Plant-specific Rac/Rop small GTPases function as molecular switches for numerous signal transduction events, including defense responses. To understand the function of each of the seven Rac/Rop family members in rice, we studied the tissue-specific expression patterns of Rac/Rop genes by semi-quantitative reverse transcription-PCR (RT-PCR), and also Rac/Rop subcellular localization using green fluorescent protein (GFP) fusion proteins in transient expression systems. We also investigated the roles of these genes in disease resistance by testing single Rac/Rop-RNAi (RNA interference) plants against the rice blast pathogen Magnaporthe grisea. Our studies show that expression of OsRac2, 6 and 7 is very low in leaf blades, and reveal a strong correlation between the number of lysine and/or arginine (KR) residues in the polybasic region of Rac/Rop GTPases and their subcellular distribution in vivo. Infection assays showed that OsRac1 is a positive regulator of blast resistance, confirming previous observations, whereas OsRac4 and OsRac5 are negative regulators of blast resistance. OsRac6 may make minor contributions to disease resistance, while OsRac3 and OsRac7 are probably not involved in defense. Therefore, our study suggests that the rice Rac/Rop family plays multiple roles in diverse cellular activities and has both positive and negative functions in disease resistance.

Root hair is considered to play important roles in water and nutrient uptake and anchoring the plant to the soil. To gain further knowledge of root hair morphogenesis in rice, we isolated root hairless 2 (rth2) mutant from the mutant panel of Nipponbare. Positional cloning and complementation test revealed that the causal gene of rth2 was Cellulose Synthase-Like D1 (OsCSLD1). rth2 has a premature stop codon in exon 1 as a result of two consecutive nucleotide substitutions and is predicted to produce truncated proteins lacking the D, D, D, QxxRW motif and 8 transmembrane domains. In rth2, bulges were normally initiated from asymmetric divisions of root epidermal cells, but bulges did not elongate. Therefore, rth2 shows completely roothairless phenotype. qRT-PCR analysis revealed that OsCSLD1 was expressed not only in root but also in shoot. In situ hybridization showed that OsCSLD1 was expressed not only in root hairs but also in epidermal and cortex cell walls except for stele. Agronomic character evaluation in pot experiments showed that rth2 did not differ significantly from Nipponbare in all characters examined except for root dry weight, which showed a significant increase in rth2. In paddy field experiment, rth2 was significantly inferior compared with Nipponbare in agronomic performance.