Multiple Links between HD-Zip Proteins and Hormone Networks

HD-Zip proteins are unique to plants, and contain a homeodomain closely linked to a leucine zipper motif, which are involved in dimerization and DNA binding. Based on homology in the HD-Zip domain, gene structure and the presence of additional motifs, HD-Zips are divided into four families, HD-Zip I–IV. Phylogenetic analysis of HD-Zip genes using transcriptomic and genomic datasets from a wide range of plant species indicate that the HD-Zip protein class was already present in green algae. Later, HD-Zips experienced multiple duplication events that promoted neo- and sub-functionalizations. HD-Zip proteins are known to control key developmental and environmental responses, and a growing body of evidence indicates a strict link between members of the HD-Zip II and III families and the auxin machineries. Interactions of HD-Zip proteins with other hormones such as brassinolide and cytokinin have also been described. More recent data indicate that members of different HD-Zip families are directly involved in the regulation of abscisic acid (ABA) homeostasis and signaling. Considering the fundamental role of specific HD-Zip proteins in the control of key developmental pathways and in the cross-talk between auxin and cytokinin, a relevant role of these factors in adjusting plant growth and development to changing environment is emerging.

Arabidopsis HD-Zip II proteins regulate the exit from proliferation during leaf development in canopy shade

The shade avoidance response is mainly evident as increased plant elongation at the expense of leaf and root expansion. Despite the advances in understanding the mechanisms underlying shade-induced hypocotyl elongation, little is known about the responses to simulated shade in organs other than the hypocotyl. In Arabidopsis, there is evidence that shade rapidly and transiently reduces the frequency of cell division in young first and second leaf primordia through a non-cell-autonomous mechanism. However, the effects of canopy shade on leaf development are likely to be complex and need to be further investigated. Using combined methods of genetics, cell biology, and molecular biology, we uncovered an effect of prolonged canopy shade on leaf development. We show that persistent shade determines early exit from proliferation in the first and second leaves of Arabidopsis. Furthermore, we demonstrate that the early exit from proliferation in the first and second leaves under simulated shade depends at least in part on the action of the Homeodomain-leucine zipper II (HD-Zip II) transcription factors ARABIDOPSIS THALIANA HOMEOBOX2 (ATHB2) and ATHB4. Finally, we provide evidence that the ATHB2 and ATHB4 proteins work in concert. Together the data contribute new insights on the mechanisms controlling leaf development under canopy shade.

Multiple Pathways in the Control of the Shade Avoidance Response

To detect the presence of neighboring vegetation, shade-avoiding plants have evolved the ability to perceive and integrate multiple signals. Among them, changes in light quality and quantity are central to elicit and regulate the shade avoidance response. Here, we describe recent progresses in the comprehension of the signaling mechanisms underlying the shade avoidance response, focusing on Arabidopsis, because most of our knowledge derives from studies conducted on this model plant. Shade avoidance is an adaptive response that results in phenotypes with a high relative fitness in individual plants growing within dense vegetation. However, it affects the growth, development, and yield of crops, and the design of new strategies aimed at attenuating shade avoidance at defined developmental stages and/or in specific organs in high-density crop plantings is a major challenge for the future. For this reason, in this review, we also report on recent advances in the molecular description of the shade avoidance response in crops, such as maize and tomato, and discuss their similarities and differences with Arabidopsis.

Homeodomain-Leucine Zipper II family of transcription factors to the limelight: central regulators of plant development

The Arabidopsis genome encodes 10 Homeodomain-Leucine Zipper (HD-Zip) II transcription factors that can be subdivided into 4 clades (α-δ). All the γ (ARABIDOPSIS THALIANA HOMEOBOX 2 [ATHB2], HOMEOBOX ARABIDOPSIS THALIANA 1 [HAT1], HAT2) and δ (HAT3, ATHB4) genes are regulated by light quality changes (Low Red [R]/Far-Red [FR]) that induce the shade avoidance response in most of the angiosperms. HD-Zip IIγ and HD-Zip IIδ transcription factors function as positive regulators of shade avoidance, and there is evidence that at least ATHB2 is directly positively regulated by the basic Helix-Loop-Helix (bHLH) proteins PHYTOCHROME INTERACTING FACTOR 4 (PIF4) and PIF5. Recent evidence demonstrate that, in addition to their function in shade avoidance, HD-Zip IIγ and HD-Zip IIδ proteins play an essential role in plant development from embryogenesis onwards in a white light environment.

Dynamics of the shade-avoidance response in Arabidopsis

Shade-intolerant plants perceive the reduction in the ratio of red light (R) to far-red light (FR) as a warning of competition with neighboring vegetation and display a suite of developmental responses known as shade avoidance. In recent years, major progress has been made in understanding the molecular mechanisms underlying shade avoidance. Despite this, little is known about the dynamics of this response and the cascade of molecular events leading to plant adaptation to a low-R/FR environment. By combining genome-wide expression profiling and computational analyses, we show highly significant overlap between shade avoidance and deetiolation transcript profiles in Arabidopsis (Arabidopsis thaliana). The direction of the response was dissimilar at the early stages of shade avoidance and congruent at the late ones. This latter regulation requires LONG HYPOCOTYL IN FAR RED1/SLENDER IN CANOPY SHADE1 and phytochrome A, which function largely independently to negatively control shade avoidance. Gene network analysis highlights a subnetwork containing ELONGATED HYPOCOTYL5 (HY5), a master regulator of deetiolation, in the wild type and not in phytochrome A mutant upon prolonged low R/FR. Network analysis also highlights a direct connection between HY5 and HY5 HOMOLOG (HYH), a gene functionally implicated in the inhibition of hypocotyl elongation and known to be a direct target of the HY5 transcription factor. Kinetics analysis show that the HYH gene is indeed late induced by low R/FR and that its up-regulation depends on the action of HY5, since it does not occur in hy5 mutant. Therefore, we propose that one way plants adapt to a low-R/FR environment is by enhancing HY5 function.

Arabidopsis HD-Zip II transcription factors control apical embryo development and meristem function

The Arabidopsis genome encodes ten Homeodomain-Leucine zipper (HD-Zip) II proteins. ARABIDOPSIS THALIANA HOMEOBOX 2 (ATHB2), HOMEOBOX ARABIDOPSIS THALIANA 1 (HAT1), HAT2, HAT3 and ATHB4 are regulated by changes in the red/far red light ratio that induce shade avoidance in most of the angiosperms. Here, we show that progressive loss of HAT3, ATHB4 and ATHB2 activity causes developmental defects from embryogenesis onwards in white light. Cotyledon development and number are altered in hat3 athb4 embryos, and these defects correlate with changes in auxin distribution and response. athb2 gain-of-function mutation and ATHB2 expression driven by its promoter in hat3 athb4 result in significant attenuation of phenotypes, thus demonstrating that ATHB2 is functionally redundant to HAT3 and ATHB4. In analogy to loss-of-function mutations in HD-Zip III genes, loss of HAT3 and ATHB4 results in organ polarity defects, whereas triple hat3 athb4 athb2 mutants develop one or two radialized cotyledons and lack an active shoot apical meristem (SAM). Consistent with overlapping expression pattern of HD-Zip II and HD-Zip III gene family members, bilateral symmetry and SAM defects are enhanced when hat3 athb4 is combined with mutations in PHABULOSA (PHB), PHAVOLUTA (PHV) or REVOLUTA (REV). Finally, we show that ATHB2 is part of a complex regulatory circuit directly involving both HD-Zip II and HD-Zip III proteins. Taken together, our study provides evidence that a genetic system consisting of HD-Zip II and HD-Zip III genes cooperates in establishing bilateral symmetry and patterning along the adaxial-abaxial axis in the embryo as well as in controlling SAM activity.

Plant adaptation to dynamically changing environment: the shade avoidance response

The success of competitive interactions between plants determines the chance of survival of individuals and eventually of whole plant species. Shade-tolerant plants have adapted their photosynthesis to function optimally under low-light conditions. These plants are therefore capable of long-term survival under a canopy shade. In contrast, shade-avoiding plants adapt their growth to perceive maximum sunlight and therefore rapidly dominate gaps in a canopy. Daylight contains roughly equal proportions of red and far-red light, but within vegetation that ratio is lowered as a result of red absorption by photosynthetic pigments. This light quality change is perceived through the phytochrome system as an unambiguous signal of the proximity of neighbors resulting in a suite of developmental responses (termed the shade avoidance response) that, when successful, result in the overgrowth of those neighbors. Shoot elongation induced by low red/far-red light may confer high relative fitness in natural dense communities. However, since elongation is often achieved at the expense of leaf and root growth, shade avoidance may lead to reduction in crop plant productivity. Over the past decade, major progresses have been achieved in the understanding of the molecular basis of shade avoidance. However, uncovering the mechanisms underpinning plant response and adaptation to changes in the ratio of red to far-red light is key to design new strategies to precise modulate shade avoidance in time and space without impairing the overall crop ability to compete for light.

The Arabidopsis homeodomain-leucine zipper II gene family: diversity and redundancy

The Arabidopsis genome contains 10 genes belonging to the HD-Zip II family including ATHB2 and HAT2. Previous work has shown that ATHB2 is rapidly and strongly induced by light quality changes that provoke the shade avoidance response whereas HAT2 expression responds to auxin. Here, we present a genome-wide analysis of the HD-Zip II family. Phylogeny reconstruction revealed that almost all of the HD-Zip II genes can be subdivided into 4 clades (alpha-delta), each clade comprising 2-3 paralogs. Gene expression studies demonstrated that all the gamma and delta genes are regulated by light quality changes. Kinetics of induction, low R/FR/high R/FR reversibility and auxin response analyses strongly suggested that HAT1, HAT3 and ATHB4, as ATHB2, are under the control of the phytochrome system whereas HAT2 is up-regulated by low R/FR as a consequence of the induction of the auxin signaling pathway provoked by FR-rich light. Root and shoot digital in situ revealed that gamma and delta genes are also tightly regulated during plant development with both distinct and overlapping patterns. Phenotypes of gain of function and dominant negative lines demonstrated that one or more of the HD-Zip II gamma genes negatively regulate cell proliferation during leaf development in a high R/FR light environment. Finally, target gene analysis using a chimeric transcription factor (HD-Zip2-V-G), known to activate ATHB2 target genes in a glucocorticoid-dependent manner, revealed that all the 10 HD-Zip II genes can be recognized by the HD-Zip 2 domain in vivo, implying an intricate negative feedback network.

A novel regulatory circuit underlying plant response to canopy shade

A plant growing in the field has the unique ability to sense the presence of other plants growing near by and adjust its growth rate accordingly. This ability to detect neighbors, which is referred to as shade avoidance response, is mediated by members of the phytochrome family which detect light in the red (R) and far-red (FR) region of the spectrum. Work done by several laboratories has shown that low R/FR provides the signal for shade avoidance response during which the elongation of stem-like organs occurs at the expense of leaf development. However, the mechanism by which the low R/FR signal is transduced to attenuate leaf development has remained largely unknown. In the August issue of Genes and Development, we have shown that low R/FR rapidly and transiently arrests the growth of the leaf primordium. By exploiting mutant analysis in combination with genome wide expression profiling, we have identified a novel regulatory circuit underlying plant response to canopy shade. Together, the data demonstrate that the growth arrest induced by low R/FR depends on auxin-induced cytokinin breakdown in pre-procambial cells of developing primordia. In this addendum, we discuss open questions to be addressed in the future.

Canopy shade causes a rapid and transient arrest in leaf development through auxin-induced cytokinin oxidase activity

A plant grown under canopies perceives the reduction in the ratio of red (R) to far-red (FR) light as a warning of competition, and enhances elongation growth in an attempt to overgrow its neighbors. Here, we report that the same low R/FR signal that induces hypocotyl elongation also triggers a rapid arrest of leaf primordium growth, ensuring that plant resources are redirected into extension growth. The growth arrest induced by low R/FR depends on auxin-induced cytokinin breakdown in incipient vein cells of developing primordia, thus demonstrating the existence of a previously unrecognized regulatory circuit underlying plant response to canopy shade.

A dynamic balance between gene activation and repression regulates the shade avoidance response in Arabidopsis

Plants grown under dense canopies perceive through the phytochrome system a reduction in the ratio of red to far-red light as a warning of competition, and this triggers a series of morphological changes to avoid shade. Several phytochrome signaling intermediates acting as positive regulators of accelerated elongation growth and induction of flowering in shade avoidance have been identified. Here we report that a negative regulatory mechanism ensures that in the presence of far-red-rich light an exaggerated plant response does not occur. Strikingly, this unpredicted negative regulatory mechanism is centrally involved in the attenuation of virtually all plant responses to canopy shade.

Twilight-zone and canopy shade induction of the Athb-2 homeobox gene in green plants

We present evidence that a novel phytochrome (other than phytochromes A and B, PHYA and PHYB) operative in green plants regulates the "twilight-inducible" expression of a plant homeobox gene (Athb-2). Light regulation of the Athb-2 gene is unique in that it is not induced by red (R)-rich daylight or by the light-dark transition but is instead induced by changes in the ratio of R to far-red (FR) light. These changes, which normally occur at dawn and dusk (end-of-day FR), also occur during the daytime under the canopy (shade avoidance). By using pure light sources and phyA/phyB null mutants, we demonstrated that the induction of Athb-2 by changes in the R/FR ratio is mediated for the most part by a novel phytochrome operative in green plants. Furthermore, PHYB plays a negative role in repressing the accumulation of Athb-2 mRNA in the dark and a minor role in the FR response. The strict correlation of Athb-2 expression with FR-induced growth phenomena suggests a role for the Athb-2 gene in mediating cell elongation. This interpretation is supported by the finding that the Athb-2 gene is expressed at high levels in rapidly elongating etiolated seedlings. Furthermore, as either R or FR light inhibits cell elongation in etiolated tissues, they also down-regulate the expression of Athb-2 mRNA. Thus, these data support the notion that changes in light quality perceived by a novel phytochrome regulate plant development through the action of the Athb-2 homeobox gene.

Ectopic expression of the Arabidopsis transcriptional activator Athb-1 alters leaf cell fate in tobacco

The Arabidopsis thaliana Athb-1 is a homeobox gene of unknown function. By analogy with homeobox genes of other organisms, its gene product, Athb-1, is most likely a transcription factor involved in developmental processes. We constructed a series of Athb-1-derived genes to examine the roles of Athb-1 in transcriptional regulation and plant development. Athb-1 was found to transactivate a promoter linked to a specific DNA binding site by transient expression assays. In transgenic tobacco plants, overexpression of Athb-1 or its chimeric derivatives with heterologous transactivating domains of the yeast transcription factor GAL4 or herpes simplex virus transcription factor VP16 conferred deetiolated phenotypes in the dark, including cotyledon expansion, true leaf development, and an inhibition of hypocotyl elongation. Expression of Athb-1 or the two chimeric derivatives also affected the development of palisade parenchyma under normal growth conditions, resulting in light green sectors in leaves and cotyledons, whereas other organs in the transgenic plants remained normal. Both developmental phenotypes were induced by glucocorticoid in transgenic plants expressing a chimeric transcription factor comprising the Athb-1 DNA binding domain, the VP16 transactivating domain, and the glucocorticoid receptor domain. Plants with severe inducible phenotypes showed additional abnormality in cotyledon expansion. Our results suggest that Athb-1 is a transcription activator involved in leaf development.

Ectopic expression of the Arabidopsis transcriptional activator Athb-1 alters leaf cell fate in tobacco

The Arabidopsis thaliana Athb-1 is a homeobox gene of unknown function. By analogy with homeobox genes of other organisms, its gene product, Athb-1, is most likely a transcription factor involved in developmental processes. We constructed a series of Athb-1-derived genes to examine the roles of Athb-1 in transcriptional regulation and plant development. Athb-1 was found to transactivate a promoter linked to a specific DNA binding site by transient expression assays. In transgenic tobacco plants, overexpression of Athb-1 or its chimeric derivatives with heterologous transactivating domains of the yeast transcription factor GAL4 or herpes simplex virus transcription factor VP16 conferred deetiolated phenotypes in the dark, including cotyledon expansion, true leaf development, and an inhibition of hypocotyl elongation. Expression of Athb-1 or the two chimeric derivatives also affected the development of palisade parenchyma under normal growth conditions, resulting in light green sectors in leaves and cotyledons, whereas other organs in the transgenic plants remained normal. Both developmental phenotypes were induced by glucocorticoid in transgenic plants expressing a chimeric transcription factor comprising the Athb-1 DNA binding domain, the VP16 transactivating domain, and the glucocorticoid receptor domain. Plants with severe inducible phenotypes showed additional abnormality in cotyledon expansion. Our results suggest that Athb-1 is a transcription activator involved in leaf development.

The Arabidopsis Athb-2 and -4 genes are strongly induced by far-red-rich light

The isolation and characterization of the Athb-2 gene, which codes for a HD-Zip protein in Arabidopsis thaliana is reported. Interestingly, Southern analysis has established that the Arabidopsis genome contains sequences which are highly related to the Athb-2 HD-Zip coding region. One of these genes, designated Athb-4, has been cloned and partially sequenced. Amino acid sequence comparison revealed that Athb-2 and -4 are members of a small family of HD-Zip related proteins. Athb-2 mRNA analysis showed that this gene is expressed during the vegetative and reproductive phases of plant growth. A significant increment in the amount of Athb-2 transcripts was observed in flowering plants. A higher steady-state level of the Athb-2 mRNA was also found in dark-adapted plants. Remarkably, far-red-rich light treatment of Arabidopsis plants results in a rapid and strong induction of the Athb-2 expression. This light treatment also induces the accumulation of Athb-4 transcripts, suggesting a similar role for the two members of this HD-Zip family.

[Enteroparasitoses in the V Region, Chile. A study of rural school children from Santo Domingo, 1987]

The results of a new enteroparasitological survey carried out by the authors are analyzed with the aim of contributing to the knowledge of the situation of these infections in the V Region, Chile. In 1987 the children of five rural schools of Santo Domingo were studied by means of the modified Telemann method, the Ziehl-Neelsen stain and seried Graham test. The parasites more frequently found were: E. vermicularis (50.4%) and G. lamblia (10.8%). No E. histolytica was found in these children. The most frequent commensal was E. nana (21.9%). Cryptosporidium sp. presented a low frequency (0.9%), a figure that in these asymptomatic subjects is in accordance with that found in outpatients with chronic diarrhea in Valparaíso, V Region.

[Cryptosporidiosis in the V Region Chile. III. Study of malnourished patients, 1985-1987]

Frequency of infection by Cryptosporidium sp. in 1,039 faecal smears stained by Ziehl-Neelsen, obtained from undernourished patients of a Nutritional Recovery Center and an ambulatory undernourished center from the Fifth Region, Chile, were studied. All underwent a coproparasitological examination by the modified Telemann method. Thirty eight (3.7%) patients infected by the parasite were detected, with an overall frequency of 8.5% among patients of the Nutritional Recovery Center and 1.9% among the ambulatory patients; this difference was statistically significant. The highest percentage of positive results were detected among the younger milk feeding infants. Also the percentage of difference among these and the older milk feeding infants (3.7%) was statistically significant. Association of Cryptosporidium sp. and Giardia lamblia was observed in 6 ambulatory patients (2.3%).