Gene expression related to seed shattering and the cell wall in cultivated and weedy rice.

Seed shattering is an evolutionary trait that is essential to the survival of wild and weedy rice. Discovery of the qSH1 gene in rice subspecies Japonica and Sh4 in the rice subspecies Indica indicated the possibility that seed shattering is governed by major genes in a qualitative manner. However, observation of the large variability of seed shattering in weedy rice has led us to hypothesise that other genes related to abscission layer integrity could also be important in the regulation of seed shattering in rice. Gene expression 10 days after pollination and nucleotide composition revealed that qSH1 and Sh4 that are described as major players in seed shattering were not important in weedy rice. High expression of the gene OsCPL1 was positively associated with the occurrence of high seed shattering in weedy rice, which did not concur in previous studies of cultivated rice. This result is related to the absence of four SNPs and an indel in the OsCPL1 gene in weedy rice that are related to seed shattering in previous studies. Analysis of the expression of six genes related to cell wall synthesis/degradation revealed the importance of the genes OsXTH8 and OsCel9D in seed shattering in weedy rice. Therefore, in addition to qSH1 and Sh4, the genes OsCPL1, OsXTH8 and OsCel9D should be considered in studies of rice evolution and in the development of mitigation approaches of gene flow in transgenic rice.

Genetic divergence among Brazilian garlic cultivars based on morphological characters and AFLP markers.

Outside its centers of origin, garlic propagates only asexually. Since asexual reproduction leads to the absence of meiotic recombination, the main garlic cultivars available for cultivation have arisen from the accumulation of somatic mutations in early cultivars. Thus, it is common for a single clone to have different names in different regions. This study aimed to evaluate the genetic diversity of 20 garlic cultivars by using morphological characters and amplified fragment length polymorphism (AFLP) markers to identify possible duplicate cultivars. We assessed 28 morphological characters related to the leaves, bulbs, and bulbils of the garlic plant and divided them into two categories: quantitative and qualitative (14 characters each). For molecular marker-based analysis, we used three AFLP primer combinations. Genetic divergence was calculated using the Jaccard coefficient; the cultivars were grouped using unweighted pair-group mean analysis. The average genetic divergence detected using the morphological characters was 2.30 (range, 0.45-4.70). Plant height and coat adhesion exhibited the highest divergence among the cultivars. The average genetic diversity based on AFLP data was 43% (range, 0-79%). Dendrograms derived from both techniques divided the cultivars into two groups: noble and semi-noble. Together with the divergence within groups, the correlation between morphological and molecular data suggested that the cultivars in the noble group had greater phenotypic stability than those in the semi-noble group. Analysis of Jonas and Quitéria cultivars using these two techniques revealed only slight differences, suggesting that these cultivars may be clones or have a high degree of kinship.

Attenuation of phosphate starvation responses by phosphite in Arabidopsis.

When inorganic phosphate is limiting, Arabidopsis has the facultative ability to metabolize exogenous nucleic acid substrates, which we utilized previously to identify insensitive phosphate starvation response mutants in a conditional genetic screen. In this study, we examined the effect of the phosphate analog, phosphite (Phi), on molecular and morphological responses to phosphate starvation. Phi significantly inhibited plant growth on phosphate-sufficient (2 mM) and nucleic acid-containing (2 mM phosphorus) media at concentrations higher than 2.5 mM. However, with respect to suppressing typical responses to phosphate limitation, Phi effects were very similar to those of phosphate. Phosphate starvation responses, which we examined and found to be almost identically affected by both anions, included changes in: (a) the root-to-shoot ratio; (b) root hair formation; (c) anthocyanin accumulation; (d) the activities of phosphate starvation-inducible nucleolytic enzymes, including ribonuclease, phosphodiesterase, and acid phosphatase; and (e) steady-state mRNA levels of phosphate starvation-inducible genes. It is important that induction of primary auxin response genes by indole-3-acetic acid in the presence of growth-inhibitory Phi concentrations suggests that Phi selectively inhibits phosphate starvation responses. Thus, the use of Phi may allow further dissection of phosphate signaling by genetic selection for constitutive phosphate starvation response mutants on media containing organophosphates as the only source of phosphorus.

Conditional identification of phosphate-starvation-response mutants in Arabidopsis thaliana.

Plants have evolved elaborate metabolic and developmental adaptations to low phosphorus availability. Biochemical responses to phosphate limitation include increased production and secretion of phosphate-acquisition proteins such as nucleases, acid phosphatases, and high-affinity phosphate transporters. However, the signal transduction pathways that sense phosphate availability and integrate the phosphate-starvation response in plants are unknown. We have devised a screen for conditional mutants in Arabidopsis thaliana (L.) Heynh. to dissect signaling of phosphate limitation. Our genetic screen is based on the facultative ability of wild-type Arabidopsis plants to metabolize exogenous DNA when inorganic phosphate is limiting. After screening 50,000 M2 seedlings, we isolated 22 confirmed mutant lines that showed severely impaired growth on medium containing DNA as the only source of phosphorus, but which recovered on medium containing soluble inorganic phosphate. Characterization of nine such mutant lines demonstrated an inability to utilize either DNA or RNA. One mutant line, psr1 (phosphate starvation response), had significantly reduced activities of phosphate-starvation-inducible isoforms of ribonuclease and acid phosphatase under phosphate-limiting conditions. The data suggest that a subset of the selected mutations impairs the expression of more than one phosphate-starvation-inducible enzyme required for utilization of exogenous nucleic acids, and may thus affect regulatory components of a Pi starvation response pathway in higher plants.