Investigation of the role of cell-cell interactions in division plane determination during maize leaf development through mosaic analysis of the tangled mutation

Cellular perspectives for improving mesophyll conductance

After entering the leaf, CO2 faces an intricate pathway to the site of photosynthetic fixation embedded within the chloroplasts. The efficiency of CO2 flux is hindered by a number of structural and biochemical barriers which, together, define the ease of flow of the gas within the leaf, termed mesophyll conductance. Previous authors have identified the key elements of this pathway, raising the prospect of engineering the system to improve CO2 flux and, thus, to increase leaf photosynthetic efficiency. In this review, we provide a perspective on the potential for improving the individual elements that contribute to this complex parameter. We lay particular emphasis on generation of the cellular architecture of the leaf which sets the initial boundaries of a number of mesophyll conductance parameters, incorporating an overview of the molecular transport processes which have been proposed as major facilitators of CO2 flux across structural boundaries along the pathway. The review highlights the research areas where future effort might be invested to increase our fundamental understanding of mesophyll conductance and leaf function and, consequently, to enable translation of these findings to improve the efficiency of crop photosynthesis.

Multicellular genesis of leaf primordium was demonstrated via chimaeric transgenic plant of maize (Zea mays L.) regenerated from Type II calli

Type-II embryonic calli were induced from immature embryos of maize (Zea mays L.) genotype YD and bombarded with beta-glucuronidase gene. Bombarded calli were proliferated on normal N6 medium for 2 weeks at 26°C in the dark and selected on N6 medium containing 1 mg/l 2,4-dichlorophenoxyacetic acid (2,4-D) and 5 mg/l phosphinothricin (PPT) but without casamino acids and proline under the same conditions for 14 days. Regeneration was carried out on hormone-free MS medium containing 5 mg/l phosphinothricin at 26°C under 3000 lux illumination. Plants over 8 cm were transplanted into soil and sprayed with 250 mg/l phosphinothricin when two new leaves appeared. Except normal transgenic plants, chimaeric transgenics also were regenerated in the present work. The expression pattern of beta-glucuronidase gene in leaves of chimaeric transgenic plant revealed that more than one cell formed leaf primordium at the initial stage, and filial cells stemed from each cell in leaf primordium arranged in a row longitudinally from leaf base to leaf apex. There was a clear boundary as a straight line between the area formed by transformed cells and the area formed by normal cells. A hypothesis was put forward that the primitive cells in leaf primordium divided in a longitudinal style, resulted in leaf elongation, then the filial cells divided transversally and synchronously toward the outside to broaden the leaf.

MAP65-3 microtubule-associated protein is essential for nematode-induced giant cell ontogenesis in Arabidopsis

The infection of plants by obligate parasitic nematodes constitutes an interesting model for investigating plant cytoskeleton functions. Root knot nematodes have evolved the ability to manipulate host functions to their own advantage by redifferentiating root cells into multinucleate and hypertrophied feeding cells. These giant cells result from repeated rounds of karyokinesis without cell division. Detailed functional analyses demonstrated that Arabidopsis thaliana Microtubule-Associated Protein65-3 (MAP65-3) was essential for giant cell ontogenesis and that cytokinesis was initiated but not completed in giant cells. In developing giant cells, MAP65-3 was associated with a novel kind of cell plate-the giant cell mini cell plate-that separates daughter nuclei. In the absence of functional MAP65-3, giant cells developed but failed to fully differentiate and were eventually destroyed. These defects in giant cells impaired the maturation of nematode larvae. Thus, MAP65-3 is essential for giant cell development during root knot nematode infection. Subcellular localization of MAP65-3 and analysis of microtubule organization in the dyc283 T-DNA map65-3 mutant demonstrated that MAP65-3 played a critical role in organizing the mitotic microtubule array during both early and late mitosis in all plant organs. Here, we propose a model for the role of MAP65-3 in giant cell ontogenesis.

tie-dyed1 Functions non-cell autonomously to control carbohydrate accumulation in maize leaves

The tie-dyed1 (tdy1) mutant of maize (Zea mays) produces chlorotic, anthocyanin-accumulating regions in leaves due to the hyperaccumulation of carbohydrates. Based on the nonclonal pattern, we propose that the accumulation of sucrose (Suc) or another sugar induces the tdy1 phenotype. The boundaries of regions expressing the tdy1 phenotype frequently occur at lateral veins. This suggests that lateral veins act to limit the expansion of tdy1 phenotypic regions by transporting Suc out of the tissue. Double mutant studies between tdy1 and chloroplast-impaired mutants demonstrate that functional chloroplasts are needed to generate the Suc that induces the tdy1 phenotype. However, we also found that albino cells can express the tdy1 phenotype and overaccumulate Suc imported from neighboring green tissues. To characterize the site and mode of action of Tdy1, we performed a clonal mosaic analysis. In the transverse dimension, we localized the function of Tdy1 to the innermost leaf layer. Additionally, we determined that if this layer lacks Tdy1, Suc can accumulate, move into adjacent genetically wild-type layers, and induce tdy1 phenotypic expression. In the lateral dimension, we observed that a tdy1 phenotypic region did not reach the mosaic sector boundary, suggesting that wild-type Tdy1 acts non-cell autonomously and exerts a short-range compensatory effect on neighboring mutant tissue. A model proposing that Tdy1 functions in the vasculature to sense high concentrations of sugar, up-regulate Suc transport into veins, and promote tissue differentiation and function is discussed.

Microtubule cytoskeleton: a track record

The plant microtubule cytoskeleton forms unique arrays during cell division and morphogenesis. Recent studies have addressed the biogenesis, turnover, spatio-temporal organisation and cellular function of microtubules. The results suggest that both conserved eukaryotic mechanisms and plant-specific modifications determine microtubule dynamics and function.