Can we improve the chilling tolerance of maize photosynthesis through breeding?

A deficient CP24 allele defines variation for dynamic nonphotochemical quenching and photosystem II efficiency in maize

Maize (Zea mays L.) is a global crop species in which CO2 assimilation occurs via the C4 pathway. C4 photosynthesis is typically more efficient than C3 photosynthesis under warm and dry conditions; however, despite this inherent advantage, considerable variation remains in photosynthetic efficiency for C4 species that could be leveraged to benefit crop performance. Here, we investigate the genetic architecture of nonphotochemical quenching (NPQ) and photosystem II (PSII) efficiency using a combination of high-throughput phenotyping and quantitative trait loci (QTL) mapping in a field-grown Multi-parent Advanced Generation Inter-Cross (MAGIC) mapping population. QTL mapping was followed by the identification of putative candidate genes using a combination of genomics, transcriptomics, protein biochemistry, and targeted physiological phenotyping. We identified four genes with a putative causal role in the observed QTL effects. The highest confidence causal gene was found for a large effect QTL for photosynthetic efficiency on chromosome 10, which was underpinned by allelic variation in the expression of the minor PSII antenna protein light harvesting complex photosystem II subunit (LHCB6 or CP24), mainly driven by poor expression associated with the haplotype of the F7 founder line. The historical role of this line in breeding for early flowering time may suggest that the presence of this deficient allele could be enriched in temperate maize germplasm. These findings advance our understanding of the genetic basis of NPQ and PSII efficiency in C4 plants and highlight the potential for breeding strategies aimed at optimizing photosynthetic efficiency in maize.

Osmoregulation is a crucial factor for methyl jasmonate to enhance chilling tolerance of Jatropha curcas L

Methyl jasmonate (MeJA) is a vital regulator of plant growth and plays a crucial role in chilling tolerance. However, the mechanism through which MeJA enhances chilling tolerance in plants remains unclear. Therefore, this study conducted hydroponic experiments to evaluate the effects of exogenous MeJA (0-125 μmol L-1) on osmoregulation and chilling tolerance of Jatropha curcas L. seedlings under chilling (5 °C) stress. The seedlings under chilling stress were treated with MeJA and morphological changes, physiological traits, osmoprotectants (proline, betaine and trehalose) contents, activities of key enzymes involved in osmoprotectants metabolism and expression of related genes were investigated. The results showed that treatment with 75 μmol L-1 MeJA alleviated leaf wilting and growth inhibition; significantly decreased water potential, electrolyte leakage and malondialdehyde content; and enhanced tissue vitality, water content, total chlorophyll content, net photosynthetic rate, stomatal conductance, intercellular CO2 concentration and transpiration rate in J. curcas seedlings, thereby improving chilling tolerance. Under chilling stress, 4 days of MeJA treatment remarkably increased the contents of proline, betaine and trehalose in the leaves of J. curcas seedlings by activating their biosynthesis pathways and inhibiting the degradation pathway of proline. The substantial accumulation of osmoprotectants reduced the cellular water potential, maintained the cellular water balance and stabilized the cell membrane. Furthermore, 1-4 days of MeJA treatment led to increased levels of jasmonic acid and ethylene and upregulation of JcMYC2 expression in J. curcas seedlings under chilling stress. This suggested that the JA/MeJA-MYC2 signaling pathway, along with ethylene signaling, may contribute to MeJA-induced chilling tolerance in J. curcas. Our findings suggested that exogenous MeJA treatment increases the capacity for osmoregulation and chilling tolerance in J. curcas seedlings under chilling stress and that osmoregulation is a crucial component of MeJA-induced chilling tolerance.

Diverse coping modes of maize in cool environment at early growth

BACKGROUND

Maize cultivation has considerably expanded beyond its place of origin in Central America. The successful adaptation of maize to temperate climates can be achieved by selecting genotypes that demonstrate tolerance to low temperatures, especially in cold springs. In maize, cold tolerance at the early growth stages enables early sowing, a long growing season, and eventually high yields, even in temperate climates. Maize adaptation during early growth has not been thoroughly investigated; therefore, we tested the working hypothesis that several distinct and independent adaptation strategies may be involved in maize habituation to cool temperate climates during seedling establishment.

RESULTS

We studied the effect of mild cold stress (day/night 16/12 °C) on early growth stage followed by regrowth at optimal daily temperatures (24/21 °C). Automated plant phenotyping was performed on 30 inbred lines selected from a diverse genetic pool during preliminary studies. As a result, we generated time series based on selected morphological parameters, spectral parameters, and spectral vegetation indices. These curves were clustered and four classes of maize with clearly contrasting growth modes and changes in their physiological status were distinguished at low temperatures and during regrowth. Two classes comprised either cold-sensitive (slow growth and poor physiological status in cold) or cold-tolerant (moderately fast growth and good physiological status in cold) lines. However, two other classes showed that growth rate and physiological status at low temperature is not necessarily related, for instance one class included lines with small seedlings but good physiological status and the other grouped seedlings with rapid growth despite poor physiological status. These classes clearly exhibited different modes of cold adaptation. Moreover, a class containing cold-sensitive inbred lines may represent a distinct and novel type of cold-adaptation strategy related to the arrest of coleoptile emerge related with ability to recover rapidly under favourable conditions.

CONCLUSIONS

Our results support the hypothesis that maize may have several adaptation strategies to cold environments at early growth stages based on independent mechanisms. These findings suggest that maize adaptability to adverse environments is likely more complex than previously understood.

Embracing native diversity to enhance the maximum quantum efficiency of photosystem II in maize

The sustainability of maize cultivation would benefit tremendously from early sowing, but is hampered by low temperatures during early development in temperate climates. We show that allelic variation within the gene encoding subunit M of the NADH-dehydrogenase-like (NDH) complex (ndhm1) in a European maize landrace affects several quantitative traits that are relevant during early development in cold climates through NDH-mediated cyclic electron transport around photosystem I, a process crucial for photosynthesis and photoprotection. Beginning with a genome-wide association study for maximum potential quantum yield of photosystem II in dark-adapted leaves (Fv/Fm), we capitalized on the large phenotypic effects of a hAT transposon insertion in ndhm1 on multiple quantitative traits (early plant height [EPH], Fv/Fm, chlorophyll content, and cold tolerance) caused by the reduced protein levels of NDHM and associated NDH components. Analysis of the ndhm1 native allelic series revealed a rare allele of ndhm1 that is associated with small albeit significant improvements of Fv/Fm, photosystem II efficiency in light-adapted leaves (ΦPSII), and EPH compared with common alleles. Our work showcases the extraction of favorable alleles from locally adapted landraces, offering an efficient strategy for broadening the genetic variation of elite germplasm by breeding or genome editing.

Transcriptome dynamics in Artemisia annua provides new insights into cold adaptation and de-adaptation

Plants adapt to cold stress through a tightly regulated process involving metabolic reprogramming and tissue remodeling to enhance tolerance within a short timeframe. However, the precise differences and interconnections among various organs during cold adaptation remain poorly understood. This study employed dynamic transcriptomic and metabolite quantitative analyses to investigate cold adaptation and subsequent de-adaptation in Artemisia annua, a species known for its robust resistance to abiotic stress. Our findings revealed distinct expression patterns in most differentially expressed genes (DEGs) encoding transcription factors and components of the calcium signal transduction pathway within the two organs under cold stress. Notably, the long-distance transport of carbon sources from source organs (leaves) to sink organs (roots) experienced disruption followed by resumption, while nitrogen transport from roots to leaves, primarily in the form of amino acids, exhibited acceleration. These contrasting transport patterns likely contribute to the observed differences in cold response between the two organs. The transcriptomic analysis further indicated that leaves exhibited increased respiration, accumulated anti-stress compounds, and initiated the ICE-CBF-COR signaling pathway earlier than roots. Differential expression of genes associated with cell wall biosynthesis suggests that leaves may undergo cell wall thickening while roots may experience thinning. Moreover, a marked difference was observed in phenylalanine metabolism between the two organs, with leaves favoring lignin production and roots favoring flavonoid synthesis. Additionally, our findings suggest that the circadian rhythm is crucial in integrating temperature fluctuations with the plant's internal rhythms during cold stress and subsequent recovery. Collectively, these results shed light on the coordinated response of different plant organs during cold adaptation, highlighting the importance of inter-organ communication for successful stress tolerance.

ZmASR1 negatively regulates drought stress tolerance in maize

Abscisic acid-, stress-, and ripening-induced (ASR) proteins in plants play a significant role in plant response to diverse abiotic stresses. However, the functions of ASR genes in maize remain unclear. In the present study, we identified a novel drought-induced ASR gene in maize (ZmASR1) and functionally characterized its role in mediating drought tolerance. The transcription of ZmASR1 was upregulated under drought stress and abscisic acid (ABA) treatment, and the ZmASR1 protein was observed to exhibit nuclear and cytoplasmic localization. Moreover, ZmASR1 knockout lines generated with the CRISPR-Cas9 system showed lower ROS accumulation, higher ABA content, and a higher degree of stomatal closure than wild-type plants, leading to higher drought tolerance. Transcriptome sequencing data indicated that the significantly differentially expressed genes in the drought treatment group were mainly enriched in ABA signal transduction, antioxidant defense, and photosynthetic pathway. Taken together, the findings suggest that ZmASR1 negatively regulates drought tolerance and represents a candidate gene for genetic manipulation of drought resistance in maize.

Cellular dynamics in the maize leaf growth zone during recovery from chilling depends on the leaf developmental stage

KEY MESSAGE

A novel non-steady-state kinematic analysis shows differences in cell division and expansion determining a better recovery from a 3-day cold spell in emerged compared to non-emerged maize leaves. Zea mays is highly sensitive to chilling which frequently occurs during its seedling stage. Although the direct effect of chilling is well studied, the mechanisms determining the subsequent recovery are still unknown. Our goal is to determine the cellular basis of the leaf growth response to chilling and during recovery of leaves exposed before or after their emergence. We first studied the effect of a 3-day cold spell on leaf growth at the plant level. Then, we performed a kinematic analysis to analyse the dynamics of cell division and elongation during recovery of the 4th leaf after exposure to cold before or after emergence. Our results demonstrated cold more strongly reduced the final length of non-emerged than emerged leaves (- 13 vs. - 18%). This was not related to growth differences during cold, but a faster and more complete recovery of the growth of emerged leaves. This difference was due to a higher cell division rate on the 1st and a higher cell elongation rate on the 2nd day of recovery, respectively. The dynamics of cell division and expansion during recovery determines developmental stage-specific differences in cold tolerance of maize leaves.

Nitrogen supply alleviates cold stress by increasing photosynthesis and nitrogen assimilation in maize seedlings

Cold stress inhibits the early growth of maize, leading to reduced productivity. Nitrogen (N) is an essential nutrient that stimulates maize growth and productivity, but the relationship between N availability and cold tolerance is poorly characterized. Therefore, we studied the acclimation of maize under combined cold stress and N treatments. Exposure to cold stress caused a decline in growth and N assimilation, but increased abscisic acid (ABA) and carbohydrate accumulation. The application of different N concentrations from the priming stage to the recovery period resulted in the following observations: (i) high N supply alleviated cold stress-dependent growth inhibition, as shown by increased biomass, chlorophyll and Rubisco content and PSII efficiency; (ii) cold stress-induced ABA accumulation was repressed under high N, presumably due to enhanced stomatal conductance; (iii) the mitigating effects of high N on cold stress could be due to the increased activities of N assimilation enzymes and improved redox homeostasis. After cold stress, the ability of maize seedlings to recover increased under high N treatment, indicating the potential role of high N in the cold stress tolerance of maize seedlings.

Genomic-regions associated with cold stress tolerance in Asia-adapted tropical maize germplasm

Maize is gaining impetus in non-traditional and non-conventional seasons such as off-season, primarily due to higher demand and economic returns. Maize varieties directed for growing in the winter season of South Asia must have cold resilience as an important trait due to the low prevailing temperatures and frequent cold snaps observed during this season in most parts of the lowland tropics of Asia. The current study involved screening of a panel of advanced tropically adapted maize lines to cold stress during vegetative and flowering stage under field conditions. A suite of significant genomic loci (28) associated with grain yield along and agronomic traits such as flowering (15) and plant height (6) under cold stress environments. The haplotype regression revealed 6 significant haplotype blocks for grain yield under cold stress across the test environments. Haplotype blocks particularly on chromosomes 5 (bin5.07), 6 (bin6.02), and 9 (9.03) co-located to regions/bins that have been identified to contain candidate genes involved in membrane transport system that would provide essential tolerance to the plant. The regions on chromosome 1 (bin1.04), 2 (bin 2.07), 3 (bin 3.05-3.06), 5 (bin5.03), 8 (bin8.05-8.06) also harboured significant SNPs for the other agronomic traits. In addition, the study also looked at the plausibility of identifying tropically adapted maize lines from the working germplasm with cold resilience across growth stages and identified four lines that could be used as breeding starts in the tropical maize breeding pipelines.

Response of waxy maize (Zea mays L. var. ceratina Kulesh) leaf photosynthesis to low temperature during the grain-filling stage

Low temperature (LT) during the grain-filling stage is an important factor that affects the source-sink relationship and leads to yield loss in maize (Zea mays L). In this study, field and pot trials were conducted to investigate the effects of LT during the grain-filling stage on leaf photosynthesis, antioxidant system, hormones, and grain yield of waxy maize cultivars Suyunuo 5 (S5) and Yunuo 7 (Y7). The results showed that LT treatment inhibited the chlorophyll biosynthesis and reduced the photosynthetic pigment levels during grain-filling stage. Ribulose-1,5-bisphosphate carboxylase and phosphoenolpyruvate carboxylase activities, photosynthetic rate, transpiration rate, and stomatal conductance decreased under LT treatment during the grain-filling stage. Furthermore, LT treatment increased the contents of malondialdehyde and reactive oxygen species, and decreased the activities of catalase, superoxide dismutase, peroxidase, and ascorbate peroxidase in the ear leaves, which accelerated the oxidative damage of leaf. The LT treatment also raised abscisic acid content and reduced indole acetic acid content in the ear leaves during grain-filling stage. The results of field and pot trials were verified by each other, but the field effect was greater than that of pot. Overall, LT treatment reduced the waxy maize dry matter accumulation after silking by affecting the physiological and biochemical processes of leaves, and ultimately decreased grain yield.

Integrative Proteome and Phosphoproteome Profiling of Early Cold Response in Maize Seedlings

Cold limits the growth and yield of maize in temperate regions, but the molecular mechanism of cold adaptation remains largely unexplored in maize. To identify early molecular events during cold shock, maize seedlings were treated under 4 °C for 30 min and 2 h, and analyzed at both the proteome and phosphoproteome levels. Over 8500 proteins and 19,300 phosphopeptides were quantified. About 660 and 620 proteins were cold responsive at protein abundance or site-specific phosphorylation levels, but only 65 proteins were shared between them. Functional enrichment analysis of cold-responsive proteins and phosphoproteins revealed that early cold response in maize is associated with photosynthesis light reaction, spliceosome, endocytosis, and defense response, consistent with similar studies in Arabidopsis. Thirty-two photosynthesis proteins were down-regulated at protein levels, and 48 spliceosome proteins were altered at site-specific phosphorylation levels. Thirty-one kinases and 33 transcriptional factors were cold responsive at protein, phosphopeptide, or site-specific phosphorylation levels. Our results showed that maize seedlings respond to cold shock rapidly, at both the proteome and phosphoproteome levels. This study provides a comprehensive landscape at the cold-responsive proteome and phosphoproteome in maize seedlings that can be a significant resource to understand how C₄ plants respond to a sudden temperature drop.