Expression of genes associated with the photosynthesis pathway
The key genes associated with leaf photosynthetic pathway were quantified at the early and late growth stages, respectively (Fig. 1). At the early growth stage, the transcript levels of LhcpII (encoding light-harvesting complex II), psaA and psbA (encoding two reaction-center polypeptides of PSI and PSII, respectively), petA and petG (encoding cytochrome f apoprotein and cytochrome b6-f complex small subunit, respectively), atpA (encoding chloroplast ATP synthase α subunit), rbcL and RbcS (encoding large subunit and small subunit of ribulose-1,5-bisphosphatecarboxylase/oxygenase, respectively), were significantly reduced in val1 mutant compared with the WT, whereas were significantly increased in VAL1-OE than those in WT. The exception is that there was no difference in the transcript level of RbcS between val1 mutant and WT (Fig. 1A). At the late growth stage, the transcript levels of Cab2R (encoding light harvesting chlorophyll a/b-binding protein), psaA, psbA psbD and psbO (encoding two reaction-center polypeptides of PSI and PSII, respectively), petC (encoding cytochrome b6-f complex small subunit), atpB (encoding chloroplast ATP synthase β subunit), rbcL and RbcS were significantly increased in val1 mutant compared with the WT. In addition, there was no difference in the transcript levels of LhcpII, Cab1R (encoding light harvesting chlorophyll a/b-binding protein), petG, and atpA between val1 mutant and WT (Fig. 1B).
Differences in photosynthetic pigment content
There were significant differences in photosynthetic pigment content of leaves among WT, val1 mutant and VAL1-OE at the early and late growth stages (Fig. 2). At the early growth stage, the contents of Ca, Cb, Ca+b and Car per area in val1 mutant were reduced by 42.22%, 39.06%, 41.78% and 40.98% than those in WT, respectively (P < 0.05), whereas the contents of Ca, Cb, Ca+b and Car per area in VAL1-OE leaves were significantly higher than those in WT (P < 0.05), which increased by 20.36%, 72.70%, 27.58% and 83.40%, respectively (Fig. 2A). Compared with WT, the contents of Ca, Cb, Ca+b and Car per area in val1 mutant significantly increased by 63.59%, 62.92%, 63.49% and 57.28% at the late growth stage, respectively (Fig. 2B).
Differences in plant type characteristics
There were significant differences in plant type characteristics, including plant height, tiller number, leaf width, LMA, LAP, leaf-total weight ratio and RS in WT, val1 mutant and VAL1-OE rice at the early and late growth stage (Fig. 3). At the early growth stage, the RS of val1 mutant was significantly higher than that of WT, which was enhanced by 76.14% (P < 0.05, Fig. 3G). The plant height, tiller number, leaf width, LMA, LAP and leaf-total weight ratio in val1 mutant significantly decreased by 38.28%, 54.01%, 53.29%, 82.99%, 23.67% and 17.17% than those in WT, respectively (P < 0.05). In addition, the LMA in VAL1-OE was significantly higher than that in WT, which increased by 36.95% (P < 0.05, Fig. 3D). The leaf width and LAP in VAL1-OE were significantly lower than those in WT, which reduced by 8.11% and 31.13%, respectively (P < 0.05, Fig. 3C&E). Moreover, there was no significant difference in plant height, tiller number, leaf-total weight ratio and RS between VAL1-OE and WT. At the late growth stage, the leaf-total weight ratio and RS in val1 mutant were significantly higher than those in WT, which increased by 12.13% and 125.85%, respectively (P < 0.05, Fig. 3M&N). The leaf width of val1 mutant significantly reduced by 13.95% than that of WT (P < 0.05, Fig. 3J). Meanwhile, there was no significant difference in plant height, tiller number, LMA and LAP between val1 mutant and WT.
Differences in photosynthetic characteristics
There were significant differences in photosynthetic light response curve among WT, val1 mutant and VAL1-OE at the early growth stage (Fig. 4). The A was significantly less in val1 mutant than WT under both low and high PPFD conditions, whereas it was significantly higher in VAL-OE than WT under different light conditions (Fig. 4A). In addition, the maximum net photosynthesis rate (Amax) significantly decreased by 64.78% in val1 mutant than WT (P < 0.05), and it significantly increased by 95.55% in VAL1-OE than WT (Fig. 4A). At the late growth stage, the A was significantly higher in val1 mutant than WT under both low and high PPFD conditions (Fig. 4B). Moreover, Amax significantly increased by 38.58% in val1 mutant than WT (P < 0.05, Fig. 4B).
Y(II) represents the energy absorbed by Photosystem II and used for photochemical reactions. Y(NPQ) represents the energy dissipated as heat through a regulated light protection mechanism. And Y(NO) represents the energy that is passively dissipated as heat and fluorescence. The results showed that, at the early growth stage, Y(II) in val1 mutant was lower than that in WT, whereas Y(II) in VAL1-OE was higher than that in WT under both low and high PPFD conditions. On the contrary, the Y(NPQ) was higher in val1 mutant than WT, while it was lower in VAL1-OE than WT under both low and high PPFD conditions. In addition, Y(NO) was lower in val1 mutant than WT only under low PPFD condition, and there was no difference in Y(NO) between VAL1-OE and WT (Fig. 5A-C). At the late growth stage, Y(II) in val1 mutant was higher than that in WT, whereas Y(NPQ) was lower in val1 mutant than WT under low and high PPFD conditions. And there was no difference in Y(NO) between val1 mutant and WT (Fig. 5D&E).
Y(I) represents the energy used for photochemical reactions in photosystem I. Y(ND) represents quantum yield of non-photochemical energy dissipation due to donor-side limitations. And Y(NA) represents quantum yield of non-photochemical energy dissipation due to acceptor-side limitations. The results showed that, at the early growth stage, Y(I) in val1 mutant was lower than that in WT, whereas Y(I) in VAL1-OE was higher than that in WT under both low and high PPFD conditions. In addition, the Y(NA) was higher in val1 mutant than WT under high PPFD condition, while it was lower in VAL1-OE than WT under both low and high PPFD conditions. Y(ND) was higher in val1 mutant than WT under both low and high PPFD conditions, and it was lower in VAL1-OE than WT (Fig. 6A-C). At the late growth stage, Y(I) in val1 mutant was higher than that in WT, whereas Y(NA) was lower in val1 mutant than WT under both low and high PPFD conditions. And there was no difference in Y(ND) between val1 mutant and WT (Fig. 6D&E).
Photosynthetic electron transport is a crucial physiological process in photosynthesis pathway. At the early growth stage, the ETR(II) was significantly less in val1 mutant than WT under both low and high PPFD conditions, whereas it was significantly higher in VAL-OE than WT under different light conditions (Fig. 7A). The ETR(I) was significantly less in val1 mutant than WT only under high PPFD condition, while it was significantly higher in VAL-OE than WT under different light conditions (Fig. 7C). In addition, the maximum ETR(II) and ETR(I) of val1 mutant significantly decreased by 19.51% and 21.88% than WT, respectively. And maximum ETR(II) and ETR(I) in VAL1-OE significantly increased by 35.43% and 33.97% than those in WT, respectively. At the late growth stage, the ETR(II) and ETR(I) were significantly higher in val1 mutant than WT under high PPFD condition, and the maximum ETR(II) and ETR(I) of val1 mutant significantly increased by 14.98% and 32.02% than WT, respectively (Fig. 7B&D).
In the present study, the Vcmax was significantly lower in val1 mutant, showing a decrease of 38.90% compared to WT. And the Vcmax in VAL1-OE significantly increased by 59.79% higher than WT at the early growth stage (P < 0.05, Fig. 8A). At the late growth stage, the Vcmax of val1 mutant significantly increased by 84.68% than WT (Fig. 8A).
Photoprotective mechanisms
The non-photochemical quenching (NPQ) represents the ability to dissipate excess light energy as heat in plants. CEF refers to the cyclic electron transport pathway in photosystem I by the ferredoxin, plastoquinone, and the cytochrome b6f (cytb6f) complex isoelectronic transport back to photosystem I. And Pr is an essential component of plant central metabolism responsible for recycling by-products of photosynthesis-related oxygen uptake (Fu et al. 2023). The results showed that, at the early growth stage, the NPQ of val1 mutant was significantly higher than that of WT under low PPFD condition. And the NPQ was significantly lower in VAL1-OE than that in WT under both low and high PPFD conditions. In addition, the CEF was significantly higher in val1 mutant than that in WT under low PPFD condition, whereas it was significantly lower than that in WT under high PPFD condition. Meanwhile, the CEF of VAL1-OE was always higher than that of WT. The Pr was significantly lower in val1 mutant than that in WT under low and high PPFD conditions. Moreover, The Pr was significantly lower in VAL1-OE than that in WT under low PPFD condition. At the late growth stage, the NPQ and CEF of val1 mutant were significantly higher than those of WT. And the Pr was significantly lower in val1 mutant than that in WT only under high PPFD condition (PPFD > 1500 µmol m-2 s-1).
Light use efficiency and yield components
Light use efficiency is an important index for evaluating photosynthetic potential in plants. The results showed that the LUE in WT, val1 mutant and VAL1-OE all increased rapidly and then decreased gradually with the increase of PPFD. In addition, the peak of LUE in all materials occurred around 250 µmol photons m-2 s-1. At the early growth stage, the LUE of the val1 mutant was consistently lower than that of WT, with the maximum LUE being 47.76% lower than that of WT. In contrast, the LUE of VAL1-OE was consistently higher than that of WT, with the maximum LUE being 85.74% higher than that of WT (Fig. 10). At the late growth stage, the LUE of val1 mutant was consistently higher than that of WT, and the maximum LUE increased by 79.99% compared to WT.
In this study, the dry matter weight in val1 mutant was significantly lower than that in WT at the tillering stage and heading stage. And the panicle length, grain number per panicle, panicle number, 1000-grain weight and yield per plant did not show significant differences compared to those in WT. In addition, there was no significant difference in dry matter weight, yield and its components between VAL1-OE and WT.
Tabel 1 dry matter accumulations, yield, and its components of rice
material | dry matter weight at the tillering stage (g) | dry matter weight at the heading stage (g) | panicle length (cm) | grain number per panicle | panicle number | 1000-grain weight (g) | yield per plant (g) |
WT | 29.23 ± 4.54a | 61.26 ± 4.16a | 25.93 ± 0.89a | 132.25 ± 11.20a | 8.56 ± 1.51a | 24.51 ± 0.77a | 27.61 ± 4.76a |
val1 | 11.28 ± 2.14b | 49.41 ± 1.41b | 25.64 ± 1.09a | 134.76 ± 19.13a | 7.44 ± 1.01a | 24.40 ± 0.65a | 24.16 ± 2.34a |
VAL1-OE | 30.10 ± 8.51a | 63.09 ± 6.97a | 26.91 ± 1.39a | 132.82 ± 10.27a | 8.32 ± 2.02a | 24.54 ± 0.54a | 30.80 ± 4.83a |