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author:("Ré, delfia A")
1.  Arabidopsis AtHB7 and AtHB12 evolved divergently to fine tune processes associated with growth and responses to water stress 
BMC Plant Biology  2014;14:150.
Background
Arabidopsis AtHB7 and AtHB12 transcription factors (TFs) belong to the homeodomain-leucine zipper subfamily I (HD-Zip I) and present 62% amino acid identity. These TFs have been associated with the control of plant development and abiotic stress responses; however, at present it is not completely understood how AtHB7 and AtHB12 regulate these processes.
Results
By using different expression analysis approaches, we found that AtHB12 is expressed at higher levels during early Arabidopsis thaliana development whereas AtHB7 during later developmental stages. Moreover, by analysing gene expression in single and double Arabidopsis mutants and in transgenic plants ectopically expressing these TFs, we discovered a complex mechanism dependent on the plant developmental stage and in which AtHB7 and AtHB12 affect the expression of each other. Phenotypic analysis of transgenic plants revealed that AtHB12 induces root elongation and leaf development in young plants under standard growth conditions, and seed production in water-stressed plants. In contrast, AtHB7 promotes leaf development, chlorophyll levels and photosynthesis and reduces stomatal conductance in mature plants. Moreover AtHB7 delays senescence processes in standard growth conditions.
Conclusions
We demonstrate that AtHB7 and AtHB12 have overlapping yet specific roles in several processes related to development and water stress responses. The analysis of mutant and transgenic plants indicated that the expression of AtHB7 and AtHB12 is regulated in a coordinated manner, depending on the plant developmental stage and the environmental conditions. The results suggested that AtHB7 and AtHB12 evolved divergently to fine tune processes associated with development and responses to mild water stress.
doi:10.1186/1471-2229-14-150
PMCID: PMC4064807  PMID: 24884528
AtHB7; AtHB12; Homeodomain-leucine zipper (HD-Zip I); Moderate water stress; Yield; Plant growth
2.  Nicotiana attenuata NaHD20 plays a role in leaf ABA accumulation during water stress, benzylacetone emission from flowers, and the timing of bolting and flower transitions 
Journal of Experimental Botany  2010;62(1):155-166.
Homeodomain-leucine zipper type I (HD-Zip I) proteins are plant-specific transcription factors associated with the regulation of growth and development in response to changes in the environment. Nicotiana attenuata NaHD20 was identified as an HD-Zip I-coding gene whose expression was induced by multiple stress-associated stimuli including drought and wounding. To study the role of NaHD20 in the integration of stress responses with changes in growth and development, its expression was silenced by virus-induced gene silencing (VIGS), and control and silenced plants were metabolically and developmentally characterized. Phytohormone profiling showed that NaHD20 plays a positive role in abscisic acid (ABA) accumulation in leaves during water stress and in the expression of some dehydration-responsive genes including ABA biosynthetic genes. Moreover, consistent with the high levels of NaHD20 expression in corollas, the emission of benzylacetone from flowers was reduced in NaHD20-silenced plants. Additionally, bolting time and the opening of the inflorescence buds was decelerated in these plants in a specific developmental stage without affecting the total number of flowers produced. Water stress potentiated these effects; however, after plants recovered from this condition, the opening of the inflorescence buds was accelerated in NaHD20-silenced plants. In summary, NaHD20 plays multiple roles in N. attenuata and among these are the coordination of responses to dehydration and its integration with changes in flower transitions.
doi:10.1093/jxb/erq252
PMCID: PMC2993906  PMID: 20713465
ABA; benzylacetone; corolla; HD-Zip; Nicotiana

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