Expanding wheat yields with expansin

This article is a Commentary on Calderini et al. (2021), 230: 629-640. ‘What α-expansin and how does its expression increase grain size without triggering compensatory reduction in number?’ Expansin proteins were first discovered as catalysts of ‘acid growth’ (McQueen-Mason al., 1992), the pH-dependent cell growth wall extensibility that occurs under many circumstances, classically case auxin-stimulated hypocotyls coleoptiles (Rayle & Cleland, 1992). Auxin activates proton pumping across plasma membrane, driving down extracellular pH (Fendrych 2016). action mediated by canonical signaling pathway for auxin, involving SAURs (SMALL AUXIN UP-REGULATED RNA) which inhibit PP2C.D protein phosphatases which, turn, modulate activity membrane H+-ATPases (Spartz 2014). The resulting low α-expansins, facilitating yielding stress relaxation stretched leading to water uptake enlargement. scenario typical rapidly growing organs like hypocotyls, where cells enlarge many-fold, filling up with water-filled vacuole they do so. But endosperm–the large storage tissue wheat seed–has different pattern, described later. There an odd historical twist use upregulation yields because higher ‘Green Revolution’ likely involved downregulation α-expansins reduce stem elongation, permitting increases fertilizers increasing lodging, attendant yield losses. Selection short stems Green Revolution commonly gibberellin pathways, e.g. Petti (2015). Gibberellin modulates elongation part through (Cho Kende, 1997). Thus, while transgenic approach upregulation, but targeted so endosperm. Endosperm cells–which make bulk seed–have very ontogeny from elongating eventually fill starch bodies rather than vacuole. Wheat seeds reach full well before endosperm fills (Rogers Quatrano, 1983), this study resulted larger more volume bodies, making heavier grains. authors speculate their was successful highly selective promoter (PinB) transiently directs outer layers seed (aleurone pericarp) at early stage development, time point after number determined set. timepoint may be key circumventing number/weight trade-off, promoters other temporal patterns not tested study. It also crucial active spatially seed, just endosperm, whose unique it insensitive wall-loosening α-expansin. enlarges multinuclear syncytium only later partitioned walls (Sabelli Larkins, 2009). undergo expansion tissues plant body whether are loosened apparently has been studied. cereal grasses predominantly comprised arabinoxylans structured surface enlargement; thus unlikely responsive loosening facilitate turgor-driven slippage or axial sliding between cellulose microfibrils (Fig. 1; Cosgrove, 2018). In land plants expansins encoded two multigene families, (EXPAs) β-expansins (EXPBs). Experimental characterization activities these limited, studies date show highest whereas generally β-expansins. More work needed confirm differences, hindered technical difficulties expressing heterologous systems. β-Expansins good candidates walls, some preferentially loosen arabinoxylan-rich (Sampedro 2015). As aside, have additional connection production grasses: phylogenetically distinct group β-expansins–known group-1 grass pollen allergens–are expressed specifically pollen. They weaken middle lamella maternal (the stigma style) thereby tube penetration style. expedites delivery sperm ovule double fertilization event creates triploid (Valdivia processes manipulated I now turn to. hypothesized limit enlargement (Beauzamy 2016), much epidermis restrains organs. hypothesis concerning explored experimentally Arabidopsis, something similar grain. Targeting ectopic α-expansin, done seems promising strategy; might strategy, perhaps synergistically α-expansins. Expansins turned genome-wide association (GWASs) various agronomic traits. One example particularly relevant here GWAS rice, (OsEXPA16) associated drought tolerance (Pantaliao Many shown gene upregulated during deficits, beginning analysis maize root responses mild deficits (Wu 2001) followed numerous species, Jin (2020). considered adaptive response, enabling roots continue despite reduced turgor pressure. effect : shoot ratio, allowing explore soil limiting leaf area lost atmosphere. Other reported overexpression resistance environmental stresses, Narayan (2021). possible acted developing unavoidable field. If one expect effects transgene obvious conditions stress. Consistent above scenario, recent identified major determinant kernel length (Gong 2021). strategy expansin volume, then filled valuable materials. Similar strategies applied bioproducts, including starch-filled potato tubers, oil-filled oil (e.g. Brassica, Helianthus), lignocellulose (secondary fiber cells), materials energy fields. Work author’s laboratory funded US Department Energy, Office Science, Basic Energy Sciences, grant DE-FG02-84ER13179.