Developing Maize Kernels Research Articles

Environmental factors related to fungal infection and fumonisin accumulation during the development and drying of white maize kernels

In Southern Europe where whole maize kernels are ground and used for making bread and other food products, infection of the kernels by Fusarium verticillioides and subsequent fumonisin contamination pose a serious safety issue. The influence of environmental factors on this fungal infection and mycotoxin accumulation as the kernel develops has not been fully determined, especially in such food grade maize. The objectives of the present study were to determine which environmental factors may contribute to kernel invasion by F. verticillioides and fumonisin accumulation as kernels develop and dry in naturally infected white maize. Three maize hybrids were planted at two different sowing dates and kernel samples were collected 20, 40, 60, 80 and 100days after silking. The percentage of kernels infected, and ergosterol and fumonisin contents were recorded for each sampling. F. verticillioides was the most prevalent species identified as the kernels developed. Temperature and moisture conditions during the first 80days after silking favored natural kernel infection by F. verticillioides rather than by Aspergillus or Penicillium species. Fumonisin was found in kernels as early as 20days after silking however significant fumonisin accumulation above levels acceptable in the EU did not occur until after physiological maturity of the kernel indicating that kernel drying in the field poses a high risk. Our results suggest that this could be due to increasing kernel damage by insects that favor fungal development, such as the damage by the moth Sitotroga cerealella, and to the occurrence of stress conditions for F. verticillioides growth that could trigger fumonisin biosynthesis, such as exposure to suboptimal temperatures for growth simultaneously with low water activity.

Relationship of Ear Differentiation with Kernel Development and Barrenness in Maize ( Zea mays L.)

Two cultivars ZD958 and DH661 were used to analyze the effect of different planting densities (45 000 plant ha–1, 75 000 plant ha?1, 10 5000 plant ha?1) on kernel development and barrenness in maize (Zea mays L.). The results showed that increasing plant density had little effects on the time of ear primordia differentiation (the ear primordia presented at 28–29 days after sowing under different densities) and the total number of floret primordia during silking. However, the differentiation of ear was postponed under high density, the number of abortive florets and unfertilized florets increased with the increase of plant density. Compared with low-density, the high density reduced number of normal florets per ear by 100.0 for DH661 and by 76.4 for ZD958. Additionally, the anthesis-silking interval (ASI) was elongated, the ratio of the silked-plants was decreased (ratio of DH661 was 93.64% and that of ZD958 was 81.80% under high density), the time of silking was delayed, the number of silked florets per ear was reduced and the pollen shedding duration was shortened with increasing plant density, which results in the increment of abortion after fertilization. Interestingly, kernel abortion would also occur during grain-filling, especially in 10 days after anthesis. The kernel number per ear had a positive correlation with the bottom transmittance at silking, 10 d after silking and 20 d after silking, while the ratio of abortive floret was negatively correlated with the bottom transmittance before flowering significantly.

Effects of Low-light Stress on Kernel Setting and Metabolism of Carbon and Nitrogen in Different Maize ( Zea mays L.)Genotypes

Light is one of the important factors influencing kernel growth and development of maize(Zea mays L.).A split plot experiment was conducted under field conditions to study the effects of low-light stress and light recovery on kernel setting,and carbon and nitrogen metabolism of different maize genotypes in 2011 and 2012.Light treatments(natural light and 50% shading from three days before tasselling to ten days after silking)were the main plots and cultivars(low light sensitive hybrid Yuyu 22 and low light tolerance hybrid Zhengdan 958.)were sub-plots.The results showed that low-light stress delayed the growth and development of kernels,raised the number of abortive kernel,reduced kernel volume and dry weight.Contents of soluble sugar,sucrose and starch of apical kernel were increased while the content of nitrogen and C/N ratio decreased.Starch grain density in kernel endosperm cells of Yuyu 22 reduced while that of Zhengdan 958 almost remained the same.Differences between treatments of Yuyu 22 were greater than those of Zhengdan 958,the reduction of kernel growth and development was greater in the apical than in the middle of ear.Zhengdan 958 showed stronger compensatory effect after light recovery,differences between light treatments and the control in kernel volume,dry weight,kernel carbon and nitrogen contents and C/N ratio of Zhengdan 958 were lower than that of Yuyu 22.The declines of the starch synthesis and C/N ratio are the primary causes leading to kernel abortion.

Chlorogenic Acid and Maize Ear Rot Resistance: A Dynamic Study InvestigatingFusarium graminearumDevelopment, Deoxynivalenol Production, and Phenolic Acid Accumulation

Fusarium graminearum is the causal agent of Gibberella ear rot and produces trichothecene mycotoxins. Basic questions remain unanswered regarding the kernel stages associated with trichothecene biosynthesis and the kernel metabolites potentially involved in the regulation of trichothecene production in planta. In a two-year field study, F. graminearum growth, trichothecene accumulation, and phenolic acid composition were monitored in developing maize kernels of a susceptible and a moderately resistant variety using quantitative polymerase chain reaction and liquid chromatography coupled with photodiode array or mass spectrometry detection. Infection started as early as the blister stage and proceeded slowly until the dough stage. Then, a peak of trichothecene accumulation occurred and infection progressed exponentially until the final harvest time. Both F. graminearum growth and trichothecene production were drastically reduced in the moderately resistant variety. We found that chlorogenic acid is more abundant in the moderately resistant variety, with levels spiking in the earliest kernel stages induced by Fusarium infection. This is the first report that precisely describes the kernel stage associated with the initiation of trichothecene production and provides in planta evidence that chlorogenic acid may play a role in maize resistance to Gibberella ear rot and trichothecene accumulation.

The dent stage of maize kernels is the most conducive for fumonisin biosynthesis under field conditions.

The fungal pathogen Fusarium verticillioides infects maize ears and produces fumonisins, known for their adverse effects on human and animal health. Basic questions remain unanswered regarding the kernel stage(s) associated with fumonisin biosynthesis and the kernel components involved in fumonisin regulation during F. verticillioides-maize interaction under field conditions. In this 2-year field study, the time course of F. verticillioides growth and fumonisin accumulation in developing maize kernels, along with the variations in kernel pH and amylopectin content, were monitored using relevant and accurate analytical tools. In all experiments, the most significant increase in fumonisin accumulation or in fumonisin productivity (i.e., fumonisin production per unit of fungus) was shown to occur within a very short period of time, between 22/32 and 42 days after inoculation and corresponding to the dent stage. This stage was also characterized by acidification in the kernel pH and a maximum level of amylopectin content. Our data clearly support published results based on in vitro experiments suggesting that the physiological stages of the maize kernel play a major role in regulating fumonisin production. Here we have validated this result for in planta and field conditions, and we demonstrate that under such conditions the dent stage is the most conducive for fumonisin accumulation.

Resistant-Starch Formation in High-Amylose Maize Starch during Kernel Development

The objective of this study was to understand the resistant-starch (RS) formation during kernel development of a high-amylose maize, GEMS-0067 line. The RS content of the starch, determined using AOAC method 991.43 for total dietary fiber, increased with kernel maturation and increase in the amylose/intermediate component (IC) content of the starch. Gelatinization of the native starches showed a major thermal transition with peak temperature at 76.6-81.0 degrees C. An additional peak ( approximately 97.1 degrees C) first appeared 20 days after pollination and then developed into a significant peak on later dates. After removal of lipids from the starch, this peak disappeared, but the conclusion gelatinization temperature remained the same. The proportion of the enthalpy change of the thermal transition above 95 degrees C, calculated from the thermogram of the defatted starch, increased with kernel maturation and was significantly correlated with the RS content of the starch (r = 0.98). These results showed that the increase in crystallites of amylose/IC long-chain double helices in the starch resulted in the increase in the RS content of the starch during kernel development.

Onset of grain filling is associated with a change in properties of linker histone variants in maize kernels

In maize kernel development, the onset of grain-filling represents a major developmental switch that correlates with a massive reprogramming of gene expression. We have isolated chromosomal linker histones from developing maize kernels before (11 days after pollination, dap) and after (16 dap) initiation of storage synthesis. Six linker histone gene products were identified by MALDI-TOF mass spectrometry. A marked shift of around 4 pH units was observed for the linker histone spot pattern after 2D-gel electrophoresis when comparing the proteins of 11 and 16 dap kernels. The shift from acidic to more basic protein forms suggests a reduction in the level of post-translational modifications of linker histones during kernel development. Analysis of their DNA-binding affinity revealed that the different linker histone gene products bind double-stranded DNA with similar affinity. Interestingly, the linker histones isolated from 16 dap kernels consistently displayed a lower affinity for DNA than the proteins isolated from 11 dap kernels. These findings suggest that the affinity for DNA of the linker histones may be regulated by post-translational modification and that the reduction in DNA affinity could be involved in a more open chromatin during storage synthesis.

ZmPIN1-Mediated Auxin Transport Is Related to Cellular Differentiation during Maize Embryogenesis and Endosperm Development

To study the influence of PINFORMED1 (PIN1)-mediated auxin transport during embryogenesis and endosperm development in monocots, the expression pattern of the three identified ZmPIN1 genes was determined at the transcript level. Localization of the corresponding proteins was also analyzed during maize (Zea mays) kernel development. An anti-indole-3-acetic acid (IAA) monoclonal antibody was used to visualize IAA distribution and correlate the direction of auxin active transport, mediated by ZmPIN1 proteins, with the actual amount of auxin present in maize kernels at different developmental stages. ZmPIN1 genes are expressed in the endosperm soon after double fertilization occurs; however, unlike other tissues, the ZmPIN1 proteins were never polarly localized in the plasma membrane of endosperm cells. ZmPIN1 transcripts and proteins also colocalize in developing embryos, and the ZmPIN1 proteins are polarly localized in the embryo cell plasma membrane from the first developmental stages, indicating the existence of ZmPIN1-mediated auxin fluxes. Auxin distribution visualization indicates that the aleurone, the basal endosperm transfer layer, and the embryo-surrounding region accumulate free auxin, which also has a maximum in the kernel maternal chalaza. During embryogenesis, polar auxin transport always correlates with the differentiation of embryo tissues and the definition of the embryo organs. On the basis of these reports and of the observations on tissue differentiation and IAA distribution in defective endosperm-B18 mutant and in N-1-naphthylphthalamic acid-treated kernels, a model for ZmPIN1-mediated transport of auxin and the related auxin fluxes during maize kernel development is proposed. Common features between this model and the model previously proposed for Arabidopsis (Arabidopsis thaliana) are discussed.

Cloning and characterization of a putative GS3 ortholog involved in maize kernel development

The GS3 gene was the first identified gene controlling the grain size in rice. It has been proven to be involved in the evolution of grain size during domestication. We isolated the maize ortholog, ZmGS3 and investigated its role in the evolution of maize grain size. ZmGS3 has five exons encoding a protein with 198 amino acids, and has domains in common with the rice GS3 protein. Compared with teosinte, maize has reduced nucleotide diversity at ZmGS3, and the reduction is comparable to that found in neutrally evolving maize genes. No positive selection was detected along the length of the gene using either the Hudson-Kreitman-Aguadé or Tajima's D tests. Phylogenetic analysis reveals a distribution of maize sequences among two different clades, with one clade including related teosinte sequences. The nucleotide polymorphism analysis, selection test and phylogenetic analysis reveal that ZmGS3 has not been subjected to selection, and appears to be a neutrally evolving gene. In maize, ZmGS3 is primarily expressed in immature ears and kernels, implying a role in maize kernel development. Association mapping analysis revealed one polymorphism in the fifth exon that is significantly associated with kernel length in two environments. Also one polymorphism in the promoter region was found to affect hundred kernel weight in both environments. Collectively, these results imply that ZmGS3 is involved in maize kernel development but with different functional polymorphisms and thus, possibly different mechanisms from that of the rice GS3 gene.

Heterotic patterns of sugar and amino acid components in developing maize kernels

Heterosis is the superior performance of hybrids over their inbred parents. Despite its importance, little is known about the genetic and molecular basis of this phenomenon. Heterosis has been extensively exploited in plant breeding, particularly in maize (Zea mays, L.), and is well documented in the B73 and Mo17 maize inbred lines and their F1 hybrids. In this study, we determined the dry matter, the levels of starch and protein components and a total of 24 low-molecular weight metabolites including sugars, sugar-phosphates, and free amino acids, in developing maize kernels between 8 and 30 days post-pollination (DPP) of the hybrid B73 x Mo17 and its parental lines. The tissue specificity of amino acid and protein content was investigated between 16 and 30 DPP. Key observations include: (1) most of the significant differences in the investigated tissue types occurred between Mo17 and the other two genotypes; (2) heterosis of dry matter and metabolite content was detectable from the early phase of kernel development onwards; (3) the majority of metabolites exhibited an additive pattern. Nearly 10% of the metabolites exhibited nonadditive effects such as overdominance, underdominance, and high-parent and low-parent dominance; (4) The metabolite composition was remarkably dependent on kernel age, and this large developmental effect could possibly mask genotypic differences; (5) the metabolite profiles and the heterotic patterns are specific for endosperm and embryo. Our findings illustrate the power of metabolomics to characterize heterotic maize lines and suggest that the metabolite composition is a potential marker in the context of heterosis research.

Genetic manipulation of lysine catabolism in maize kernels

In plants, lysine catabolism is thought to be controlled by a bifunctional enzyme, lysine ketoglutarate reductase/saccharopine dehydrogenase (LKR/SDH). Lysine is converted to saccharopine, through condensation with alpha-ketoglutarate, by LKR, and subsequently to glutamate and alpha-aminoadipate-delta-semialdehyde by SDH. To investigate lysine catabolism in maize kernels, we generated transgenic plants with suppressed LKR/SDH activity in either endosperm or embryo. We found that the suppression of LKR/SDH in endosperm induced an increase in free lysine in developing endosperm, which peaked at 32 days after pollination. At later stages of kernel development, most of the free lysine was found in the embryo along with an elevated level of saccharopine. By combining endosperm LKR/SDH suppression with embryo LKR/SDH suppression through crosses, the saccharopine level in embryo was reduced and resulted in higher lysine accumulation in mature kernels. These results reveal new insights into how free lysine level is regulated and distributed in developing maize kernels and demonstrate the possibility of engineering high lysine corn via the suppression of lysine catabolism.

Molecular and biochemical mechanisms in maize endosperm development: The role of pyruvate-Pi-dikinase and Opaque-2 in the control of C/N ratio

A combined transcriptomic, proteomic and metabolic analysis provided an overview of the main changes occurring in gene expression during maize kernel development. It allowed identifying genes expressed at each developmental stage and the shift occurring from one stage to the other. A major change occurred at the transition from lag phase where final grain size is established to grain filling where starch and protein are accumulated in the endosperm storage tissue. Although the expression of enzymes involved in storage product synthesis is dominant in the accumulation phase, the proportion of protein destination and protein synthesis gene products is still important. Detailed proteomic analysis of metabolism shows an upsurge of the pyruvate-Pi-dikinase (PPDK) in the late filing period (21 DAP onwards) that is interpreted as a switch in the starch/protein balance. This hypothesis is based on biochemical arguments involving the negative effect of PPi on the ADP-glucose pyrophosphorylase (Agpase), a key-enzyme of starch synthesis, and the role of phospho enolpyruvate (PEP) in aromatic amino acid synthesis. It is substantiated by the data on the Opaque-2 gene encoding a transcription factor with pleiotropic effect affecting lysine content and carbohydrate metabolism, thus acting indirectly on starch/amino acid ratio. The direct effect of O2 on PPDK gene expression provides a clue for explaining the competition between C and N metabolisms. This epistatic relationship between PPDK and O2 is further supported by quantitative and association genetics. To cite this article: J.-L. Prioul et al., C. R. Biologies 331 (2008).

Genome-wide analysis of gene expression profiles during the kernel development of maize ( Zea mays L.)

Maize kernel is an important source of food, feed, and industrial raw materials. The elucidation of the molecular mechanisms of maize kernel development will be helpful for the manipulation of maize improvements. A microarray with approximately 58,000 probes was used to study dynamic gene expression during kernel development from fertilization to physiological maturity. By comparing six consecutive time points, 3445 differentially expressed genes were identified. These genes were then grouped into 10 clusters showing specific expression patterns using a K-means clustering algorithm. An investigation of function and expression patterns of genes elucidate the regulation mechanism underlying the important developmental processes cell division and kernel filling. The differential expression of genes involved in plant hormone signaling pathways suggested that phytohormone might play a critical role in the kernel developmental process. Moreover, regulation of some transcription factors and protein kinases might be involved in the whole developmental process.

Starch Accumulation and Enzyme Activities Associated with Starch Synthesis in Maize Kernels

Abstract The filling rate and the starch accumulation in developing maize kernel were analyzed. The changes of enzyme activities associated with sucrose metabolism and starch biosynthesis were investigated. The purpose is to discuss the enzymatic mechanisms responsible for starch synthesis. Two types of maize cultivars ( Zea mays), high starch maize (Feiyu 3) and normal maize (Yuyu 22), were grown in a corn field. The factors involved in starch synthesis were performed during the growth period. The kernel filling rate, the sucrose content, the starch accumulating rates and the activities of SS (sucrose synthase), GBSS (granule-bound starch synthase), SBE (starch branching enzyme) of Feiyu 3, which has high starch content, were significantly higher than those of Yuyu 22, which has low starch content, after 10 DAP (days after pollination). Correlation analysis indicated that ADPGPPase (ADP-glucose pyrophosphorylase) and DBE (starch debranching enzyme) were not correlated with the starch accumulating rates and the kernel filling rate, but the SS activity at the middle and late period were highly significantly correlated with the starch accumulating rates and the kernel filling rate. The GBSS activity was highly significantly correlated with the amylose accumulating rate, but not correlated with the kernel filling rate. The SBE activity was highly significantly correlated with the amylopectin accumulating rate and the kernel filling rate. It was not ADPGPPase and DBE, but SS was the rate-limiting factor of starch biosynthesis in developing maize kernels. GBSS had an important effect on amylose accumulation, and SBE had a significant effect on amylopectin accumulation.

Phenolic Compounds Accumulate Specifically in Maternally‐Derived Tissues of Developing Maize Kernels

ABSTRACTPhenolic compounds function in natural resistance of plants to insects, pathogens, and abiotic stresses. We investigated the flavonoid and phenolic acid content in developing maize kernels from 8 to 28 days after pollination and found a complex and developmentally dynamic mixture of compounds. LC‐MS analysis showed no detectable levels of the flavonoid maysin in any of the kernel tissue tested. Accumulation of the three most abundant analytes was developmentally and spatially regulated and independent of the regulatory transcription factor pericarp color 1 (P1). Major analytes have UV characteristics similar to hydroxycinnamic acids. LC‐MS indicates molecular weights of 410, 440, and 862 mass units, representing coumaroylferuloylputrescine (CFP), diferuloylputrescine (DFP), and a novel compound, respectively. There are large differences in the abundance of these compounds between genetic backgrounds, and accumulation increases with age. We dissected and analyzed various tissue types and demonstrated that the majority of these compounds are present in pedicels, with lesser amounts in pericarp and placento‐chalazal regions. These data indicate that complex phenolic acids and amides rather than flavonoids are the major phenolic constituents in developing kernels, and accumulation of these compounds is developmentally and spatially regulated, with the greatest abundance in maternal tissues surrounding the kernel.

Zein Transcription and Endoreduplication in Maize Endosperm are Differentially Affected by Heat Stress

High temperature stress imposed during the cell division stage of maize (Zea mays L.) kernel development adversely affects growth and mature mass. However, the processes affected by heat stress are not known. The goal of this study was to determine the mechanisms by which heat stress affects early zein accumulation in maize kernels. Intact ears of growth chamber–grown plants were subjected to heat stress (continuous 35°C) for 2 or 4 d, starting at 5 d after pollination (DAP). Both the 27 kDa and cluster 1 zeins of subfamily 4 (ZSF4C1) zein mRNA steady‐state levels were significantly delayed by 4 d of heat stress (DHS), but were not affected by 2 DHS. Similarly, transcription rates of both zeins were reduced in the endosperm of kernels exposed to 4 DHS treatment up to 17 DAP. The 2 DHS treatment significantly delayed endosperm endoreduplication, up to 17 DAP, whereas 4 DHS significantly repressed it. The lack of coordinate changes among mRNA steady‐state levels, transcription rates, and endoreduplication during heat stress indicates that the effects of heat stress on zein transcription rates may not be directly related to alterations in endoreduplication. Instead, zein transcription is most likely affected by a delay in endosperm development.

Robustness of central carbohydrate metabolism in developing maize kernels

The central carbohydrate metabolism provides the precursors for the syntheses of various storage products in seeds. While the underlying biochemical map is well established, little is known about the organization and flexibility of carbohydrate metabolic fluxes in the face of changing biosynthetic demands or other perturbations. This question was addressed in developing kernels of maize ( Zea mays L.), a model system for the study of starch and sugar metabolism. 13C-labeling experiments were carried out with inbred lines, heterotic hybrids, and starch-deficient mutants that were selected to cover a wide range of performances and kernel phenotypes. In total, 46 labeling experiments were carried out using either [U- 13C 6]glucose or [U- 13C 12]sucrose and up to three stages of kernel development. Carbohydrate flux distributions were estimated based on glucose isotopologue abundances, which were determined in hydrolysates of starch by using quantitative 13C-NMR and GC-MS. Similar labeling patterns in all samples indicated robustness of carbohydrate fluxes in maize endosperm, and fluxes were rather stable in response to glucose or sucrose feeding and during development. A lack of ADP-glucose pyrophosphorylase in the bt2 and sh2 mutants triggered significantly increased hexose cycling. In contrast, other mutations with similar kernel phenotypes had no effect. Thus, the distribution of carbohydrate fluxes is stable and not determined by sink strength in maize kernels.

Changes in flux pattern of the central carbohydrate metabolism during kernel development in maize

Developing kernels of the inbred maize line W22 were grown in sterile culture and supplied with a mixture of [U– 13C 6]glucose and unlabeled glucose during three consecutive intervals (11–18, 18–25, or 25–32 days after pollination) within the linear phase of starch formation. At the end of each labeling period, glucose was prepared from starch and analyzed by 13C isotope ratio mass spectrometry and high-resolution 13C NMR spectroscopy. The abundances of individual glucose isotopologs were calculated by computational deconvolution of the NMR data. [1,2- 13C 2]-, [5,6- 13C 2]-, [2,3- 13C 2]-, [4,5- 13C 2]-, [1,2,3- 13C 3]-, [4,5,6- 13C 3]-, [3,4,5,6- 13C 4]-, and [U– 13C 6]-isotopologs were detected as the major multiple-labeled glucose species, albeit at different normalized abundances in the three intervals. Relative flux contributions by five different pathways in the primary carbohydrate metabolism were determined by computational simulation of the isotopolog space of glucose. The relative fractions of some of these processes in the overall glucose cycling changed significantly during maize kernel development. The simulation showed that cycling via the non-oxidative pentose phosphate pathway was lowest during the middle interval of the experiment. The observed flux pattern could by explained by a low demand for amino acid precursors recruited from the pentose phosphate pathway during the middle interval of kernel development.

Response of maize inbred lines to a defoliation treatment inducing tolerance to cold at germination

Defoliation during maize (Zea mays L.) kernel development has been observed to induce tolerance to cold of germinating seeds in responsive genotypes. The objectives of this study were to evaluate the response to defoliation of immature embryo and mature seed germinability at cold and to verify if the response was influenced by the developmental stage at which the treatment was applied. In three environments, six inbred lines (B73, IABO78, Lo1016, Lo964, Mo17, Os420) were defoliated (D) approximately 20 days after pollination (DAP) or not defoliated (ND). Immature embryos were excised three days after defoliation and germinated in vitro at 9 or 25 ∘C. At maturation, kernel germination was tested at the same temperatures. Defoliation improved cold tolerance and mean time to germination (MTG) at 9 ∘C of both embryos and kernels of Lo1016. To study the effect of kernel developmental stage on response to defoliation, plants of B73, Lo1016 and Lo964 were defoliated at 15, 18, 21, 24, 27, 30, 33, 36, and 39 DAP, or not defoliated. At the same DAP, immature grains were analyzed for dry weight, water and abscisic acid (ABA) content. In Lo1016, low amounts of kernel ABA were detected at all stages, while in Lo964 and B73 ABA increased during development. Lo1016 mature kernels showed an improvement of cold tolerance due to defoliation at all times, while the other genotypes did not. In conclusion inbred lines showed variability for mature seed and immature embryo tolerance to cold at germination and for the ability to acquire tolerance after defoliation.

Phytosulphokine gene regulation during maize (Zea mays L.) reproduction*

The sulphated pentapeptide phytosulphokine (PSK) was identified as a substance that promotes cell division in low-density suspension cultures and has been implicated in various aspects of tissue differentiation in plants. The peptide is derived from PSK precursor proteins that are encoded by small gene families. The physiological roles of PSK are still not clearly defined and little is known about expression of members of the PSK precursor gene family in any plant species. In this study, highly regulated tissue and cell type-specific expression are described for four PSK genes from maize (Zea mays L.) in female and male gametophytes, and during seed development. ZmPSK1 and ZmPSK3 were specifically and differentially expressed in cells of female and male gametophytes and in female and male gametes. In anthers ZmPSK1 or ZmPSK3 transcripts were found, for example, at high levels in secretory tapetal cells which support developing microspores. ZmPSK1 mRNA was abundant in mature pollen including sperm cells. ZmPSK1 and ZmPSK3 transcripts were also detected in egg and central cells of the female gametophyte and ZmPSK1 mRNA was present in synergids, indicating that the PSK peptide probably plays a role during gametogenesis, pollen germination, and fertilization. In developing maize kernels all four ZmPSK genes were expressed, albeit with striking differences in their expression patterns. It is proposed here that PSK is required for numerous but defined processes during gametophyte and early sporophyte development. In general, PSK availability appears to be controlled through transcriptional regulation in a tissue and cell type-specific and development-dependent manner.