Genetic dissection of the roles of β-hydroxylases in carotenoid metabolism, photosynthesis, and plant growth in tetraploid wheat (Triticum turgidum L.)

Abstract

Functional redundancy and subfunctionalization of β-hydroxylases in tetraploid wheat tissues open up opportunities for manipulation of carotenoid metabolism for trait improvement. The genetic diversity provided by subgenome homoeologs in allopolyploid wheat can be leveraged for developing improved wheat varieties with modified chemical traits, including profiles of carotenoids, which play critical roles in photosynthesis, photoprotection, and growth regulation. Carotenoid profiles are greatly influenced by hydroxylation catalyzed by β-hydroxylases (HYDs). To genetically dissect the contribution of HYDs to carotenoid metabolism and wheat growth and yield, we isolated loss-of-function mutants of the two homoeologs of HYD1 (HYD-A1 and HYD-B1) and HYD2 (HYD-A2 and HYD-B2) from the sequenced ethyl methanesulfonate mutant population of the tetraploid wheat cultivar Kronos, and generated various mutant combinations. Although functional redundancy between HYD1 and HYD2 paralogs was observed in leaves, HYD1 homoeologs contributed more than HYD2 homoeologs to carotenoid β-ring hydroxylation in this tissue. By contrast, the HYD2 homoeologs functioned toward production of lutein, the major carotenoid in mature grains, whereas HYD1 homoeologs had a limited role. These results collectively suggested subfunctionalization of HYD genes and homoeologs in different tissues of tetraploid wheat. Despite reduced photoprotective responses observed in the triple hyd-A1 hyd-B1 hyd-A2 and the quadruple hyd-A1 hyd-B1 hyd-A2 hyd-B2 combinatorial mutants, comprehensive plant phenotyping analysis revealed that all mutants analyzed were comparable to the control for growth, yield, and fertility, except for a slight delay in anthesis and senescence as well as accelerated germination in the quadruple mutant. Overall, this research takes steps toward untangling the functions of HYDs in wheat and has implications for improving performance and consumer traits of this economically important global crop.

Fig. 1

Identification of tetraploid wheat hyd mutants. a A simplified carotenoid biosynthetic pathway in wheat. Enzymes are indicated in bold. PSY, phytoene synthase; PDS, phytoene desaturase; ZISO, ζ-carotene isomerase; ZDS, ζ-carotene desaturase; CRTISO, carotenoid isomerase; LCYb, lycopene β-cyclase; LCYe, lycopene ε-cyclase; HYD, β-hydroxylase (non-heme di-iron type); CYP, cytochrome P450 type carotenoid hydroxylase; CCD, carotenoid cleavage dioxygenase. Enzyme activities blocked in the mutants are indicated in red. b Location of mutations within the HYD homoeologs in the tetraploid wheat mutants. The mutated nucleotides are highlighted in red. c Derived Cleaved Amplified Polymorphic Sequence (dCAPS) markers of HYD-A1 and HYD-B1 (WT: Wild type, HT: Heterozygous mutant, HM: Homozygous mutant). Markers of HYD-A2 and HYD-B2 were published previously (Yu et al. 2022)

Fig. 2

Induction and relaxation of non-photochemical quenching (NPQ) in dark-adapted leaves of hyd mutants and control plants. Mean values and standard deviations of 4–6 biological replicates are shown. The statistical analysis of the NPQ values during light induction is shown in Table S3

Fig. 3

Plant growth phenotyping analysis of hyd mutants and control plants. a Image of hyd mutants and control plants during the grain filling stage b Plant heights c. Days to anthesis d Days to senescence. Mean values and standard deviations are displayed from 15 plants for each genotype

Fig. 4

Rates of seed germination and leaf water loss of the quadruple mutant (hyd-A1 hyd-B1 hyd-A2 hyd-B2) and the control plants. a Cumulative germination rate of 100 seeds over time at ambient room temperature (~ 24 °C). Values are means ± standard deviations of 5 independent experiments each consisting of 100 seeds. *, P < 0.05. b Leaf water loss. Leaves of two-week-old plants were detached and left at ambient room temperature (~ 24 °C). They were weighed initially (0 h) and in 1-h intervals. Values shown are means ± standard deviations (n = 24). ***, P < 0.001

Fig. 5

Yield analysis of the hyd mutant and control plants grown in a semi-controlled (greenhouse) environment. a Average weight of individual grains per plant b Grains per plant c Harvest index d Spikes per plant e Spike fertility index (grain number per gram of spike chaff) f Spikelet fertility rate (% of fertile spikelets as a portion of total spikelets per plant) g Grains per spikelet (grain number divided by total spikelets per plant). The data shown in a and c were collected from 15 plants for each genotype, whereas the data presented in b and d–g were collected from fertile spikes of 4–6 plants for each genotype