Time-resolution of the shoot and root growth of the model cereal Brachypodium in response to inoculation with Azospirillum bacteria at low phosphorus and temperature

Martino Schillaci,Kerstin A Nagel,Ute Roessner,Penelope M C Smith,Robert Walker,Michelle Watt,Borjana Arsova

doi:10.1007/s10725-020-00675-4

Abstract

A non-invasive plant phenotyping platform, GrowScreen-PaGe, was used to resolve the dynamics of shoot and root growth of the model cereal Brachypodium (Brachypodium distachyon Bd21-3) in response to the plant growth promoting (PGP) bacteria Azospirillum (Azospirillum brasilense Sp245). Inoculated Brachypodium plants had greater early vigor and higher P use efficiency than non-inoculated Brachypodium at low P and low temperature conditions. Root systems were imaged non-invasively at eight time points and data combined with leaf area, shoot biomass and nutrient content from destructive subsamples at 7, 14 and 21 days after inoculation (DAI). Azospirillum colonisation of roots improved Brachypodium shoot and, to a greater degree, root growth in three independent experiments. Inoculation promoted P use efficiency in shoots but not P concentration or uptake, despite increased total root length. Longer roots in inoculated plants arose from twofold faster branch root growth but slower axile root growth, detected at 11 DAI. Analysis of the spatio-temporal phenotypes indicated that the effects of Azospirillum inoculation increased as shoot P concentration declined, but the magnitude depended on the time after inoculation and growth rate of branch roots compared to axile roots. High throughput plant phenotyping platforms allow the details of plant-microorganism symbioses to be resolved, offering insights into the timing of changes in different tissues to allow molecular mechanisms to be determined.

Highlights

Cereals are consumed in all societies and retain a leading position in diets by providing almost one half of the calories consumed by humans (FAO 2018)
The DNA extraction, amplification with strain specific A. brasilense Sp245 specific primers and sequencing of the obtained amplicons confirmed that in the pilot experiments and at 7, 14 and 21 days after inoculation (DAI) in the high throughput experiment (Online resource 1) bacteria-inoculated samples contained A. brasilense Sp245 DNA, while no products were amplified from DNA extracted from the non-inoculated samples
Inoculated plants supplied with lower P (7 μM KH2PO4) had 17% greater leaf area at 21 DAI than inoculated plants provided with 25 μM KH2PO4

Summary

Introduction

Cereals are consumed in all societies and retain a leading position in diets by providing almost one half of the calories consumed by humans (FAO 2018). Increasing agricultural production while availability of arable land decreases is one of the largest challenges facing modern agriculture, for cereal crops. Good soil fertility depends on availability of phosphorus (P) for crop growth. P is essential for a multitude of plant processes and is a significant limitation to production in many areas of the world due to decreasing availability of rock mineral phosphates, leading to increasing market price (Cordell et al 2011). Phosphorus use efficiency by cereals is typically low, with a large proportion of P applied to a crop as fertilizer creating bonds with other elements such as Ca, Fe and Al, becoming unavailable for plant uptake (Dhillon et al 2017). Many cereals germinate in winter, and the early stages of their growth often are

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