GROWTH REGULATING FACTOR5 stimulates Arabidopsis chloroplast division, photosynthesis, and leaf longevity.

Liesbeth Vercruyssen,Agnieszka Bielach,Liesbeth De Milde,Frank Van Breusegem,Vanesa B Tognetti,Judith Van Dingenen,Riet De Rycke,Nathalie Gonzalez,Dirk Inzé

doi:10.1104/pp.114.256180

Abstract

Arabidopsis (Arabidopsis thaliana) leaf development relies on subsequent phases of cell proliferation and cell expansion. During the proliferation phase, chloroplasts need to divide extensively, and during the transition from cell proliferation to expansion, they differentiate into photosynthetically active chloroplasts, providing the plant with energy. The transcription factor GROWTH REGULATING FACTOR5 (GRF5) promotes the duration of the cell proliferation period during leaf development. Here, it is shown that GRF5 also stimulates chloroplast division, resulting in a higher chloroplast number per cell with a concomitant increase in chlorophyll levels in 35S:GRF5 leaves, which can sustain higher rates of photosynthesis. Moreover, 35S:GRF5 plants show delayed leaf senescence and are more tolerant for growth on nitrogen-depleted medium. Cytokinins also stimulate leaf growth in part by extending the cell proliferation phase, simultaneously delaying the onset of the cell expansion phase. In addition, cytokinins are known to be involved in chloroplast development, nitrogen signaling, and senescence. Evidence is provided that GRF5 and cytokinins synergistically enhance cell division and chlorophyll retention after dark-induced senescence, which suggests that they also cooperate to stimulate chloroplast division and nitrogen assimilation. Taken together with the increased leaf size, ectopic expression of GRF5 has great potential to improve plant productivity.

Highlights

Arabidopsis (Arabidopsis thaliana) leaf development relies on subsequent phases of cell proliferation and cell expansion
Several CYTOKININ RESPONSE FACTORS (CRFs) were identified as immediate-early cytokinin response targets, which interact with the ARABIDOPSIS HISTIDINE PHOSPHOTRANSFER proteins (AHPs) and in turn regulate the transcription of a large portion of cytokinin response genes, many of which are differentially regulated by B-type ARABIDOPSIS RESPONSE REGULATORS (ARRs) (Rashotte et al, 2006; Cutcliffe et al, 2011; Brenner et al, 2012)
Besides being larger (Horiguchi et al, 2005; Gonzalez et al, 2010), a remarkable feature of the leaves of Arabidopsis plants constitutively overexpressing GROWTH REGULATING FACTOR5 (GRF5) (35S: GRF5) is their darker green color compared with wildtype leaves, which can be observed when the plants are grown both in vitro and in soil (Fig. 1A)

Summary

Introduction

Arabidopsis (Arabidopsis thaliana) leaf development relies on subsequent phases of cell proliferation and cell expansion. The B-type ARRs function as transcription factors that induce the expression of the primary cytokinin response genes, including the A-type ARRs (Mason et al, 2005; Kim et al, 2006; Taniguchi et al, 2007; Argyros et al, 2008; Brenner et al, 2012). The latter are negative feedback regulators of cytokinin signaling (Kiba et al, 2003; To et al, 2004; Dortay et al, 2006; Lee et al, 2008). Several CYTOKININ RESPONSE FACTORS (CRFs) were identified as immediate-early cytokinin response targets, which interact with the AHPs and in turn regulate the transcription of a large portion of cytokinin response genes, many of which are differentially regulated by B-type ARRs (Rashotte et al, 2006; Cutcliffe et al, 2011; Brenner et al, 2012)

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Conclusion