Comparative analysis of drought-responsive physiological and transcriptome in broomcorn millet (Panicum miliaceum L.) genotypes with contrasting drought tolerance

Climate change has caused severe drought, affecting global crop production. Broomcorn millet is a drought-tolerant crop preferred for water-saving agriculture because of its short life cycle and high water use efficiency. This study evaluated the drought tolerance of 300 broomcorn millet varieties from 21 sources under well-watered, semi-arid conditions (Yulin, Shaanxi, China) and unwatered, arid conditions (Dunhuang, Gansu, China). Two broomcorn millet varieties with contrasting drought tolerance attributes, DT 43 (drought-tolerant) and DS 190 (drought-sensitive), were selected for comparative physiological and transcriptional assessment under the two drought stress conditions (polyethylene glycol 6000 [PEG-6000] and soil drought) and corresponding melatonin treatments. The two forms of drought stress decreased photosynthetic capacity and triggered transcriptome reprogramming in both broomcorn millet cultivars. However, PEG induced a more 'severe' and 'rapid' drought stress than the 'milder' and 'slower' soil moisture drought stress. Moreover, PEG stress caused severe growth arrest and photosynthesis inhibition, especially for DS190. About 61.38% and 48.78% differentially expressed genes (DEGs) were up-regulated in DT 43 under PEG and soil drought stresses, respectively. Moreover, 74.31% and 54.59% DEGs were up-regulated in DS 190 under PEG and soil drought stresses, respectively. Most DEGs in DT 43 were significantly enriched in hormone signal transduction, mitogen-activated protein kinase (MAPK) signaling, and carbon metabolism pathways. However, most DEGs in DS 190 were enriched in plant photosynthesis, chlorophyll metabolism, and nitrogen metabolism pathways. Moreover, melatonin enhanced the drought resistance of the two genotypes, increasing photosynthetic and antioxidant enzyme activity and thus mitigating transcription response. Therefore, these unique mechanisms of enhancing drought resistance can improve bioenergy crops, especially for the cultivation of drought-tolerant varieties.