Understanding physiological and molecular mechanisms of citrus rootstock seedlings in response to root zone hypoxia by RNA-Seq

Citrus plants are sensitive to waterlogging because it frequently causes root zone hypoxia (RZH), a serious abiotic stress to its growth. As root is the first organ to be affected by hypoxia stress, its ability to sense and respond to hypoxia stress is crucial to survive. In this study, the response of citrus plant to RZH stress was investigated at morphological, physiological and molecular levels. Our data showed that RZH stress significantly decreased root length, dry weight, and the number of root tips, and not obviously affected shoot growth. Under RZH stress, the malondialdehyde (MDA) contents in leaf and root increased, and the enzyme activities of SOD, POD and APX significantly enhanced in root, while in leaf just POD activity increased with an opposite trend to CAT. RZH stress increased IAA and decreased ABA contents in root, and in leaf all hormones including IAA, ABA and GAs were increased. Despite the leaf chlorophyll content unchanged, several photosynthesis-related parameters were significantly affected by RZH stress, including ABS/RC, TRo/RC and DIo/RC etc. The root soluble sugar and lignin contents were remarkably decreased and increased by RZH stress, respectively. Besides these, RZH stress also reduced nutrient uptake, reflecting in the root decreased Fe, Mn and Zn contents. RNA-Seq analysis revealed that most of the genes were up-regulated by RZH stress, and the differentially expressed genes (DEGs) mainly linked to carbon metabolism, reactive oxygen species scavenging, nutrient uptake and transport, and hormone synthesis/signaling. Taken together, our data obtained in this study will be helpful in further studies regarding hypoxia tolerance in citrus or other crops.