Isopentenyltransferases as master regulators of crop performance: their function, manipulation, and genetic potential for stress adaptation and yield improvement

Abstract

Isopentenyltransferase (IPT) in plants regulates a rate-limiting step of cytokinin (CTK) biosynthesis. IPTs are recognized as key regulators of CTK homeostasis and phytohormone crosstalk in both biotic and abiotic stress responses. Recent research has revealed the regulatory function of IPTs in gene expression and metabolite profiles including source-sink modifications, energy metabolism, nutrient allocation and storage, stress defence and signalling pathways, protein synthesis and transport, and membrane transport. This suggests that IPTs play a crucial role in plant growth and adaptation. In planta studies of IPT-driven modifications indicate that, at a physiological level, IPTs improve stay-green characteristics, delay senescence, reduce stress-induced oxidative damage and protect photosynthetic machinery. Subsequently, these improvements often manifest as enhanced or stabilized crop yields and this is especially apparent under environmental stress. These mechanisms merit consideration of the IPTs as 'master regulators' of core cellular metabolic pathways, thus adjusting plant homeostasis/adaptive responses to altered environmental stresses, to maximize yield potential. If their expression can be adequately controlled, both spatially and temporally, IPTs can be a key driver for seed yield. In this review, we give a comprehensive overview of recent findings on how IPTs influence plant stress physiology and yield, and we highlight areas for future research.

Keywords: IPT; abiotic stress; biotic stress; cytokinin; phytohormone; plant yield; stress response.

Figure 1

An overview of CTK biosynthesis (a); CTK translocation (b); general characteristics of two known CTKs biosynthesis pathways in angiosperms (c) and, the mode of action of IPTs in drought tolerance in Arabidopsis (d). (a) CTK types derived from two known pathways, adenylate (ATP/ADP‐IPTs and AMP‐IPTs) and tRNA types (adapted from Hirose et al. (2008), further details in Sakakibara (2006)). ADP: adenosine 5’‐diphosphate; ATP: adenosine 5’‐triphosphate; tRNA: Transfer ribonucleic acid; DMAPP: dimethylallyl diphosphate; IPT: isopentenyl transferase; CYP735A: cytochrome P450 monooxygenase, family 735, subfamily A (cytokinin trans‐hydroxylase); LOG: cytokinin phosphoribohydrolase ‘Lonely guy’; cZ: cis‐Zeatin, cZR: cZ riboside; cZRMP: cZ riboside 5’‐monophosphate, DZ: Dihydrozeatin, DZR: DZ riboside; DZRMP: DZ riboside 5’‐monophosphate; iP: isopentenyladenine; iPR: iP riboside; iPRMP: isopentenyladenosine‐5’–monophosphate; iPRDP: isopentenyladenosine‐5'‐diphosphate; iPRTP: isopentenyladenosine‐5'‐triphosphate; tZ: trans‐Zeatin, tZR: tZ riboside, tZRDP: tZ riboside 5’‐diphosphate; tZRMP: tZ riboside 5’‐monophosphate; tZRTP: tZ riboside 5’‐triphosphate. (b) Translocation of CTKs in plant. As multifunctional and mobile signalling molecules, active CTKs and their derivatives contribute to various developmental processes depending on CTK transport proteins across vascular tissues. Briefly, three kinds of CTK transporters have been systematically characterized: equilibrate nucleoside transporters (ENT), purine permeases (PUP), and G subfamily ATP‐binding cassette (ABCG) transporters (Durán‐Medina et al., 2017). tZR is the primary form of xylem transported CTKs, and iPR and cZR are major forms of phloem CTKs (Osugi and Sakakibara, 2015). Arabidopsis plant vector is adapted, with permission, from Figshare (Bouché, 2018). (c) General mode of action of two IPT GFMs (ATP/ADP‐IPTs and tRNA‐IPTs) in angiosperms (Köllmer et al ; Miyawaki et al ; Wang et al., 2020a). The investigation in Arabidopsis mutants showed that ATP/ADP‐IPT GFMs control biosynthesis of iP‐ and tZ‐type CTKs while tRNA‐type IPT genes regulate cZ‐type CTKs (Köllmer et al., ; Miyawaki et al., 2006). In the basal angiosperm Amborella trichopoda (Amborellaceae), and in Fragaria vesca (wild strawberry), eudicot woodland strawberry, transcriptomic analysis indicated that tRNA‐IPTs are constitutively expressed throughout the plant, whereas the expression of ATP/ADP‐IPTs is tissue‐specific and rapidly down‐regulated by abiotic stresses (Wang et al., 2020a). Generally, tRNA‐IPTs and the associated cZ‐type CTKs play a housekeeping role, whereas ATP/ADP‐IPTs and associated iP/tZ‐type CTKs play regulatory roles in organ development and stress responses in angiosperms (Köllmer et al., ; Miyawaki et al., , ; Wang et al., 2020a). (d) The negative regulatory role of IPT‐repressed CTKs in osmotic stress tolerance in Arabidopsis. Arrowheads represent activation, and perpendicular bars indicate inhibition. Studies of drought stress tolerance using CTK‐deficient plants, such as the quadruple ATP/ADP‐ipt1,3,5,7 loss‐of‐function mutants, CTK signalling mutants [AHP mutants (Arabidopsis Histidine Phosphotransfer Proteins 2,3,5) and ARR mutants (type B Arabidopsis Response Regulators 1,10,12)], and down‐regulated expression of five ATP/ADP‐IPT genes (IPT1, IPT4, IPT5, IPT6, IPT8) found in the AtMYB2 (ABA‐dependent signalling pathway) overexpressor, have indicated that CTK‐depleted mutants have improved drought acclimation/adaptation (Guo and Gan, ; Nguyen et al., ; Nishiyama et al., , ; Werner et al., 2010). Briefly, reducing endogenous CTK lowers the output of the CTK signalling cascades (i.e. quadruple ipt1,3,5,7 down‐regulates expression of AHP2, AHP3, AHP5) leading to drought acclimation/adaptation. CTK: cytokinin; AHK: Arabidopsis histidine kinase; AHP: Arabidopsis histidine phosphotransfer; ARR: Arabidopsis response regulator; ROS: reactive oxygen species.

Figure 2

(a) Site‐specific expression of IPT GFMs in developmental tissues in planta, including Arabidopsis, rice, Chinese cabbage, strawberry, apple, soybean, pea, maize (Brugière et al., ; Dolgikh et al., ; Le et al., ; Liu et al., ; Mi et al., ; Miyawaki et al., ; Takei et al., ; Tan et al., ; Tsai et al., 2012). The names of the polyploid BrIPT GFMs follow the Liu et al. (2013b) naming system. (b) IPT GFMs in Arabidopsis are involved in a variety of environmental responses and developmental processes (Ghosh et al., ; Hirose et al., ; Markovich et al., ; Miyawaki et al., ; Nishiyama et al., ; Takei et al., ; Woo et al., 2012). Color boxes indicate distinct transcriptional profiles of each IPT GFM in response to the noted stress factors or biological process Figure design is adapted from (Hallmark and Rashotte, ; Victor et al., 2019). The Arabidopsis plant vector is adapted, with permission, from Figshare (Bouché, 2018).

Figure 3

Functional specificities of IPT and their potential utilization for crop improvement. Manipulation of specific IPTs can help to obtain a certain trait, enhancing the feasibility of goal‐directed molecular design (Kuppu et al., ; Nishiyama et al., ; Peleg et al., ; Rivero et al., ; Skalák et al., ; Xu and Huang, 2017). (a) Schematic diagram representing contribution of IPTs in the regulation of CTKs with other plant hormone biosynthesis and signalling pathways. The results of in planta studies report that overexpression of IPTs changes endogenous CTK and other phytohormone levels. Manipulation of specific IPTs modifies transcriptional activities of hormone biosynthesis and signalling components. Cytokinin: CTK; IPT: isopentenyl transferase; CYP735A: cytochrome P450 monooxygenase, family 735, subfamily A (cytokinin trans‐hydroxylase); LOG: cytokinin phosphoribohydrolase ‘Lonely guy’; HK: histidine kinase; HP: histidine phosphotransfer; RRA/RRB: response regulator type A/B. Abscisic acid: ABA; ZEP: zeaxanthin epoxidase; ABA1,2: zeaxanthin epoxidase ABA deficient 1/2; ABA3: molybdenum cofactor (MoCo) synthase; ABA4: unidentified enzyme; AAO3: abscisic aldehyde oxidase 3; PYR: pyrabactin resistance; PYLs: pyrabactin 1‐Like; PP2Cs: protein phosphatase 2Cs; SnKRs: sucrose non‐fermenting1‐related protein kinase2; ABREs: ABA‐responsive elements; ABF: ABRE‐binding factors. Ethylene‐ET; ACS: 1‐aminocyclopropane‐1‐carboxylic acid (ACC) synthase; ACO: ACC oxidase; ETR: ET receptor; CTR1: constitutive triple response 1; EBFs: ET‐intensive (EIN3) binding F‐box proteins; ERF: ethylene response factor. Jasmonic acid: JA; AOS: Allene Oxide Synthase; AOC: Allene Oxide Cyclase; OPR: 12‐oxo‐phytodienoic acid (OPDA) reductase; JAR1: JA resistant 1; COL: constant‐like; JAZ: jasmonate ZIM‐domain; MYCs: basic helix–loop–helix (bHLH) transcription factors. Gibberellin: GA; CPS: ent‐copalyl diphosphate synthase; KS: ent‐kaurene synthase; KO: ent‐kaurene oxidase; KAO: ent‐kaurenoic acid oxidase; GA20ox: GA 20‐oxidases; GA3ox: GA 3‐oxidases; GID1: Gibberellin insensitive dwarf1 (GA receptor); DELLA: GA signalling repressors; PIFs: phytochrome interacting factors. Brassinosteroids: BR; DWF4: Dwaft4 (a cytochrome P450); CPD: BR biosynthetic cytochrome P450; BR6ox2: BR‐6‐oxidase2; BRI1: BR‐insensitive1; BAK1: BRI1‐associated‐kinase1; BIN2: BR insensitive2; BZR: Brassinazole‐resistant1. Auxin: AU; TAR: tryptophan aminotransferase‐related; YUC: YUCCA enzyme; TIR1: Transport inhibitor response1; AFB2: Auxin signalling F‐box2; IAAs: Indole‐3‐acetic acid transcriptional repressors; ARFs: Auxin response factors. Figure design adapted from (Nishiyama et al., 2018). (b) At the protein level, increasing endogenous CTK levels through the controlled expression of IPT GFMs can target various substrates to regulate specific functions. Arrowheads represent activation. AA: ascorbic acid; CAT: catalase, SOD: superoxide dismutase, POD: peroxidase; RuBisCO GAPDH: Ribulose‐1,5‐bisphosphate carboxylase/oxygenase; GAPDH: Glyceraldehyde‐3‐phosphate dehydrogenase; Fd‐GOGAT: Ferredoxin‐dependent glutamate synthase. (c) At the transcriptional level, IPTs act upstream of various genes to regulate specific functions. Arrowheads represent activation. ROS: reactive oxygen species; TFs: transcription factors; bHLH148: basic helix–loop–helix 148; MYB4/4: MYB domain protein 4; WRKY28/53/71: TFs containing highly conserved WRKY domain; Pr ToxA: proteinaceous host‐selective toxin; LRR: leucine‐rich repeat; USP: universal stress protein; GSK: glycogen synthase kinase; ABC: ATP‐binding cassette transporters; SAG: senescence‐associated gene.