Abstract
Climate change with increasing periods of drought is expected to reduce the yield of biomass crops such as poplars. To combat yield loss, it is important to better understand the molecular mechanisms that control growth under drought. Here, the goal was to resolve the drought-induced changes of active cytokinins, a main growth hormone in plants, at the tissue level in different cell types and organs of poplars (Populus × canescens) in comparison with growth, biomass, leaf shedding, photosynthesis and water potential. Since cytokinin response is mediated by type-A response regulators, ARR5::GUS reporter lines were used to map cytokinin activity histochemically. The expression of PtaRR3 and PtaRR10 was examined in different stem sections. Young leaves showed strong cytokinin activity in the veins and low staining under drought stress, accompanied by diminished leaf expansion. Leaf scars, at positions where drought-shedding occurred, showed strong reduction of cytokinin activity. The pith in the differentiation zone of stem showed high cytokinin activity with distinct, very active parenchymatic cells and enhanced activity close to primary xylem. This pattern was maintained under drought but the cytokinin activity was reduced. Mature phloem parenchymatic cells showed high cytokinin activity and mature wood showed no detectable cytokinin activity. Cytokinin activity in the cambium was apparent as a clear ring, which faded under drought. Xylem-localized cytokinin activities were also mirrored by the relative expression of PtaRR3, whereas PtaRR10 showed developmental but no drought-induced changes. Primary meristems exhibited high cytokinin activity regardless of drought stress, supporting a function of this phytohormone in meristem maintenance, whereas declining cytokinin activities in apical pith tissues and cambium of drought-stressed poplars linked cytokinin in these cell types with the control of primary and secondary growth processes. Changes in cytokinin activity further imply a role in drought avoidance mechanisms of poplars, especially in the reduction of leaf area.
Highlights
Cytokinins form a group of phytohormones that have roles in promoting cell division, shoot initiation, growth and the regulation of vascular development (Hwang et al 2012; Kieber and Schaller 2014)
Our results suggest that the cytokinin activity was kept up in the buds under drought stress, most likely because cytokinins have an important role in protecting and maintaining the shoot apical meristem (Werner et al 2001)
The tissue- and cell-specific localization is attributable to different functions of cytokinins: in the primary meristems high cytokinin activities under drought support a role of this phytohormone in meristem maintenance (Leibfried et al 2005)
Summary
Cytokinins form a group of phytohormones that have roles in promoting cell division, shoot initiation, growth and the regulation of vascular development (Hwang et al 2012; Kieber and Schaller 2014). Cytokinin perception and signalling in plants involves a His-Asp phosphorelay mediating the transmission of the signal (Mizuno 2005; Schaller et al 2008) In this pathway, type-A ARRs (Arabidopsis response regulators) are the primary response genes for cytokinins in Arabidopsis (D’Agostino et al 2000). A total of type-A RR genes are present in Arabidopsis (D’Agostino et al 2000; Schaller et al 2008; Pils and Heyl 2009) and were reported in Populus trichocarpa (Ramírez-Carvajal et al 2008; Immanen et al 2013) The expression of these genes is transcriptionally regulated and induced by exogenous cytokinin (Taniguchi et al 1998; D’Agostino et al 2000; Paul et al 2016)
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