Abstract
The current knowledge regarding transcriptomic changes induced by alkalinity on plants is scarce and limited to studies where plants were subjected to the alkaline salt for periods not longer than 48 h, so there is no information available regarding the regulation of genes involved in the generation of a new homeostatic cellular condition after long-term alkaline stress. Lotus japonicus is a model legume broadly used to study many important physiological processes including biotic interactions and biotic and abiotic stresses. In the present study, we characterized phenotipically the response to alkaline stress of the most widely used L. japonicus ecotypes, Gifu B-129 and MG-20, and analyzed global transcriptome of plants subjected to 10 mM NaHCO3 during 21 days, by using the Affymetrix Lotus japonicus GeneChip®. Plant growth assessment, gas exchange parameters, chlorophyll a fluorescence transient (OJIP) analysis and metal accumulation supported the notion that MG-20 plants displayed a higher tolerance level to alkaline stress than Gifu B-129. Overall, 407 and 459 probe sets were regulated in MG-20 and Gifu B-129, respectively. The number of probe sets differentially expressed in roots was higher than that of shoots, regardless the ecotype. Gifu B-129 and MG-20 also differed in their regulation of genes that could play important roles in the generation of a new Fe/Zn homeostatic cellular condition, synthesis of plant compounds involved in stress response, protein-degradation, damage repair and root senescence, as well as in glycolysis, gluconeogenesis and TCA. In addition, there were differences between both ecotypes in the expression patterns of putative transcription factors that could determine distinct arrangements of flavonoid and isoflavonoid compounds. Our results provided a set of selected, differentially expressed genes deserving further investigation and suggested that the L. japonicus ecotypes could constitute a useful model to search for common and distinct tolerance mechanisms to long-term alkaline stress response in plants.
Highlights
Plant salt stress represents a large economic problem worldwide, which has been the subject of countless studies
When stress treatment was extended to 2 months, some alkalinized Gifu B-129 plants died while MG-20 looked well adapted
The most highly induced of these transcripts in MG-20 is similar to an Arabidopsis nitrate transporter (AT1G59740) which functions in nitrate removal from xylem sap [58]. These results suggest a better nitrate assimilation in MG-20 compared to Gifu B-129, which is in line with the fact that Gifu B-129 plants experienced a higher level of active Fe-deprivation than MG-20 ones, since nitrate acquisition was shown to be limited under Fe deficiency in cucumber [59]
Summary
Plant salt stress represents a large economic problem worldwide, which has been the subject of countless studies. Soils owing its alkalinity to high Na2CO3 and NaHCO3 contents are extended throughout practically all climatic regions. In these soils, the stressor factors for plant growth are high pH, high exchangeable sodium percent, poor fertility, dispersed physical properties, and very low water infiltration capacity. The same technique has allowed the identification of genes that respond to alkalinity in Glycine soja [6,7], Glycine max [8], Leymus chinensis [9], Puccinellia tenuiora [10,11] and Limonium bicolor [12]
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