Cytokinin glucosyl transferases, key regulators of cytokinin homeostasis, have potential value for wheat improvement

Abstract

The cytokinins, which are N 6 -substituted adenine derivatives, control key aspects of crop productivity. Cytokinin levels are controlled via biosynthesis by isopentenyl transferase (IPT), destruction by cytokinin oxidase/dehydrogenase (CKX), and inactivation via glucosylation by cytokinin glucosyl transferases (CGTs). While both yield components and tolerance to drought and related abiotic stressors have been positively addressed via manipulation of IPT and/or CKX expression, much less attention has been paid to the CGTs. As naming of the CGTs has been unclear, we suggest COGT, CNGT, CONGT and CNOGT to describe the O-, N-and dual function CGTs. As specific CGT mutants of both rice and arabidopsis showed impacts on yield components, we interrogated the wheat genome database, IWGSC RefSeq v1.0 & v2.0, to investigate wheat CGTs. Besides providing unambiguous names for the 53 wheat CGTs, we show their expression patterns in 70 developmental tissues and their response characteristics to various stress conditions by reviewing more than 1000 RNA-seq data sets. These revealed various patterns of responses and showed expression generally being more limited in reproductive tissues than in vegetative tissues. Multiple cis-regulatory elements are present in the 3 kb upstream of the start codons of the 53 CGTs. Elements associated with abscisic acid, light and methyl jasmonate are particularly over-represented, indicative of the responsiveness of CGTs to the environment. These data sets indicate that CGTs have potential value for wheat improvement and that these could be targeted in TILLING or gene editing wheat breeding programmes.genes controlling the formation of the deactivated cytokinin Oand N-glucosides. After introducing the cytokinins more generally, this article focuses on the cytokinin O-and N-glucosyl transferases and their potential roles in modifying yield in bread wheat, under ambient and stress conditions. Cytokinin biosynthesis and metabolismNaturally occurring cytokinins, which are adenine derivatives, fall into two groups: those with an isoprenoid side chain and those with an aromatic one. The isoprenoid cytokinins are considered to be the predominant forms (Jaworek et al., 2019;Sakakibara, 2006). The aromatics are not covered in this review as, while not exhaustively looked for in all cereals, they were not detected in either maize leaves (Lacuesta et al., 2018) or in rice (Kudo et al., 2012), although low levels of 6-benzylamino purine were recently reported in wheat spikes (Jabło nski et al., 2020) and low levels of various aromatic metabolites have been detected in germinating seeds of maize and oats (Stirk et al., 2012).

Genotype-Dependent Effect of Silencing of TaCKX1 and TaCKX2 on Phytohormone Crosstalk and Yield-Related Traits in Wheat

“…The same trait in silenced TaCKX1 lines of Kontesa is down- or up-regulated by TaCKX11 and TaCKX9 , which coordinate cZOG metabolism [ 7 ]. cZOG is a product of reversible inactivation of cZ by O-glucosyl transferase [ 13 ] and was not detected in 7 DAP spikes of Ostka. However, in both cultivars, seedling root weight is upregulated mainly by cZ, which is metabolized in a different way.…”

“…Inactivation occurs via O - and N -glucosylation by cytokinin glucosyl transferases (CGTs) to storage forms. A review on IPTs , their function and regulation of stress and yield improvement in plants [ 4 ], and reviews on wheat TaCKX and TaCGT gene family members (GFMs) and processes in wheat have been recently reported [ 12 , 13 ]. Both processes of inactivation and destruction via CKX enzymes, which are especially important in the regulation of cytokinin level in cereal species, are organ- and species-specific.…”

A New RING Finger Protein, PLANT ARCHITECTURE and GRAIN NUMBER 1, Affects Plant Architecture and Grain Yield in Rice

“…Moreover, O-glycosylation is catalyzed by cytokinin glucosyltransferases and is reversed by β-glucosidases, whereas N-glycosylation is nonreversible [42]. A recent study revealed the potential utility of cytokinin glucosyltransferases for wheat improvement [46]. In our study, we detected differences in the cytokinin glucosyltransferase activity between the wild-type and OsPAGN1 loss-of-function transgenic plants.…”

Plant Growth Regulators INCYDE and TD-K Underperform in Cereal Field Trials

“…Similarly, an in silico TILLING resource is being generated for the most widely grown Chinese bread wheat variety, Jimai 22. Within this population, multiple point mutants for not only all CKX GFMs but also the zeatin O-glucosyl transferase (ZOGT) GFMs have been identified [9,41]. Importantly, while CKX GFMs have been the target of much research [9], the high levels of cytokinin glucosides in wheat and barley, and the negative relationship of ZOGT gene expression with yield in wheat [41,91], indicate that the ZOGT GFMs warrant further investigation, which is beyond the tools offered by the CKX inhibiting PGRs.…”

“…An alternative to the application of cytokinin has been the targeting of the enzymes that either deactivate cytokinin through glucosylation [41], or irreversibly degrade cytokinin via CKX [42,43]. Targeting CKX expression and/or activity has been suggested as a potential strategy to enhance yield [28,31,[44][45][46], and CKX gene family members (GFMs) have been identified as being important for determining yield in both wheat and barley ( [9], and references therein).…”