Overcoming the trade‐off between grain weight and number in wheat by the ectopic expression of expansin in developing seeds leads to increased yield potential

Abstract

•Wheat is the most widely grown crop globally, providing 20 % of all human calories and protein. Achieving step changes in genetic yield potential is crucial to ensure food security, but efforts are thwarted by an apparent trade-off between grain size and number. Expansins are proteins that play important roles in plant growth by enhancing stress relaxation in the cell wall, which constrains cell expansion. •Here, we describe how targeted over-expression of an α-expansin in early developing wheat seeds leads to a significant increase in grain size without a negative effect on grain number, resulting in a yield boost under field conditions. •The best-performing transgenic line yielded 12.3 % higher average grain weight than the control, and this translated to an increase in grain yield of 11.3 % in field experiments using an agronomically appropriate plant density. •This targeted transgenic approach provides an opportunity to overcome a common bottleneck to yield improvement across many crops.

Expanding wheat yields with expansin

“…This selective wall loosening activity expedites delivery of the two sperm to the ovule for the double fertilization event that creates the triploid endosperm (Valdivia et al ., 2009). This action is very different from the seed growth processes manipulated by Calderini et al ., which I now turn to.…”

“…Because wheat provides one‐fifth of the calories and protein in the global human diet, pressures to increase yields will surely mount as the human population expands, and as climate change reduces agricultural productivity. Calderini et al . report >10% increases in grain yields with targeted expression of an α‐expansin gene in the young seed, at a developmental time point preceding the bulk of grain filling.…”

“…Although the developmental mechanisms for this trade‐off are unclear, work published in this issue of New Phytologist by Calderini et al . (2021; pp. 629‐640) demonstrates a means around this yield bottleneck by transgenic expression of a cell wall loosening protein in the developing seed.…”

“…Gibberellin modulates stem elongation in part through expression of α‐expansins (Cho & Kende, 1997). Thus, yield increases in the Green Revolution involved downregulation of α‐expansin while the transgenic approach by Calderini et al . involved its upregulation, but targeted so as to enlarge the endosperm.…”

“…This hypothesis concerning seed growth has been explored experimentally in Arabidopsis, but something similar may limit growth of the wheat grain. Targeting these outer layers of the seed for ectopic expression of α‐expansin, as done by Calderini et al ., thus seems like a promising strategy; β‐expansins might also work in a similar strategy, perhaps synergistically with α‐expansins.…”