Molecular Cloning of Early-expressed cDNAs from Rice in Response to Infection by Rice Blast Fungus Magnaporthe grisea

Compatible and incompatible reactions in rice plants ( Oryza sativa L. cv. Shenxianggen No.4) were resulted from inoculation with two different virulent races of rice blast fungus ( Magnaporthe grisea (Hebert) Barr), and thus an effective infecting system was established between rice plants and the rice blast pathogen. Two cDNA clones that showed induced and temporal patterns in expression in the very early stage in response to infection of the fungus were obtained from the plants by use of differential display. Of the two cDNA clones, Fastresp_a was induced to express in both compatible and incompatible interactions although it was expressed earlier in the former reaction. The second one, Fastresp_b , was only expressed in incompatible interaction. Southern blot analysis of the rice genomic DNA indicated that both of the two clones were from genome of the plant. No significant homology to the two genes was found from the rice gene database. This suggested that they were novel genes in rice and may play important roles in rice resistant response to infection of rice blast fungus.

Local HY5 Activity Mediates Hypocotyl Growth and Shoot-to-Root Communication

Plants optimize their growth in fluctuating environments using information acquired by different organs.This information is then transmitted through the rest of the plant using both short-and long-distance signals,including hormones and mobile proteins.Although a few of these signals have been characterized,long-distance signaling is not well understood in plants.Recently,the light-regulated transcription factor HY5 was reported to move from the shoot to the root to regulate root growth.We generated a cell-type specifically expressed HY5 fusion protein that could not be detected outside the tissue in which it was targeted.By expressing this DOF-HY5 protein in specific cell types of the hypocotyl,we showed that its local activity was sufficient to regulate hypocotyl growth.We also found that,although DOF-HY5 was expressed specifically in the shoot and not detected in the roots,it could rescue hy5 growth defects in primary roots but not in lateral roots.We therefore conclude that HY5 protein mobility is not required in the hypocotyl or for shoot-to-root communication.Our results indicate that a signal downstream of,or in parallel with,HY5 in the shoot is mobile and links shoot and root growth.

Overexpression of the bacterial tryptophan oxidase RebO affects auxin biosynthesis and Arabidopsis development

Both tryptophan （Trp） and auxin are essential for plant growth and Trp is a precursor for auxin biosynthesis. Concentrations of Trp and auxin need to be tightly con- trolled to ensure optimal growth and development. It has been very difficult to study the homeostasis of these two essential and inter-dependent compounds. Auxin is mainly synthesized from Trp via a two-step pathway using indole- 3-pyruvate （IPA） as the intermediate. Here we used a bacterial Trp oxidase RebO, which does not exist in Ara- bidopsis and which converts Trp to the imine form of IPA, to modulate IPA levels in Arabidopsis. Our results demonstrate that Arabidopsis plants use two strategies to ensure that no excess IPA is made from Trp. IPA is made from Trp by the TAA family of aminotransferases, which we show catalyzes the reverse reaction when IPA level is high. Moreover, excess IPA is converted back to Trp by the YAS1 aminotransferase. We show that the VASl-cat- alyzed reaction is very important for Trp homeostasis. This work not only elucidates the intricate biochemical mech- anisms that control the homeostasis of Trp, IPA, and auxin, but also provides novel tools for further biochemical studies on Tip metabolism and auxin biosynthesis in plants.