Plant physiology and proteomics reveals the leaf response to drought in alfalfa (Medicago sativa L.)

Abstract

Despite its relevance, protein regulation, metabolic adjustment, and the physiological status of plants under drought is not well understood in relation to the role of nitrogen fixation in nodules. In this study, nodulated alfalfa plants were exposed to drought conditions. The study determined the physiological, metabolic, and proteomic processes involved in photosynthetic inhibition in relation to the decrease in nitrogenase (Nase) activity. The deleterious effect of drought on alfalfa performance was targeted towards photosynthesis and Nase activity. At the leaf level, photosynthetic inhibition was mainly caused by the inhibition of Rubisco. The proteomic profile and physiological measurements revealed that the reduced carboxylation capacity of droughted plants was related to limitations in Rubisco protein content, activation state, and RuBP regeneration. Drought also decreased amino acid content such as asparagine, and glutamic acid, and Rubisco protein content indicating that N availability limitations were caused by Nase activity inhibition. In this context, drought induced the decrease in Rubisco binding protein content at the leaf level and proteases were up-regulated so as to degrade Rubisco protein. This degradation enabled the reallocation of the Rubisco-derived N to the synthesis of amino acids with osmoregulant capacity. Rubisco degradation under drought conditions was induced so as to remobilize Rubisco-derived N to compensate for the decrease in N associated with Nase inhibition. Metabolic analyses showed that droughted plants increased amino acid (proline, a major compound involved in osmotic regulation) and soluble sugar (D-pinitol) levels to contribute towards the decrease in osmotic potential (Ψs). At the nodule level, drought had an inhibitory effect on Nase activity. This decrease in Nase activity was not induced by substrate shortage, as reflected by an increase in total soluble sugars (TSS) in the nodules. Proline accumulation in the nodule could also be associated with an osmoregulatory response to drought and might function as a protective agent against ROS. In droughted nodules, the decrease in N2 fixation was caused by an increase in oxygen resistance that was induced in the nodule. This was a mechanism to avoid oxidative damage associated with reduced respiration activity and the consequent increase in oxygen content. This study highlighted that even though drought had a direct effect on leaves, the deleterious effects of drought on nodules also conditioned leaf responsiveness.

Effect of temporary drought at different growth stages on snap bean pod quality and yield

“…Increased or decreased levels of protein in response to drought stress depend on plant species and organ (Sharma and Dubey, 2011). Drought stress increased protein concentrations in chickpea shoot (Rai et al, 1983), the whole barley plant (Bole and Pittman, 1978), in tomato leaves (Chao et al, 1999), and in the alfalfa shoot (Aranjuelo et al, 2011). However, levels of protein were reduced in immature and mature pods of broad bean (Ouzounidou et al, 2014), leaves of common bean (Lazcano-Ferrat and Lovatt, 1999), the wheat plant (Kulshrestha et al, 1987) and in shoots and roots of rice (Sharma and Dubey, 2005).…”

Photosynthetic carbon and nitrogen metabolism and the relationship between their metabolites and lipid peroxidation in dwarf bamboo (Fargesia rufa Yi) during drought and subsequent recovery

“…Moreover, a decline in the activities of nitrate reductase (NR) and glutamine synthetase (GS) was observed under drought (Xu and Zhou 2006). Although recent studies refer to a more detailed characterization of C and N metabolism (Aranjuelo et al 2011;Kang et al 2011), changes in key enzyme activities and metabolites involved in C and N metabolism induced by drought are still being debated due to plant species (Sánchez-Rodríguez et al 2011), stress intensity, and duration Zhou 2006, 2007). Many metabolites are believed simply to accumulate under drought conditions (Muller et al 2011), and metabolite patterns remain unclear.…”

Alfalfa and Its Symbiosis Responses to Osmotic Stress

“…This stress afects not only the biomass of the nodules but also their functioning. In addition, several studies have reported that biological N 2 ixing (BNF) is conditioned by the active interaction of nodules with the leaves and roots [40][41][42].…”