The researchers examined fertilized plant hypocotyls to identify potential genetic controls. 

They identified a gene called AtBG_ppap that was upregulated exclusively in fertilized hypocotyls and identified its role in dissolving callose. When they modified hypocotyls to overexpress AtBG_ppap, the gateway remained permanently in the open gate state, increasing nutrient uptake. 

“This led us to the realization that keeping the gateway permanently open could enlarge seeds,” Kasahara said. “When we tested this theory with rice seeds, we made seeds that were 9% bigger. With seeds from other species, we succeeded in increases of as much as 16.5%.”  

Their findings represent a significant advancement in seed enhancement in plant breeding. Maintaining a permanently open state could substantially increase yields of important crops. 

Kasahara also believes these findings will enhance understanding of plant evolution, particularly why flowering plants (angiosperms) dominate today’s flora. “Since an unfertilized hypocotyl cannot become a seed in the first place, feeding it would be ‘wasteful’ for the plant,” he said. “Therefore, angiosperms may have been able to survive until modern times by feeding the embryo body using this mechanism to ensure that they only give resources to fertilized seeds.”