Root engineering in maize by increasing cytokinin degradation causes enhanced root growth and leaf mineral enrichment

Eswarayya Ramireddy,Jan Erik Leuendorf,Mieke Van Lijsebettens,Hilde Nelissen,Dirk Inzé,Thomas Schmülling

doi:10.1007/s11103-021-01173-5

Abstract

Key messageRoot-specific expression of a cytokinin-degrading CKX gene in maize roots causes formation of a larger root system leading to higher element content in shoot organs.The size and architecture of the root system is functionally relevant for the access to water and soil nutrients. A great number of mostly unknown genes are involved in regulating root architecture complicating targeted breeding of plants with a larger root system. Here, we have explored whether root-specific degradation of the hormone cytokinin, which is a negative regulator of root growth, can be used to genetically engineer maize (Zea mays L.) plants with a larger root system. Root-specific expression of a CYTOKININ OXIDASE/DEHYDROGENASE (CKX) gene of Arabidopsis caused the formation of up to 46% more root dry weight while shoot growth of these transgenic lines was similar as in non-transgenic control plants. The concentration of several elements, in particular of those with low soil mobility (K, P, Mo, Zn), was increased in leaves of transgenic lines. In kernels, the changes in concentration of most elements were less pronounced, but the concentrations of Cu, Mn and Zn were significantly increased in at least one of the three independent lines. Our data illustrate the potential of an increased root system as part of efforts towards achieving biofortification. Taken together, this work has shown that root-specific expression of a CKX gene can be used to engineer the root system of maize and alter shoot element composition.

Highlights

Roots fulfil important functions for plants, including anchoring in the soil and providing access to soil nutrients and water
Generation of transgenic maize plants with enhanced CYTOKININ OXIDASE/DEHYDROGENASE (CKX) gene expression in roots We chose the RCc3 promoter of rice (Xu et al 1995) to achieve root-specific expression of the Arabidopsis CKX1 gene in the maize inbred line B104 (Coussens et al 2012)
The CKX1 gene was chosen as its root-specific expression enhanced root growth in tobacco, Arabidopsis (Werner et al 2010) and barley (Ramireddy et al 2018a)

Summary

Introduction

Roots fulfil important functions for plants, including anchoring in the soil and providing access to soil nutrients and water. Only eight maize genes were identified that are involved in regulating root growth and development but their individual relevance for plant performance is as yet not clear (Hochholdinger et al 2018). CKX transgenic plants with a larger root system were shown to respond less sensitive than the cognate wild-type plants to drought (Werner et al 2010; Ramireddy et al 2018a) underpinning the beneficial effect of a larger root system under water deficit (Comas et al 2013; Gao and Lynch 2016; Klein et al 2020). Transgenic maize lines formed a larger root system without reducing shoot growth demonstrating the potential of root engineering in maize The shoots of these plants contained higher concentrations of several essential elements underpinning the role of cytokinin in regulating mineral nutrition

Results

Discussion

B9 C4 RCc3:CKX1 lines

Experimental procedures