Global reprogramming of transcription and metabolism in Medicago truncatula during progressive drought and after rewatering.

Medicago truncatula is a model legume forage crop native to the arid and semi-arid environments of the Mediterranean. Given its drought-adapted nature, it is an ideal candidate to study the molecular and biochemical mechanisms conferring drought resistance in plants. Medicago plants were subjected to a progressive drought stress over 14 d of water withholding followed by rewatering under controlled environmental conditions. Based on physiological measurements of plant water status and changes in morphology, plants experienced mild, moderate and severe water stress before rehydration. Transcriptome analysis of roots and shoots from control, mildly, moderately and severely stressed, and rewatered plants, identified many thousands of genes that were altered in expression in response to drought. Many genes with expression tightly coupled to the plant water potential (i.e. drought intensity) were identified suggesting an involvement in Medicago drought adaptation responses. Metabolite profiling of drought-stressed plants revealed the presence of 135 polar and 165 non-polar compounds in roots and shoots. Combining Medicago metabolomic data with transcriptomic data yielded insight into the regulation of metabolic pathways operating under drought stress. Among the metabolites detected in drought-stressed Medicago plants, myo-inositol and proline had striking regulatory profiles indicating involvement in Medicago drought tolerance.

Zhang JY ，Cruz DE Carvalho MH ，Torres-Jerez I ，Kang Y ，Allen SN ，Huhman DV ，Tang Y ，Murray J ，Sumner LW ，Udvardi MK ... -  《-》

Medicago truncatula genotypes Jemalong A17 and R108 show contrasting variations under drought stress.

Drought is one of the most significant abiotic stresses that restrict crop productivity. Medicago truncatula is a model legume species with a wide genetic diversity. We compared the differential physiological and molecular changes of two genotypes of M. truncatula (Jemalong A17 and R108) in response to progressive drought stress and rewatering. The MtNCED and MtZEP activation and higher abscisic acid (ABA) content was observed in Jemalong A17 plants under normal conditions. Additionally, a greater increase in ABA content and expression of MtNCED and MtZEP in Jemalong A17 plants than that of R108 plants were observed under drought conditions. A more ABA-sensitive stomatal closure and a slower water loss was found in excised leaves of Jemalong A17 plants. Meanwhile, Jemalong A17 plants alleviated leaf wilting and maintained higher relative water content under drought conditions. Exposed to drought stress, Jemalong A17 plants exhibited milder oxidative damage which has less H2O2 and MDA accumulation, lower electrolyte leakage and higher chlorophyll content and PSII activity. Furthermore, Jemalong A17 plants enhanced expression of stress-upregulated genes under drought conditions. These results suggest that genotypes Jemalong A17 and R108 differed in their response and adaptation to drought stress. Given the relationship between ABA and these physiological responses, the MtNCED and MtZEP activation under normal conditions may play an important role in regulation of greater tolerance of Jemalong A17 plants to drought stress. The activation of MtNCED and MtZEP may lead to the increase of ABA content which may activate expression of drought-stress-regulated genes and cause a series of physiological resistant responses.

Luo SS ，Sun YN ，Zhou X ，Zhu T ，Zhu LS ，Arfan M ，Zou LJ ，Lin HH ... -  《-》

Kresoxim-methyl primes Medicago truncatula plants against abiotic stress factors via altered reactive oxygen and nitrogen species signalling leading to downstream transcriptional and metabolic readjustment.

Biotic and abiotic stresses, such as fungal infection and drought, cause major yield losses in modern agriculture. Kresoxim-methyl (KM) belongs to the strobilurins, one of the most important classes of agricultural fungicides displaying a direct effect on several plant physiological and developmental processes. However, the impact of KM treatment on salt and drought stress tolerance is unknown. In this study we demonstrate that KM pre-treatment of Medicago truncatula plants results in increased protection to drought and salt stress. Foliar application with KM prior to stress imposition resulted in improvement of physiological parameters compared with stressed-only plants. This protective effect was further supported by increased proline biosynthesis, modified reactive oxygen and nitrogen species signalling, and attenuation of cellular damage. In addition, comprehensive transcriptome analysis identified a number of transcripts that are differentially accumulating in drought- and salinity-stressed plants (646 and 57, respectively) after KM pre-treatment compared with stressed plants with no KM pre-treatment. Metabolomic analysis suggests that the priming role of KM in drought- and to a lesser extent in salinity-stressed plants can be attributed to the regulation of key metabolites (including sugars and amino acids) resulting in protection against abiotic stress factors. Overall, the present study highlights the potential use of this commonly used fungicide as a priming agent against key abiotic stress conditions.

Filippou P ，Antoniou C ，Obata T ，Van Der Kelen K ，Harokopos V ，Kanetis L ，Aidinis V ，Van Breusegem F ，Fernie AR ，Fotopoulos V ... -  《-》

Drought-induced senescence of Medicago truncatula nodules involves serpin and ferritin to control proteolytic activity and iron levels.

Drought is a major constraint for legume growth and yield. Senescence of nitrogen-fixing nodules is one of the early drought responses and may cause nutrient stress in addition to water stress in legumes. For nodule senescence to function as part of a drought-survival strategy, we propose that the intrinsically destructive senescence process must be tightly regulated. Medicago truncatula protease inhibitor and iron scavenger-encoding genes, possibly involved in controlling nodule senescence, were identified. RNA interference (RNAi) lines were constructed in which expression of a serpin or ferritins was knocked down. Both wild-type and RNAi lines were subjected to drought stress and nodule activity and plant physiological responses were measured. Drought caused M. truncatula to initiate nodule senescence before plant growth was affected and before an increase in papain-like proteolytic activity and free iron levels was apparent. Knock-down expression of serpin6 and ferritins caused increased protease activity, free iron levels, early nodule senescence and reduced plant growth. The results suggest that M. truncatula nodule-expressed serpin6 and ferritins mediate ordered drought-induced senescence by regulating papain-like cysteine protease activity and free iron levels. This strategy may allow the drought-stressed plants to benefit maximally from residual nitrogen fixation and nutrient recovery resulting from break down of macromolecules.

Dhanushkodi R ，Matthew C ，McManus MT ，Dijkwel PP ... -  《-》

Transcriptional profiling of Medicago truncatula under salt stress identified a novel CBF transcription factor MtCBF4 that plays an important role in abiotic stress responses.

Li D ，Zhang Y ，Hu X ，Shen X ，Ma L ，Su Z ，Wang T ，Dong J ... -  《BMC PLANT BIOLOGY》