Plant photobiology: From basic theoretical research to crop production improvement

As one of the most important environmental signals for plants, light plays a profound role in regulating virtually every aspect of plant growth and development. Light signals are perceived by plants via several families of photoreceptors, among which phytochromes are responsible for absorbing the red (R) and far-red (FR) wavelengths (600–750 nm), cryptochromes (CRYs) perceive the blue (B)/ultraviolet-A (UV-A) wavelengths (320–500 nm), and UV RESISTANCE LOCUS 8 (UVR8) is a recently-characterized UV-B (280–320 nm) photoreceptor. These photoreceptors perceive and transduce the light signals through intracellular signaling pathways, ultimately leading to adaptive physiological changes. Such adjustments of plant growth and development in response to their light environment are often mediated by hormone signaling pathways.

In this issue, Park et al. (2024) review how plant photomorphogenesis is regulated by two recently identified phytochemicals, karrikins (KARs) and strigolactones (SLs). Karrikins and SLs are structurally related butenolides, and recent accumulating data strongly support that light and KAR/SL signals act together to modulate plant growth and development and adaptive fitness to environmental stimulations. SUPPRESSOR OF MORE AXILLARY GROWTH 2 (MAX2) 1 (SMAX1) and SMAX1-LIKE (SMXL) proteins function as central negative regulators of KAR and SL signaling, and it was shown that SMAX1 and SMXL play a key role in integrating KAR/SL signals with light as well as other pathways to modulate plant growth and development. Another review by Qu et al. (2024) summarizes recent progress in understanding of the photoregulatory mechanisms of Arabidopsis CRY complexes. Particularly, the dual-action mechanism, including the “Lock-and-Key” and the “Liquid-Liquid Phase Separation” (LLPS) mechanisms, may explain, at least in part, the diversity of CRY-interacting proteins and CRY functions. The classical “Lock-and-Key” mechanism involves blue light-induced changes in the interactions between CRYs and their interacting proteins, while the recently proposed LLPS mechanism involves blue light-induced co-condensation of CRYs and their interacting proteins.

Abscisic acid (ABA) is a classical phytohormone that plays an important role in regulating plant growth and development as well as plant responses to environmental stresses. Light and ABA were shown to antagonistically regulate several plant responses or developmental processes, such as seed germination and stomatal movement. In this issue, Luo et al. (2024) showed that PHYTOCHROME-INTERACTING FACTOR4 (PIF4), a key negative regulator of photomorphogenesis, physically interacts with ABSCISIC ACID INSENSITIVE4 (ABI4), a pivotal transcription factor of ABA signaling, to form a transcriptional activator complex. The PIF4–ABI4 complex synergistically promotes the expression of its target genes including ABI4 itself, and 9-CIS-EPOXYCAROTENOID DIOXYGENASE 6 encoding a key enzyme of ABA biosynthesis. Thus, the PIF4–ABI4 transcriptional activator complex synergistically promotes seed dormancy by enhancing ABA biosynthesis and signaling.

The phytohormone jasmonates (JA) are lipid-derived signaling molecules that play a key role in regulating diverse plant defense responses. Li et al. (2024a) show that in response to UV-B irradiation, monomerized UVR8 accumulates in the nucleus, interacts with the transcription factor TCP4, and enhances TCP4 binding to the promoter of LIPOXYGENASE2 (LOX2), encoding an enzyme involved in the initial step of JA biosynthesis. Thus, UVR8 activates the expression of LOX2 in a TCP4-dependent manner. Subsequently, the increase in JA abundance promotes anthocyanin biosynthesis, leading to enhanced plant tolerance to UV-B stress.

Starch is the major energy storage compound in plants, and the seed starch synthesis directly contributes to crop yield. In this issue, Shi et al. (2024) reveals the molecular mechanism of phytochrome B (phyB)-mediated light modulation of starch synthesis in Arabidopsis leaves. The pivotal transcription factors of light signaling, ELONGATED HYPOCOTYL5 (HY5) and the PIF proteins (including PIF3, PIF4 and PIF5), antagonistically regulate starch synthesis by competing for binding to the promoters of genes encoding granule-bound starch synthase, soluble starch synthase 3 (SS3) and SS4. Under high red to far-red (R:FR) light conditions, photoactivated phyB promotes the phosphorylation and degradation of PIFs and the accumulation of HY5, leading to upregulation of starch synthesis-related genes and increased starch synthesis in leaves. By contrast, low R:FR light conditions, such as canopy shade, inhibits leaf starch synthesis by inactivating phyB and by reducing the productivity of photosynthesis. In addition, Fu et al. (2024) demonstrate that Receptors for Activated C Kinase 1A (RACK1A) functions as a flexible platform connecting multiple key components of light signaling pathways, including HY5, PIF3 and CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1). In the dark, RACK1A interacts with PIF3 to enhance its activation of a downstream target gene, BBX11. However, COP1 targets phosphorylated RACK1A in darkness for 26S proteasome-mediated degradation. In the light, RACK1A interacts with HY5 and inhibits its transcriptional activity, thereby playing a negative role in regulating photomorphogenesis.

This special issue also includes three papers on crop photobiology research. High-density planting is a major approach to increase the yield of crops, including maize. However, under high-density planting, plants compete with their neighbors for sunlight by initiating a suite of morphological adaptations called shade avoidance response, including increased plant height, reduced leaf angle, and early flowering. The review by Jafari et al. (2024) summarizes recent advances in exploiting the genetic basis of five morphological traits (plant height and ear height, leaf angle, tassel branch number, flowering time, and root system architecture) essential for maize tolerance to high-density planting. A deeper understanding of the genetic and molecular mechanisms underlying shade avoidance response in maize will offer new strategies and gene targets for breeding maize cultivars tolerant to high-density planting.

FLAVIN-BINDING, KELCH REPEAT, F-BOX 1 (FKF1), an F-box protein serving as a key component of the SKP1/CUL1/F-box (SCF)-type E3 ligase complex, is an essential circadian clock-regulated blue light receptor that play distinct roles in regulating flowering time. Two FKF1 homologous genes exist in the maize genome, designated ZmFKF1a and ZmFKF1b; however, their functions remain unclear. In this issue, Chen et al. (2024) demonstrated that ZmFKF1s interact with ZmCONZ1 and ZmGI1, thereby increasing the transcript levels of ZmCONZ1 and ZCN8. In addition, ZmFKF1b was subjected to strong selection during modern maize breeding in China, and natural variations in the coding region of ZmFKF1b in maize inbred lines Zheng58 and Chang7-2 may be related to differential flowering regulations. Notably, ZmFKF1b^Hap_C may be a novel and significant haplotype for further breeding.

Rapeseed (Brassica napus) is a globally cultivated oil crop. Li et al. (2024b) reports interesting developmental stage-dependent shade responses observed in B. napus seedlings. In contrast to the classical approach to study shade avoidance response (in which the seedlings were first fully de-etiolated under high R:FR light conditions and then subjected to low R:FR light treatments), this study transferred B. napus seedlings grown under white light for 1–4 d to white plus FR light conditions for a total of 6 d, respectively. Interestingly, it was observed that B. napus seedlings display different responses to shade depending on the timing of the shade treatment, which is different from Arabidopsis seedlings. Further analyses indicate that the temporal expression of two AUXIN/INDOLE-3-ACETIC ACID genes, BnIAA32 and BnIAA34, determines the specific response of B. napus seedlings to shade.

A deeper understanding of how plants perceive and respond to their light environment not only has important theoretical significance, but also has significant implications for improving crop tolerance to high-density planting. The papers in this issue provide important advances and summaries of multiple aspects in plant photobiology, which will facilitate the design of shade-tolerant, high-yield crops in the near future.