Salt stress alters root meristem definition, vascular differentiation and metabolome in Sorghum bicolor (L.) genotypes

Knowledge on salt tolerance requires further investigation, particularly in plants of agro-food interest. Sorghum is a potentially useful plant because it is a emerging food species that combines high levels of salt tolerance with interesting nutritional characteristics. In sorghum different genotypes respond differently to saline stress and the early events characterizing the salt stress tolerance are not yet fully understood. Moreover, the number of salt resistant genotypes needs to be extended. The genotypes Bianca and Tonkawa are two possible candidates for extending sorghum cultivation to soils characterized by high levels of salinity. The root is the first organ that responds to soil conditions, especially during the initial stages of plant developmental. The research aim was to analyse the root system responses to salt stress (NaCl) of Bianca and Tonkawa genotypes to identify the morpho-functional and metabolic changes that occur during the initial stages of the root system development and to use them as discriminating parameters for assessing the different plant’s susceptibility to the salt. The results showed that salt stress negatively affected many morphological and cyto-histological root parameters, from seed germination to root system establishment. The salt altered the root meristem organization and quiescent centre (QC) definition, but similarly in both genotypes. By contrast, it reduced primary root (PR) length and induced a more extended oxidative stress in the adventitious roots (ARs) and lateral root primordia (LRPs) of Tonkawa in comparison with Bianca. The stele area and the number of protoxylem and phloem elements in the ARs were more reduced in 150 mM NaCl-treated Tonkawa seedlings in comparison with those of Bianca. Moreover, the salt enhanced lignin deposition in protoxylem, early metaxylem and endodermis and changed the root metabolic profiles significantly increasing the levels of leucine, isoleucine, alanine, proline, trigonelline, allantoin and glutamine in Bianca compared to Tonkawa. Altogether, specific morpho-anatomical and metabolic differences between the genotypes were identified as discriminating markers of genotype salt susceptibility.