Cytokinins Stimulate Plasmodesmatal Transport in Leaves.

Wilson Horner,Jacob O Brunkard

doi:10.3389/fpls.2021.674128

Abstract

Plant cells are connected by plasmodesmata (PD), nanoscopic channels in cell walls that allow diverse cytosolic molecules to move between neighboring cells. PD transport is tightly coordinated with physiology and development, although the range of signaling pathways that influence PD transport has not been comprehensively defined. Several plant hormones, including salicylic acid (SA) and auxin, are known to regulate PD transport, but the effects of other hormones have not been established. In this study, we provide evidence that cytokinins promote PD transport in leaves. Using a green fluorescent protein (GFP) movement assay in the epidermis of Nicotiana benthamiana, we have shown that PD transport significantly increases when leaves are supplied with exogenous cytokinins at physiologically relevant concentrations or when a positive regulator of cytokinin responses, ARABIDOPSIS HISTIDINE PHOSPHOTRANSFER PROTEIN 5 (AHP5), is overexpressed. We then demonstrated that silencing cytokinin receptors, ARABIDOPSIS HISTIDINE KINASE 3 (AHK3) or AHK4 or overexpressing a negative regulator of cytokinin signaling, AAHP6, significantly decreases PD transport. These results are supported by transcriptomic analysis of mutants with increased PD transport (ise1–4), which show signs of enhanced cytokinin signaling. We concluded that cytokinins contribute to dynamic changes in PD transport in plants, which will have implications in several aspects of plant biology, including meristem patterning and development, regulation of the sink-to-source transition, and phytohormone crosstalk.

Highlights

Plasmodesmata (PD) are narrow, membrane-lined channels in plant cell walls that connect the cytosols of neighboring cells (Brunkard and Zambryski, 2017; Faulkner, 2018; Azim and Burch-Smith, 2020)
Based on the previous finding that the cytokinin signaling inhibitor, ARABIDOPSIS HISTIDINE PHOSPHOTRANSFER PROTEIN 6 (AHP6), is severely transcriptionally repressed in ise1 and ise2 (Burch-Smith et al, 2011), we explored whether the transcriptomes of ise mutants reveal any clear changes in the gene expression that could reflect enhanced cytokinin signaling, which would support the hypothesis that cytokinin signaling could contribute to the ise phenotype
We demonstrated that manipulating the cytokinin signaling pathway produces consistent observable effects on green fluorescent protein (GFP) movement in the leaf epidermal tissue

Summary

Introduction

Plasmodesmata (PD) are narrow, membrane-lined channels in plant cell walls that connect the cytosols of neighboring cells (Brunkard and Zambryski, 2017; Faulkner, 2018; Azim and Burch-Smith, 2020). Cytokinins Promote PD Transport (Kim et al, 2002; Xu et al, 2012). These screens led to the discovery and characterization of five mutants so far: four with increased PD trafficking (ise1–ise4; Kobayashi et al, 2007; Stonebloom et al, 2009; Burch-Smith and Zambryski, 2010; Burch-Smith et al, 2011; Brunkard et al, 2020) and one with decreased PD trafficking (dse1; Xu et al, 2012)

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