What type of flowering plant is maize

The genetics of inflorescence and flower development in maize and other grasses has been recently reviewed by other authors McSteen et al. Current knowledge of grass inflorescence development is briefly summarized here and the latest work is reviewed. The focus is on maize, but some discoveries in rice are also included. The basic unit of grass inflorescence architecture is the spikelet, a compact axillary branch that consists of two bracts subtending one to several reduced flowers Clifford, Maize is a monoecious plant that produces male flowers on a terminal tassel Fig.

The tassel initiates several long, indeterminate branches at the base while the ear consists of a single spike with no long branches. The tassel's main spike and branches, and the entire ear, produce short branches spikelet pairs that bear two spikelets Figs. The branches and spikelet pairs arise in the axils of small, undeveloped leaves referred to as bracts.

In maize, spikelet and spikelet pair meristems are considered determinate because they produce a defined number of organs Vollbrecht et al. Wild-type maize inflorescences. A Tassel bearing spikelet pairs inset on branches and main spike. B Ear showing rows of kernels. C Scanning electron microscopy of a tassel primordium initiating spikelet pair meristems. D Scanning electron microscopy of an ear. Spm: spikelet pair meristem.

Sm: spikelet meristem. Unlike its ancestor teosinte, which is induced to flower by short days, maize undergoes transition from the vegetative to reproductive phase after producing a fixed number of leaves Irish and Nelson, The most important known regulator of the transition to reproductive stage in maize is indeterminate1 id1.

The id1 gene encodes a zinc-finger protein that is produced in young leaves. ID1 functions non-autonomously to signal to the shoot apical meristem SAM for the transition to a reproductive stage. Mutants of id1 form many more leaves than wild-type maize and show vegetative reversion in the tassel with plantlets arising in male spikelets, often complete with roots Colasanti et al.

Phenotypic analyses of zfl1 and zfl2 mutants shows that their function is somewhat conserved in maize. While single zfl1 and zfl2 mutants have mild phenotypes, the double mutants do not undergo normal transition to reproductive development and have abnormal terminal inflorescences.

Similarly, in the tassel, paleas and lemmas are normal but stamens do not develop and are sometimes replaced by lemma or palea-like organs Bomblies et al. Mutations in these genes cause an enlargement of the shoot and flower meristems, resulting in flowers with extra floral parts Clark et al. In recent years it has become evident that much of the CLV pathway is conserved between Arabidopsis and grasses. The inflorescence phenotypes of td1 and fea2 mutants are similar. Mutations result in fasciated ears, an increase in spikelet density in the tassel due to a thicker rachis, and an increase in stamen number in male florets.

Ears have increased seed row number and are shorter and fatter than wild type. These phenotypes are consistent with the observed increase in size of all inflorescence meristems of td1 and fea2 mutants. A dominant maize mutant that causes fasciation of the ear, Fasciated ear1 Fas1 Fig.

Mutations in rice orthologues of CLV1 and CLV3 show similar phenotypes to maize td1 and fea2 mutants, especially the enlargement of shoot apical and floral meristems. Despite its broad expression pattern, mutations in FON1 only affect the floral meristems by increasing the numbers of palea, lemma, stamens, and pistils.

Mutations in FON4 , a gene with sequence similarity to CLV3 , cause enlargement of shoot apical, inflorescence, and floral meristems Chu et al. Consequently, FON4 mutants have thicker stems, additional inflorescence branches, and extrafloral organs.

FON4 is expressed in the central zone of vegetative, inflorescence, and floral meristems, similar to CLV3.

Another rice CLV3 -like gene, FON2, has a broad expression pattern, but its loss of function only affects the specification of flower organ number, while inflorescences are normal Suzaki et al. This suggests that in vegetative and inflorescence meristems, FON4 acts redundantly with FON2 to limit meristem size, but both are needed in floral meristems for proper flower formation. An opposite group of maize mutants fail to produce branches and spikelets.

Bracts are not affected, as demonstrated by a double mutant with tasselsheath , which makes enlarged bracts McSteen and Hake, Ears often fail to form and if they do, have few kernels and extra silks. Tassels have fewer branches and fewer spikelet pairs Kerstetter et al. Although the inflorescence defects are similar, the effect on vegetative branching differs.

The pattern of expression of ra2 , a marker of the very early stages of axillary branching Bortiri et al. Expression of ra2 in bif2 tassels show that this mutant initiates fewer than normal meristems, but they are of normal size. On the other hand, ba1 tassels have a normal distribution of axillary meristems as determined by the domains of ra2 expression but the size of the meristem anlagen is smaller, indicating that meristems of ba1 mutants fail to grow because they do not reach a critical size E Bortiri et al.

Spikelet pairs are a derived feature, present only in the Andropogoneae tribe, a monophyletic group that includes species such as maize, sorghum, and sugar cane LeRoux and Kellogg, ; Group, Both spikelet pairs and indeterminate branches originate from axillary meristems; however, unlike other axillary meristems of racemose inflorescences, spikelet pair meristems terminate after the production of two spikelets.

For this reason they are considered determinate. The phylogenetic placement of spikelet pairs suggests that a novel genetic programme arose in the Andropogoneae to specify the fate of determinate axillary meristems.

The ramosa mutants ra1 , ra2 , ra3 provide the starting material to study the molecular basis of the spikelet pair developmental programme. All ra mutants have axillary meristems with increased indeterminacy.

Ears of strong ra mutant alleles make branches and, in tassels, spikelet pairs are replaced by indeterminate branches. The long branches at the base of the tassel show increased degrees of branching Fig. RA2 of maize, sorghum, barley, and rice has a conserved C-terminus domain in addition to the LOB domain Bortiri et al.

In addition, ra2 expression is normal in ra1 and ra3 mutants Bortiri et al. These data suggest that ra2 and ra3 are upstream of ra1 , but in different pathways, and both are necessary for normal transcription of ra1. This finding would explain why ra1 is not expressed in branch meristems, which have ra2 but lack ra3 expression. Orthologues of ra2 and ra3 have been found in other grasses, including rice, and they have a similar expression pattern in those grass species Bortiri et al.

However, a ra1 orthologue has not been found in rice, or other grasses outside the Andropogoneae E Vollbrecht, EA Kellogg, and S Malcomber, unpublished results. Judging from the conservation of their sequence and expression patterns, it appears that RA2 and RA3 have been recruited to regulate ra1 , a gene whose function arose in the Andropogoneae, and the three act together to impose determinate fate to the spikelet pair meristems.

The function of ra2 and ra3 in other grasses is not yet known, but it is speculated that they modulate the extent of branching. Other mutants with loss of determinacy in both tassels and ears are branched silkless1 bd1 , indeterminate spikelet1 ids1 , tassel seed4 ts4 , fuzzy tassel , and the dominant mutant Tassel seed6 Ts6.

Each branch produces spikelet pair meristems, similar to the branches of the tassel. In the tassel, the spikelet meristems are also indeterminate, but produce spikelets in a distichous pattern, eventually producing fertile flowers Chuck et al.

The rice mutant, frizzy panicle , is an orthologue of bd1 with a similar mutant phenotype Komatsu et al. The ear and tassel are similar, although the ear is not fertile Chuck et al. The fuzzy tassel mutant also makes extra flowers per spikelet and multiple sterile flower parts but, unlike ids1 , it lacks normal glumes Fig. The expression of bd1 is seen as the spikelet pair meristem divides to produce a spikelet meristem.

The expression appears first at the base of one spikelet meristem, then at the base of the other. The expression marks a position between glume and spikelet meristem. Expression is not seen in the carpel and glumes Chuck et al. Ts6 and ts4 have similar phenotypes Fig. They both have feminized tassels see discussion below of sex determination , but can be male fertile depending on inbred background E Bortiri and S Hake, personal observations. Meristem determinacy is affected at different stages of inflorescence development in these two mutants.

Analysis by Erin Irish shows that in ts4, spikelet pair meristems are transformed into indeterminate branches bearing additional spikelet pairs, while in Ts6 the pedicellate spikelet meristems makes more flowers Irish, SEM analysis suggests that the branching patterns in ts4 are not as regular as seen in ids1 or bd1 mutants Irish, ; G Chuck and S Hake, unpublished results. All maize flowers initiate a palea, lemma, two lodicules, three stamens, and three carpels, which fuse to make a single pistil.

After initiation, pistil primordia in tassel flowers abort, and stamen primordia in the ear show cell-cycle arrest Dellaporta and Calderon-Urrea, In addition, the lower floret of the ear also arrests. As a result, tassel spikelets bear two functional staminate flowers, but in the ear only one pistillate flower develops to maturity.

Several mutants have been found that alter sex determination of either male or female flowers. A special class of dwarf plants is andromonoecious, with male flowers in the tassel and perfect flowers in the ear.

Most andromonoecious dwarfs are defective in GA biosynthesis Phinney, , although the dominant mutant, D8 is defective in GA response Harberd and Freeling, An understanding of how GA regulates sex determination is not known, but the finding that GA levels are fold lower in developing tassel spikelets suggests that low GA levels are required for staminate flower development and higher levels trigger stamen abortion Rood and Pharis, An additional effect of GA-deficient mutants in maize is a reduction in tassel branch number.

This is most obvious in anther ear1 , which encodes an ent-kaurene synthase, an enzyme that catalyses an early step in the GA biosynthesis Bensen et al. Thus GA may regulate not only stamen abortion in the ear, but also tassel branching. In addition, the lower floret of the ear fails to abort. Expression of ts2 is not seen in ts1 mutants, thus ts1 is thought to operate upstream of ts2 Calderon-Urrea and Dellaporta, Double mutants with ts2 show that ts2 is epistatic to sk Jones, ; Veit et al.

These results suggest that SK negatively regulates TS2 in the upper floret of the ear, thus only this floret is protected from the cell death mediated by TS2. Double mutants between ts2 and D8-Mp1 show an additive phenotype with perfect flowers stamens and pistils in both ear and tassel Veit et al.

This results shows that the TS2 and GA pathways operate independently and the pistil development in the tassel is not dependent on the loss of stamens. The ABC model of flower development was originally described for the eudicots Arabidopsis and Antirrhinum.

This model holds that A-class genes specify sepal fate in the first flower whorl, A plus B genes specify petals in the second whorl, B plus C genes give rise to stamens, and C genes alone are needed for carpel development in the fourth whorl Coen and Meyerowitz, This model has been expanded recently to incorporate D class genes, responsible for the development of ovules, and E-class genes, which are necessary for normal expression of all the above-mentioned genes. Mutations in si1 transform lodicules into bract-like organs reminiscent of paleas or lemmas Ambrose et al.

This phenotype is similar to a loss of function of B-class genes in eudicots. Later, expression is restricted to the region of the floral meristem that will give rise to lodicules and stamens. The finding that SILKY1 has biochemical properties of B-class proteins and can rescue an Arabidopsis ap3 mutant shows that B-class function is conserved between grasses and Arabidopsis Whipple et al. In ag and ple mutants, flowers produce sepals and petals in a reiterative fashion Yanofsky et al.

Maize and rice have duplicate AG -like genes and they appear to have evolved partial subfunctionalization. Mutations in zag1 cause indeterminate growth of pistil primordia giving rise to more than one silk and undifferentiated masses of tissue inside the ovary Mena et al.

In the tassel some silks occasionally develop, indicating that pistil abortion is not complete, but the stamens are normal. The expression patterns of zag1 and zmm2 are largely non-overlapping because zag1 transcript levels are higher in pistils while zmm2 is expressed in stamens, suggesting a sex organ specialization that explains the lack of phenotype in tassel flowers of zag1 Mena et al.

A similar finding has been described in rice. However, OSMADS58 appears to have a role in floral meristem determinacy because mutants consistently had indeterminate organ development. Double mutant analysis of si1 and zag1 show the expected phenotype for a BC double mutant, i. The origin of the sterile floral parts of grasses has been a mystery for many years. In the last few years it has become evident that the ABC model of flower development applies, with some modifications, to maize and rice.

For example, the phenotypes of mutations in maize B- and C-class genes, and their orthologues in rice Nagasawa et al. The interpretation of lemma and palea, however, is more difficult because homeotic transformations in maize B- and C-class mutants generate leaf-like organs that have characteristics of both Ambrose et al. One hypothesis suggests that the lemma arose from reductions and fusions of bracts that formed outside the flower in the common ancestor of grasses and sister lineages Whipple and Schmidt, Much of the natural variation in inflorescence shape observed in maize and other grass species are actually due to the cumulative effect of several loci.

Therefore, and because of the economical importance of maize and grasses in general, the study of quantitative trait loci QTL is an important field of cereal genetics aimed at yield improvement. Quantitative studies have been energized recently by the advancement of genomic tools such as the sequencing of the rice genome and the rapid development of very dense genetic maps in several grass species.

As a consequence, QTL mapping with greatly improved resolution power is now a powerful tool to uncover genes that control important traits. Recently, two reports have characterized the contribution of QTL to inflorescence architecture in grasses. Using two sorghum inbred lines with different inflorescences, Brown et al. Their findings suggest that branches of different orders are under the control of different loci.

Dw3 , which encodes a P-glyocoprotein responsible for auxin transport Multani et al. The Sbra2 gene closely co-localized to one of two QTL detected for primary branch number.

Using maize tassels, Upadyayula and colleagues identified two QTL for higher branch number, five for spikelet pair density on the central spike, and two for spikelet pair density on the branches Upadyayula et al. It is interesting that the latter two sets of QTL spikelet pairs on the central spike versus that on primary branches are non-overlapping, again indicating that different loci have prominent roles at different stages of development.

Pollen shedding from tassel starts about one or two days before the silks first appear. Pollen shedding continued for several days- about a week with peak production about the third day.

Pollination of a corn plant is a vigorous process and a healthy plant ensures that there is always pollen present for every silk that emerges. Successful pollination results in successful fertilization and optimal kernel development. Although the process of pollination may seem to be susceptible to weather conditions, most of the time it overcomes adverse situations very well.

Pollen does not get easily washed of the tassel because little to no shedding occurs when humidity is excessively high. Pollen that has landed on silk also tend to fend off the weather quite easily because silk surface is sticky and germination takes place immediately after landing of the pollen. In a growing environment, heat and water stress usually can go hand-in-hand. Stress from water deficit conditions generally have more impact on corn pollination than heat stress.

Under high temperature conditions, corn plants require more energy to maintain the normal physiological processes. This can reduce the number of potential kernel per row. When only high temperature is present, the plant can access water from deeper soil profile, in which case the direct impact of heat will likely be low. If only water stress is present silk emergence and ear elongation is slower than the rate of pollen shed which affects the number of potential kernel per row.

This can result in earl with missing kernels. Prolonged dry conditions can impact the kernel development after pollination as well.