LAZY1 controls rice shoot gravitropism through regulating polar auxin transport

Leaf expansion in grasses under salt stress

Restriction of leaf growth is among the earliest visible effects of many stress conditions, including salinity. Because leaves determine radiation interception and are the main photosynthetic organs, salinity effects on leaf expansion and function are directly related to yield constraints under saline conditions. The expanding zone of leaf blades spans from the meristem to the region in which cells reach their final length. Kinematic methods are used to describe cell division and cell expansion activities. Analyses of this type have indicated that the reduction in leaf expansion by salinity may be exerted through effects on both cell division and expansion. In turn, the components of vacuole-driven cell expansion may be differentially affected by salinity, and examination of salinity effects on osmotic and mechanical constraints to cell expansion have gradually led to the identification of the gene products involved in such control. The study of how reactive oxygen species affect cell expansion is an emerging topic in the study of salinity's regulation of leaf growth.

DWARF27, an iron-containing protein required for the biosynthesis of strigolactones, regulates rice tiller bud outgrowth

Tillering in rice (Oryza sativa) is one of the most important agronomic traits that determine grain yields. Previous studies on rice tillering mutants have shown that the outgrowth of tiller buds in rice is regulated by a carotenoid-derived MAX/RMS/D (more axillary branching) pathway, which may be conserved in higher plants. Strigolactones, a group of terpenoid lactones, have been recently identified as products of the MAX/RMS/D pathway that inhibits axillary bud outgrowth. We report here the molecular genetic characterization of d27, a classic rice mutant exhibiting increased tillers and reduced plant height. D27 encodes a novel iron-containing protein that localizes in chloroplasts and is expressed mainly in vascular cells of shoots and roots. The phenotype of d27 is correlated with enhanced polar auxin transport. The phenotypes of the d27 d10 double mutant are similar to those of d10, a mutant defective in the ortholog of MAX4/RMS1 in rice. In addition, 2'-epi-5-deoxystrigol, an identified strigolactone in root exudates of rice seedlings, was undetectable in d27, and the phenotypes of d27 could be rescued by supplementation with GR24, a synthetic strigolactone analog. Our results demonstrate that D27 is involved in the MAX/RMS/D pathway, in which D27 acts as a new member participating in the biosynthesis of strigolactones.

Control of a key transition from prostrate to erect growth in rice domestication

The transition from the prostrate growth of ancestral wild rice (O. rufipogon Griff.) to the erect growth of Oryza sativa cultivars was one of the most critical events in rice domestication. This evolutionary step importantly improved plant architecture and increased grain yield. Here we find that prostrate growth of wild rice from Yuanjiang County in China is controlled by a semi-dominant gene, PROG1 (PROSTRATE GROWTH 1), on chromosome 7 that encodes a single Cys(2)-His(2) zinc-finger protein. prog1 variants identified in O. sativa disrupt the prog1 function and inactivate prog1 expression, leading to erect growth, greater grain number and higher grain yield in cultivated rice. Sequence comparison shows that 182 varieties of cultivated rice, including 87 indica and 95 japonica cultivars from 17 countries, carry identical mutations in the prog1 coding region that may have become fixed during rice domestication.

Genetic control of rice plant architecture under domestication

The closely related wild rice species Oryza rufipogon is considered the progenitor of cultivated rice (Oryza sativa). The transition from the characteristic plant architecture of wild rice to that of cultivated rice was one of the most important events in rice domestication; however, the molecular basis of this key domestication transition has not been elucidated. Here we show that the PROG1 gene controls aspects of wild-rice plant architecture, including tiller angle and number of tillers. The gene encodes a newly identified zinc-finger nuclear transcription factor with transcriptional activity and is mapped on chromosome 7. PROG1 is predominantly expressed in the axillary meristems, the site of tiller bud formation. Rice transformation experiments demonstrate that artificial selection of an amino acid substitution in the PROG1 protein during domestication led to the transition from the plant architecture of wild rice to that of domesticated rice.

Genetic modification of plant architecture and variety improvement in rice

The structure of the aerial part of a plant, referred to as plant architecture, is subject to strict genetic control, and grain production in cereal crops is governed by an array of agronomic traits. Rice is one of the most important cereal crops and is also a model plant for molecular biological research. Recently, significant progress has been made in isolating and collecting rice mutants that exhibit altered plant architecture. In this article we summarize the recent progress in understanding the basic patterning mechanisms involved in the regulation of tillering (branching) pattern, stem structure and leaf arrangement in rice plants. We discuss the relationship between the genetic modification of plant architecture and the improvement of pivotal agronomic traits in rice.

Molecular basis of plant architecture

Higher plants display a variety of architectures that are defined by the degree of branching, internodal elongation, and shoot determinancy. Studies on the model plants of Arabidopsis thaliana and tomato and on crop plants such as rice and maize have greatly strengthened our understanding on the molecular genetic bases of plant architecture, one of the hottest areas in plant developmental biology. The identification of mutants that are defective in plant architecture and characterization of the corresponding and related genes will eventually enable us to elucidate the molecular mechanisms underlying plant architecture. The achievements made so far in studying plant architecture have already allowed us to pave a way for optimizing the plant architecture of crops by molecular design and improving grain productivity.

TAC1, a major quantitative trait locus controlling tiller angle in rice

A critical step during rice (Oryza sativa) cultivation is dense planting: a wider tiller angle will increase leaf shade and decrease photosynthesis efficiency, whereas a narrower tiller angle makes for more efficient plant architecture. The molecular basis of tiller angle remains unknown. This research demonstrates that tiller angle is controlled by a major quantitative trait locus, TAC1 (Tiller Angle Control 1). TAC1 was mapped to a 35-kb region on chromosome 9 using a large F(2) population from crosses between an indica rice, IR24, which displays a relatively spread-out plant architecture, and an introgressed line, IL55, derived from japonica rice Asominori, which displays a compact plant architecture with extremely erect tillers. Genetic complementation further identified the TAC1 gene, which harbors three introns in its coding region and a fourth 1.5-kb intron in the 3'-untranslated region. A mutation in the 3'-splicing site of this 1.5-kb intron from 'AGGA' to 'GGGA' decreases the level of tac1, resulting in a compact plant architecture with a tiller angle close to zero. Further sequence verification of the mutation in the 3'-splicing site of the 1.5-kb intron revealed that the tac1 mutation 'GGGA' was present in 88 compact japonica rice accessions and TAC1 with 'AGGA' was present in 21 wild rice accessions and 43 indica rice accessions, all with the spread-out form, indicating that tac1 had been extensively utilized in densely planted rice grown in high-latitude temperate areas and at high altitudes where japonica rice varieties are widely cultivated.