Root architecture of Medicago Truncatula: response to drought of the primary and secondary root

Abstract

Forage legumes are important in agricultural systems as feed sources for livestock and raw food materials for humans. Medicago truncatula, considered as a model legume, is phylogenetically related to some of the most economically important European legume crops. However, legumes show inconsistent production rates, mostly due to abiotic factors and particularly drought (Zahran, 1999), one of the major factor affecting crop productivity. The more severe the water deficit, the larger the number of cellular processes affected, starting with inhibition of plant cell growth, followed by inhibition of cell division, cell wall formation and protein biosynthesis (Hsiao, 1973). The second most important consequence of decreased water availability is a reduction of leaf C fixation due to either CO2 diffusion and/or metabolic disruption processes. Due to the importance of the photosynthetic process for ecosystem productivity, plant research has been highly focused to photosynthetic aboveground organs and only in the last decades root research is emerging. The present study aims to analyze the response of the root of M. truncatula to drought stress. To do so, root architecture analysis and metabolic profiling of primary and secondary roots were carried out. The primary root system of legumes has been described as a prominent, central taproot that penetrates to different depths in unrestricted soil. However, M. truncatula taproot become early branched with numerous lateral roots. Under controlled conditions, the taproot develops at the upper part of the root and represents around 5-10% of the root biomass in plants at the late vegetative stage. Preliminary results suggest that the primary and secondary root respond differently to drought. Total carbon (C) and nitrogen (N) content was not affected in the primary root of drought stressed plants when a significant decline was observed in the secondary roots. The C decline agrees with the reduction of the starch in drought stress secondary roots and the accumulation of soluble C compounds as sucrose, glucose, malate, and citrate. However, the decline of N content could not be explained in base to the soluble protein content which was unaffected. Besides, a general accumulation of amino acids was detected in the secondary roots of drought stressed plants. Amino acids, sugars and organic acids, are compatible solutes which can exert a role on turgor maintenance and also stabilizing proteins and cell structures under stress conditions. Response to drought of the different root parts of this forage legume will be discussed.