Stomatal development in Arabidopsis and grasses: differences and commonalities

Stomata, found on the epidermis of all terrestrial plants, consist of two specialized cells called guard cells, which surround a tiny pore. Major advances have been made in our understanding of the genetic control of stomatal development in Arabidopsis and grasses. In Arabidopsis, three basic-helix-loop-helix (bHLH) genes control the successive steps that lead to stomatal formation. SPEECHLESS (SPCH) drives the cell division that initiates the stomatal cell lineage, MUTE induces the formation of the immediate stomatal precursor cell, and FAMA causes the stomatal precursor cell to divide into the two guard cells. Recent results demonstrate that these genes share functions with their grass homologs, and that MUTE is expressed later in development than its grass counterparts. Other differences in stomatal development between these two plant groups are exemplified by the PANGLOSS1 (PAN1) gene of maize. PAN1, which encodes a leucine-rich repeat receptor-like kinase with an inactive kinase domain, promotes polarization of the subsidiary mother cell and orients its cell division plane. Because such events do not exist in Arabidopsis, it is likely that the PAN1-like genes of Arabidopsis and PAN1 are paralogs. Together, these results indicate that distinctions in the regulation of gene expression and protein function are both responsible for the divergence of stomatal development between Arabidopsis and grasses.     

bHLH proteins know when to make a stoma

In Arabidopsis thaliana, stomata develop through a stereotypical pattern of cell divisions. Three recent publications demonstrate that three bHLH proteins act successively in such lineages to drive the formation of stomata. SPEECHLES drives the division that initiates the stomatal-cell lineage. Then MUTE induces the formation of the immediate stomatal precursor cell. Finally, FAMA causes the stomatal precursor cell to divide into the two guard cells that surround each stomatal pore.     

The bHLH protein, MUTE, controls differentiation of stomata and the hydathode pore in Arabidopsis

Stomata are turgor-driven epidermal valves on the surface of plants that allow for efficient gas and water exchange between the plant and its environment. The Arabidopsis thaliana basic helix-loop-helix (bHLH) protein, MUTE, is a master regulator of stomatal differentiation where it is required for progression through the stomatal lineage and the differentiation of stomata. The genetic control of stomatal spacing across the epidermal surface is variable in different organs. For instance, a distinct suite of genes from those in leaves regulates stomatal patterning in hypocotyls. Here we report that regardless of organ type, MUTE controls downstream events directing stomatal differentiation, specifically the transition from meristemoid to guard mother cell. Ectopic MUTE expression is sufficient to over-ride cell fate specification in cell types that do not normally differentiate stomata. Furthermore, MUTE is required for the production of the structure evolutionarily related to stomata, the hydathode pore. Consistently, MUTE displays expression at the tip of cotyledons and leaves, thus co-localizing with the auxin maxima. However, MUTE itself was not regulated by the auxin, and the absence of hydathode pores in mute did not affect the auxin maxima. Surprisingly, our analysis revealed that the requirement for MUTE could be partially circumvented under conditions of compromised inhibitory signaling.     

The too many mouths and four lips mutations affect stomatal production in Arabidopsis

Stomata regulate gas exchange through the aerial plant epidermis by controlling the width of a pore bordered by two guard cells. Little is known about the genes that regulate stomatal development. We screened cotyledons from ethyl methanesulfonate-mutagenized seeds of Arabidopsis by light microscopy to identify mutants with altered stomatal morphology. Two mutants, designated too many mouths (tmm) and four lips (flp), were isolated with extra adjacent stomata. The tmm mutation results in stomatal clustering and increased precursor cell formation in cotyledons and a virtual absence of stomata in the inflorescence stem. The flp mutation results in many paired stomata and a small percentage of unpaired guard cells in cotyledons. The double mutant (tmm flp) exhibits aspects of both parental phenotypes. Both mutations appear to affect stomatal production more than patterning or differentiation. tmm regulates stomatal production by controlling the formation, and probably the activity, of the stomatal precursor cell.     

Stomatal development and patterning in Arabidopsis leaves

The functional unit for gas exchange between plants and the atmosphere is the stomatal complex, an epidermal structure composed of two guard cells, which delimit a stomatal pore, and their subsidiary cells. In the present work, we define the basic structural unit formed in Arabidopsis thaliana during leaf development, the anisocytic stomatal complex. We perform a cell lineage analysis by transposon excision founding that at least a small percentage of stomatal complexes are unequivocally non-clonal. We also describe the three-dimensional pattern of stomata in the Arabidopsis leaf. In the epidermal plane, subsidiary cells of most stomatal complexes contact the subsidiary cells of immediately adjacent complexes. This minimal distance between stomatal complexes allows each stoma to be circled by a full complement of subsidiary cells, with which guard cells can exchange water and ions in order to open or to close the pore. In the radial plane, stomata (and their precursors, the meristemoids) are located at the junctions of several mesophyll cells. This meristemoid patterning may be a consequence of signals that operate along the radial axis of the leaf, which establish meristemoid differentiation precisely at these places. Since stomatal development is basipetal, these radially propagated signals may be transmitted in the axial direction, thus guiding stomatal development through the basal end of the leaf.     

Structural and functional aspects of stomata

Differential cell wall thickening in developing guard cells of Polypodium vulgare L. has been studied with particular reference to guard cell protoplast deformation and the eventual formation of the stomatal pore. Concomitant studies on the development of guard cell chloroplasts and their starch inclusions during ontogeny of the stomatal complex have provided data which have been incorporated into a model to account for the formation of the pore. Guard cell starch inclusions reach a maximum density per unit volume at the same time as the guard cell walls achieve maximum differential thickening. These events coincide with the development of the pore. It is suggested that, whilst pore formation is initiated enzymatically, the mechanical forces required to bring about the separation of the two guard cells are of an osmotic nature derived from starch hydrolysis. The development of the mesophyll in relation to the epidermis is examined in respect of the formation of substomatal chambers.     

The identification of genes involved in the stomatal response to reduced atmospheric relative humidity

Stomatal pores of higher plants close in response to decreases in atmospheric relative humidity (RH). This is believed to be a mechanism that prevents the plant from losing excess water when exposed to a dry atmosphere and as such is likely to have been of evolutionary significance during the colonization of terrestrial environments by the embryophytes. We have conducted a genetic screen, based on infrared thermal imaging, to identify Arabidopsis genes involved in the stomatal response to reduced RH. Here we report the characterization of two genes, identified during this screen, which are involved in the guard cell reduced RH signaling pathway. Both genes encode proteins known to be involved in guard cell ABA signaling. OST1 encodes a protein kinase involved in ABA-mediated stomatal closure while ABA2 encodes an enzyme involved in ABA biosynthesis. These results suggest, in contrast to previously published work, that ABA plays a role in the signal transduction pathway connecting decreases in RH to reductions in stomatal aperture. The identification of OST1 as a component required in stomatal RH and ABA signal transduction supports the proposition that guard cell signaling is organized as a network in which some intracellular signaling proteins are shared among different stimuli.     

狭基巢蕨叶表皮的结构和气孔器发育的观察

狭基巢蕨Ｎｅｏｔｏｐｔｅｒｉｓａｎｔｒｏｐｈｙｏｉｄｅｓ（Ｃｈｒｉｓｔ）Ｃｈｉｎｇ叶片的上表皮无气孔器,仅具表皮细胞,下表皮由表皮细胞和气孔器组成,气孔指数为２．５。上下表皮细胞和气孔器的细胞中均含有叶绿体。每个气孔器由２个肾形的保卫细胞和２～６个副卫细胞组成,其中以３个和４个副卫细胞的占绝大多数（３细胞的占４５．１％,４细胞的占４３．５％）。从发育上看,气孔器原始细胞进行２次分裂,产生２个保卫细胞和１个同源的副卫细胞。气孔器的发育过程大体可分为４个时期：（１）气孔器原始细胞的分化和分裂期;（２）保卫细胞母细胞成熟期;（３）保卫细胞母细胞分裂和气孔器幼期;（４）气孔器成熟期。狭基巢蕨的气孔器属于中周型     

Ultrastructure of stomatal development in Arabidopsis (Brassicaceae) leaves

Stomatal development was studied in wild-type Arabidopsis leaves using light and electron microscopy. Development involves three successive types of stomatal precursor cells: meristemoid mother cells, meristemoids, and guard mother cells (GMCs). The first two types divide asymmetrically, whereas GMCs divide symmetrically. Analysis of cell wall patterns indicates that meristemoids can divide asymmetrically a variable number of times. Before meristemoid division, the nucleus and a preprophase band of microtubules become located on one side of the cell, and the vacuole on the other. Meristemoids are often triangular in shape and have evenly thickened walls. GMCs can be detected by their roughly oval shape, increased starch accumulation, and wall thickenings on opposite ends of the cells. Because these features are also found in developing stomata, stomatal differentiation begins in GMCs. The wall thickenings mark the division site in the GMC since they overlie a preprophase band of microtubules and occur where the cell plate fuses with the parent cell wall. Stomatal differentiation in Arabidopsis resembles that of other genera with kidney-shaped guard cells. This identification of stages in stomatal development in wild-type Arabidopsis provides a foundation for the analysis of relevant genes and of mutants defective in stomatal patterning, cell specification, and differentiation.     

Morphogenetic effects of various growth substances on the cotyledonary stomata of brinjal and tomato

The effect of different growth substances on the development of normal and abnormal stomata are presented. Anomocytic, paracytic, anisocytic and stoma with a single subsidiary cell are observed. Abnormal developments like persistent stomatal cells, degeneration of guard cells, unusual thickening, unequal guard cells, single guard cells and size and shape of the pore are noticed in various growth substances. The growth substances also affect the stomatal frequency, stomatal index, epidermal frequency and size of guard and epidermal cells in both the plants. The highest meristematic activity is found in MOR 100 ppm in brinjal and in GA 25 ppm in tomato. The largest size of stomata is found in COL 25 ppm in brinjal and in MH 50 ppm in tomato. The same growth substance responds differently in the two plants.