Overexpressing the HD-Zip class II transcription factor EcHB1 from Eucalyptus camaldulensis increased the leaf photosynthesis and drought tolerance of Eucalyptus

Keisuke Sasaki,Yuuki Ida,Tetsu Kawazu,Yuko T. Hanba,Takashi Hibino,Sakihito Kitajima

doi:10.1038/s41598-019-50610-5

Keisuke Sasaki, Yuuki Ida + Show 4 more

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https://doi.org/10.1038/s41598-019-50610-5

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Abstract

Alteration in the leaf mesophyll anatomy by genetic modification is potentially a promising tool for improving the physiological functions of trees by improving leaf photosynthesis. Homeodomain leucine zipper (HD-Zip) transcription factors are candidates for anatomical alterations of leaves through modification of cell multiplication, differentiation, and expansion. Full-length cDNA encoding a Eucalyptus camaldulensis HD-Zip class II transcription factor (EcHB1) was over-expressed in vivo in the hybrid Eucalyptus GUT5 generated from Eucalyptus grandis and Eucalyptus urophylla. Overexpression of EcHB1 induced significant modification in the mesophyll anatomy of Eucalyptus with enhancements in the number of cells and chloroplasts on a leaf-area basis. The leaf-area-based photosynthesis of Eucalyptus was improved in the EcHB1-overexpression lines, which was due to both enhanced CO2 diffusion into chloroplasts and increased photosynthetic biochemical functions through increased number of chloroplasts per unit leaf area. Additionally, overexpression of EcHB1 suppressed defoliation and thus improved the growth of Eucalyptus trees under drought stress, which was a result of reduced water loss from trees due to the reduction in leaf area with no changes in stomatal morphology. These results gave us new insights into the role of the HD-Zip II gene.

Highlights

One of the promising measures to improve leaf photosynthesis is to modify the leaf mesophyll anatomy
Alterations in some of the tree phenotypes were obtained for the two transgenic Eucalyptus lines EcHB1-2 and EcHB1-10 compared to the control line GUT5 (Fig. 1B)
Significant enhancements in the area-based leaf photosynthetic functions were obtained in Eucalyptus trees overexpressing EcHB1 (Fig. 3)

Summary

Introduction

One of the promising measures to improve leaf photosynthesis is to modify the leaf mesophyll anatomy. We focused on EcHB1 from Eucalyptus as a candidate for improving tree photosynthesis, growth, and drought tolerance through changes in the leaf anatomy. In our previous study[15], we generated Eucalyptus plants overexpressing EcHB1 under the control of a CaMV 35 S promoter, and reported an increase in the plant growth and alterations in flower morphology that suggest functional roles of EcHB1 on the cell growth in multiple plant organs including the leaves. The aims of the present study are to test the hypothesis that overexpression of EcHB1, an HD-Zip II transcription factor from Eucalyptus camaldulensis, improves the leaf photosynthetic function, growth, and drought tolerance through alterations in the mesophyll anatomy in Eucalyptus trees. The number of leaves and growth in response to soil drought were examined for assessing alteration in the drought tolerance of Eucalyptus trees through overexpression of EcHB1

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