In roots of Arabidopsis thaliana, Zn can be either loaded into the xylem for translocation to the shoot or stored in vacuoles. Vacuolar storage is achieved through the action of the Zn/Cd transporter HMA3 (Heavy Metal Atpase 3). The Col-0 accession has an HMA3 loss-of-function allele resulting in high shoot Cd, when compared to accession CSHL-5 which has a functional allele and low shoot Cd. Interestingly, both Col-0 and CSHL-5 have similar shoot Zn concentrations. We hypothesize that plants sense changes in cytosolic Zn that are due to variation in HMA3 function, and respond by altering expression of genes related to Zn uptake, transport and compartmentalisation, in order to maintain Zn homeostasis. The expression level of genes known to be involved in Zn homeostasis were quantified in both wild-type Col-0 and Col-0::HMA3CSHL-5 plants transformed with the functional CSHL-5 allele of HMA3. We observed significant positive correlations between expression of HMA3 and of genes known to be involved in Zn homeostasis, including ZIP3, ZIP4, MTP1, and bZIP19. The results support our hypothesis that alteration in the level of function of HMA3 is counterbalanced by the fine regulation of the Zn homeostasis gene network in roots of A. thaliana.

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-(Phytotoxic effects of fl uoride in leaf morpho-anatomy of Brachiaria brizantha (Hochst. Ex A. Rich.) Stapf and Brachiaria decumbens Stapf (Poaceae)). To assess the effects of fl uoride on signal grass, plants of Brachiaria brizantha and B. decumbens were submitted to simulated fl uoride fog (10 μg ml -1 ). Leaf fragments from apparently healthy portions were colleted 24 hours after the last exposure and used for microscopic analysis. Roots, leaves and rhizomes were submitted to fl uoride content quantifi cation. The leaves presented small, sparse necroses. The accumulation of fl uoride was observed on the leaves only. B. brizantha and B. decumbens presented, respectively, 4.4 and 5.2 times more fl uoride than the control treatment. No difference was observed concerning the accumulation potential of both species. The main micromorphological alterations observed were injuries near the trichomes, therefore, suggesting that these are the preferential routes by which pollutants enter the plants. Visually healthy leaf portions of the two species showed microscopic damages before symptoms appeared, presenting turgidity loss in the bulliform cells of B. brizantha and in all tissues of B. decumbens. Micromorphometric alterations were observed only in B. decumbens, indicating higher sensibility to fl uoride in comparison to B. brizantha. Further studies of long duration will be essential to determine the bioaccumulation potential of B. brizantha.

ResumoA identificação de caracteres morfofisiológicos associados à resistência à seca é de grande importância para a seleção de genótipos de eucalipto mais tolerantes. Além de características intrínsecas da espécie, fatores externos, como a suplementação de boro (B), podem potencializar essa tolerância. Este trabalho objetivou avaliar os efeitos da deficência hídrica em dois clones de eucalipto com tolerância diferencial à seca, bem como o potencial do B na mitigação dos danos. Mudas dos clones i144 (tolerante) e gg100 (sensível) foram cultivadas em solo, em casa de vegetação, com e sem restrição hídrica e de B. Foram realizadas avaliações fotossintéticas, do potencial hídrico foliar (Ψw leaf ) e de crescimento. Ambos os clones sofreram redução drástica do Ψw leaf quando a umidade do solo atingiu 65 % da capacidade de campo, independentemente da suplementação de B. O principal efeito do B como mitigador dos efeitos da seca no clone i144, tolerante, foi estimular o aumento da produção de raízes e redução na produção de biomassa foliar, otimizando a absorção de água e as perdas por transpiração. Em ambos os clones, as trocas gasosas foram afetadas pela seca, mas respostas diferenciais não foram observadas em função da suplementação de B. Verificou-se redução significativa da taxa de assimilação de CO 2 e na eficiência instantânea de uso da água no clone gg100, e aumento considerável da eficiência intrínseca de uso da água no clone i144. Ambos os clones apresentaram queda das taxas transpiratórias, a qual foi mais evidente no clone i144, como resultado da redução da condutância estomática. Verificou-se redução da taxa de transporte de elétrons no clone gg100 e do rendimento quântico efetivo do fotossistema II em ambos os clones, quando submetidos à seca. No clone i144, o B atenuou a dissipação de energia térmica nas plantas controle e submetidas à seca, fazendo com que, nestas últimas, os valores se igualassem aos do controle. Conclui--se que os efeitos da seca, bem como o potencial do B em mitigar seus danos, variam entre os genótipos, sendo o clone tolerante mais beneficiado pela suplementação de B.Palavras-chave: estresse hídrico, clone gg100, clone i144, fotossíntese, crescimento. AbstractThe identification of morphological-physiological traits associated with drought resistance is extremely important for the selection of the most drought-tolerant Eucalyptus genotypes. Aside from the intrinsic characteristics of the species, external factors such as boron (B) supplementation can raise tolerance. This work aimed to evaluate the effects of water deficit on two Eucalyptus clones with differential drought tolerance, and the potential of B to mitigate the damage. Plants of clones i144 (tolerant) and gg100 (sensitive) were grown in soil, in a greenhouse, with and without water and B restriction. We evaluated the photosynthetic rates, the leaf water potential (Ψw leaf ), and the growth. In both clones, Ψw leaf decreased drastically when soil moisture reached 65 % of the field capacity, regardless of B supply. The main...

Arsenic (As) is a toxic element to most organisms. Studies investigating anatomic alterations due to As exposure in plants are scarce but of utmost importance to the establishment of environmental biomonitoring techniques. So, this study aimed to investigate the effects of As on the development and initial root growth in Cajanus cajan (Fabaceae), characterize and quantify the possible damages, evaluate genotoxic effects, and identify structural markers to be used in environmental bioindication. Plants were exposed hydroponically to 0.5, 1.0, 1.5, and 2.0 mg As L(-1), as sodium arsenate. Growth parameters were measured, and in the end of the exposure, root samples were analyzed for qualitative and quantitative anatomical alterations. Arsenic genotoxicity was evaluated through analysis of the mitotic index in the root apex. Compared to the control, As-treated seedlings showed an altered architecture, with significantly decreased root length (due to the lower mitotic index in the apical meristem and reduced elongation of parenchyma cells) with darkened color, and abnormal development of the root cap. A significant increase in vascular cylinder/root diameter ratio was also detected, due to the reduction of the cellular spaces in the cortex. The secondary xylem vessel elements were reduced in diameter and had sinuous walls. The severest damage was visible in the ramification zone, where uncommon division planes of phellogen and cambium cells and disintegration of the parenchyma cells adjacent to lateral roots were observed. The high sensibility of C. cajan to As was confirmed, since it caused severe damages in root growth and anatomy. The main structural markers for As toxicity were the altered root architecture, with the reduction of the elongation zone and increase of ramification zone length, and the root primordia retained within the cortex. Our results show a new approach about As toxicity and indicate that C. cajan is a promising species to be used for bioindication of environmental contamination by As.

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