Hindered translocation of sugars within maize ear reduces grain weight under drought stress

Maize, one of the most important crops in the food chain, is facing severe threats from the escalating incidence of drought stress worldwide. Maize grain development requires assimilate uptake from the attached cob, however, the spatial distribution of sugars (the main carbon assimilate) within ear and its relationship with grain development under drought stress remain unclear. Here, we systematically investigated the dynamics of sugars and starch in different sub-tissues of cob (pith, woody ring, pedicel, and glume) and grain at different regions (the apical, middle and basal) of the developing ear under drought stress during grain filling. Results showed that the major sugar within cob was sucrose, followed by hexoses (fructose and glucose) and starch. Spatially, sucrose was highest in the woody ring and decreased toward the pedicel and then grain, whilst the hexoses were higher in the grain and then the cob center (pith) but gradually decreased towards the pedicel. Under drought, sucrose and hexoses were significantly accumulated in the pedicel but reduced in the grain, revealing that sugar uptake from the pedicel by grain was blocked. Moreover, sugars were reduced in the apical and middle regions of cob but were promoted in the basal region by drought, suggesting a drought-suppressed sugar translocation within cob. Accordingly, grain weight in the apical, middle, and basal regions were reduced by 18.9 %, 11.3 %, and 10.8 %, with starch weight decreased by 25.2 %, 21.4 %, and 2.0%, respectively, at maturity. Collectively, these results support that sugar uptake from the pedicel by the grain and upward translocation along the cob are associated with grain weight in response to drought. These findings firstly uncover the spatial patterns for sugars within ear to respond to drought stress and highlight the importance of sugar availability within cob in grain yield formation.