An integrated approach to study the diversity in molecular mechanisms of salt tolerance in tropical maize
Maize is one of the most produced crops in the world, ranking second in 2017. However, despite having higher genetic diversity than temperate germplasm, tropical maize production and research has been clearly neglected, especially by the private sector. Furthermore, production of maize is constantly confronted by the presence of biotic and abiotic stresses, including salinity, which already affects 19.5% of irrigated lands. Salinity in maize impacts all developmental stages with different strength. Tolerance is dependent on multiple biochemical and physiological pathways. Thus, it is important to understand and quantify the disorders and responses, which allows in a later stage the detection of salt tolerant plants. Results obtained over time can point out critical moments for molecular analysis, such as proteomics. In this context, our general aim was to develop a simple phenotyping protocol to characterize tropical genotypes. For that, we characterized the stress in different tropical maize genotypes to understand the reactions at different levels.
To establish our phenotyping protocol, we focused in chapter 3 on the selection of appropriate traits that are transferrable to countries where research is less intensive (“cheap and easy phenotyping technology”). We monitored the growth of three maize genotypes in control conditions: the temperate B73, and the tropical genotypes Across8023 and Across8024. Secondly, for the establishment of an appropriate stress level, the growth of those three maize genotypes was monitored in three conditions: control, 0.6% NaCl and 1.2% NaCl. A picture from the top area allows a reliable monitoring of the same individuals through all the experimental time. It is a powerful variable and together with plant height it is a low-cost non-destructive resource for growth determination. We determined 0.6% NaCl as the most appropriate level of stress to rank our genotypes.
In chapter 4 we applied the methodology determined in chapter 3 to screen more genotypes and to get an insight into the responses at the cellular level at an early stage. In addition to the sequenced genotype B73, three tropical maize genotypes were evaluated during four weeks after sowing: CML421, CML448 and CML451. Experimental chapter 4 proved that growth monitoring in terms of top area, fresh weight and ion content during salt stress is a successful strategy to guide roots proteomics analyses and study the early reactions. Proteomics responses indicated an increase in the abundance of proteins related to cellular respiration, response to oxidative and water stress, and cell redox homeostasis in roots of salt-stressed plants.
In chapter 5 we focused on the growth and molecular characterization of the roots and the leaves of more three new genotypes (CML465, CML488 and CML491) and we investigated the molecular responses at a later stage. As molecular responses, leaf proteomics pointed to a general decrease in photosynthesis, regulation of biological quality and light harvesting ATP synthase activity for all the salt-treated plants. Roots proteomics showed that salt stress affected the protein abundance more than three times fold than leaves, being related to energy synthesis, antioxidant response and protein synthesis.
In conclusion, we used a systems-level approach uniting plant responses at different scales to classify and characterize different tropical maize genotypes towards salt stress. An overall ranking of all genotypes groups Across8023, Across8024 and CML465 as the most tolerant in salt stress, followed by CML488, B73, CML 448, CML451, CML491 and, finally, CML421 as the most sensitive. A low-budget research approach was established for maize phenotyping and molecular mechanisms of salt tolerance and stress markers were identified through the applied methodology.