Bioenergy Sorghum’s Roots: A Key to Tomorrow’s Sustainable Biomass Production on Annual Cropland

Abstract

Bioenergy sorghum has a high biomass yield potential, is drought resilient, has good nitrogen use efficiency, and has a root system that contributes to the accumulation of soil organic carbon. In this dissertation, field and greenhouse grown bioenergy sorghum root systems were analyzed during the growing season under different conditions to characterize sorghum root system morphology, anatomy, and gene expression patterns. Bioenergy sorghum roots grew continuously during a 155-day growing season to produce an average of ~175 nodal roots, accumulated ~7 Mg of dry biomass per hectare, and reached >2.4 m deep in the soil profile. Nodal root buds were first observed forming in the seventh phytomer 180° from the tiller bud at the base of the pulvinus. During a ~10-week period of development, two rings of nodal root buds were formed that encircle the stem prior to nodal root outgrowth. RNA-seq data showed that early-, middle-, and late-stage nodal root buds had significantly divergent expression. Brightfield and fluorescence microscopy were used to identify anatomical root adaptations – the production of a suberized cerastele. RNA-Seq profiles of roots at different levels of water deficit indicated that the production of raffinose may be involved in cerastele production and as an osmoprotectant. Additionally, candidate transcription factors were identified that could regulate the water deficit response, under the control of abscisic acid and ethylene, in sorghum roots. The traits described, and their respective genetic regulators, are good candidates for further investigation to fully understand nodal root production; these identified traits have the potential to become breeding targets in other Poaceae crops to increase water deficit tolerance and reduce reliance on irrigation from rapidly depleting ground water reserves in these additional crops.