Cytokinin Regulates the Activity of Reproductive Meristems, Flower Organ Size, Ovule Formation, and Thus Seed Yield inArabidopsis thaliana     

Abstract

The size and activity of the shoot apical meristem is regulated by transcription factors and low molecular mass signals, including the plant hormone cytokinin. The cytokinin status of the meristem depends on different factors, including metabolic degradation of the hormone, which is catalyzed by cytokinin oxidase/dehydrogenase (CKX) enzymes. Here, we show that CKX3 and CKX5 regulate the activity of the reproductive meristems of Arabidopsis thaliana. CKX3 is expressed in the central WUSCHEL (WUS) domain, while CKX5 shows a broader meristematic expression. ckx3 ckx5 double mutants form larger inflorescence and floral meristems. An increased size of the WUS domain and enhanced primordia formation indicate a dual function for cytokinin in defining the stem cell niche and delaying cellular differentiation. Consistent with this, mutation of a negative regulator gene of cytokinin signaling, ARABIDOPSIS HISTIDINE PHOSPHOTRANSFER PROTEIN 6, which is expressed at the meristem flanks, caused a further delay of differentiation. Terminal cellular differentiation was also retarded in ckx3 ckx5 flowers, which formed more cells and became larger, corroborating the role of cytokinin in regulating flower organ size. Furthermore, higher activity of the ckx3 ckx5 placenta tissue established supernumerary ovules leading to an increased seed set per silique. Together, the results underpin the important role of cytokinin in reproductive development. The increased cytokinin content caused an ;55% increase in seed yield, highlighting the relevance of sink strength as a yield factor.

Cytokinins - recent news and views of evolutionally old molecules

“…higher-order mutants of His phosphotransfer proteins (ahp1, -2,-3, -4 and -5; Hutchison et al 2006) and Type B response regulators (arr1, -10 and -12; Mason et al 2005), or plants expressing 35S:ARR1-SRDX, a dominant repressor of response regulator ARR1 (Heyl et al 2008)). Mutation of particular CKX isoforms leading to flower organspecific enhancement of CK levels confirmed the role of CK in regulation of flower and reproductive development through control of the size and activity of floral meristems and placenta (Bartrina et al 2011). Ectopic overexpression of the CK-activating enzyme LOG showed pleiotropic phenotypes, such as promotion of cell division in embryos and leaf vascular tissues, reduced apical dominance and a delay of leaf senescence (Kuroha et al 2009).…”

“…Structural insights into how CK bindis to the CRE1/AHK4 receptor (Hothorn et al 2011) should accelerate the search for CK receptor antagonists that might have interesting applications in agriculture Nisler et al 2010) as root-growth promoting substances (Arata et al 2010). Similarly, due to the finding that CKX activity is a determinant of generative organ development and yield (Ashikari et al 2005;Bartrina et al 2011), great potential lies ahead for the practical use of deregulation of CK degradation by transgenic approaches or by chemical inhibition of CKX activity. CK perception and metabolism are involved in response to various biotic and abiotic stresses, which opens new possibilities for agricultural applications to increase the stress tolerance.…”

Arabidopsis ABCG14 protein controls the acropetal translocation of root-synthesized cytokinins

“…Cytokinins delay leaf senescence, thereby extending the photosynthetic period 5,6,21 and enhancing the plant's photosynthate production capacities. They also play key roles in regulating reproductive development and sink strength 37,38 ; thus, they are among the most important target hormones for improving crop yields. Furthermore, they are pivotal signals in communicating nitrogen availability from roots to shoots via the xylem vessels 10,33 .…”

Endoplasmic reticulum: Where nucleotide sugar transport meets cytokinin control mechanisms

“…Elevated levels of endogenous cytokinins and cytokinin response genes in rock1 inflorescences suggest that ROCK1 regulates the cytokinin activity in meristematic cells. Furthermore, as very similar changes in inflorescence meristem activity have been described for the ckx3 ckx5 mutant, 29 it is plausible that CKX isoforms expressed specifically in the shoot meristem are direct targets of ROCK1 under physiological conditions. However, this needs to be tested experimentally.…”