Abstract
BackgroundPlant roots release a variety of organic compounds into the soil which alter the physical, chemical and biological properties of the rhizosphere. Root exudates are technically challenging to measure in soil because roots are difficult to access and exudates can be bound by minerals or consumed by microorganisms. Exudates are easier to measure with hydroponically-grown plants but, even here, simple compounds such as sugars and organic acids can be rapidly assimilated by microorganisms. Sterile hydroponic systems avoid this shortcoming but it is very difficult to maintain sterility for long periods especially for larger crop species. As a consequence, studies often use small model species such as Arabidopsis to measure exudates or use seedlings of crop plants which only have immature roots systems.ResultsWe developed a simple hydroponic system for cultivating large crop plants in sterile conditions for more than 30 days. Using this system wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.) plants were grown in sterile conditions for 30 days by which time they had reached the six-leaf stage and developed mature root systems with seminal, nodal and lateral roots. To demonstrate the utility of this system we characterized the aluminium-activated exudation of malate from the major types of wheat roots for the first time. We found that all root types measured released malate but the amounts were two-fold greater from the seminal and nodal axile roots compared with the lateral roots. Additionally, we showed that this sterile growth system could be used to collect exudates from intact whole root systems of barley.ConclusionsWe developed a simple hydroponic system that enables cereal plants to be grown in sterile conditions for longer periods than previously recorded. Using this system we measured, for the first time, the aluminium-activated efflux of malate from the major types of wheat roots. We showed the system can also be used for collecting exudates from intact root systems of 30-day-old barley plants. This hydroponic system can be modified for various purposes. Importantly it enables the study of exudates from crop species with mature root systems.
Highlights
Plant roots release a variety of organic compounds into the soil which alter the physical, chemical and biological properties of the rhizosphere
TaALMT1 encodes an anion channel from the aluminium-activated malate transporter family that releases malate anions [7] while TaMATE1B encodes a transporter from the multidrug and toxic compound exudation (MATE) family that releases citrate anions [8, 9]
A system for growing large crop plants under sterile conditions for 30 days A hydroponic system was developed for growing cereals such as wheat and barley under axenic conditions for at least 30 days, which was sufficient for plants to develop mature root systems comprising seminal, nodal and lateral roots
Summary
Plant roots release a variety of organic compounds into the soil which alter the physical, chemical and biological properties of the rhizosphere. Cereal plants are estimated to release approximately 11% of their net fixed carbon from the roots as rhizodeposits [1] These deposits include sloughed off border cells and tissues, mucilage, volatile organic molecules and exudates comprised of high and low molecular weight compounds such as sugars, amino acids, phenolics, and organic anions [1,2,3]. A well-studied example is the release of organic anions from root apices in response to the toxic aluminium (Al3+) ions prevalent in acid soils. These anions protect the sensitive root apices by chelating the toxic Al3+ cations in the apoplast of root cells to form less-harmful complexes [6]. TaALMT1 requires soluble Al3+ cations to activate malate release at low pH, whereas citrate release via TaALMT1B is constitutive and independent of external pH or Al3+ concentration
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