Alterations in plant architecture related to plant height, shoot branching, stem elongation, and leaf and inflorescence morphology have been of interest for centuries, as they control several important agronomic traits like yield, physiological and biochemical processes, lodging, and resistance to environmental stress (Khush 1999; Camp 2005; Wang and Li 2006). The negative impact of current global climate change and scarcity of water and nutrients on yield could be minimized by manipulating root system architecture towards a uniform distribution of roots in the soil that efficiently optimizes water and nutrient uptake (de Dorlodot et al., 2007).
The recent studies carried out in model plants, Arabidopsis and Petunia, and crop plants including tomato, maize, and rice served as platforms for understanding the molecular basis of plant architecture (Wang and Li 2006, 2008). Recently, RNAi technology is used to improve crop productivity by modifying plant architecture. Adventitious root emergence and development were significantly inhibited in the OsPIN1 RNA interference (RNAi) transgenic plants, which were similar to the phenotype ofNPA (N-1-Naphthylphthalamic acid, an auxin transport inhibitor)-treated wild-type plants. ?-Naphthylacetic acid (?-NAA) treatment was able to rescue the mutated phenotypes occurring in the RNAi plants. Over-expression or suppression of the OsPIN1 expression through a transgenic approach resulted in changes of tiller numbers and shoot/root ratio suggesting that OsPIN1 played an important role in auxin-dependent adventitious root emergence and tillering (Xuet al., 2005).Endogenous plant hormones, including gibberellins (GAs) and brassinosteroids (BRs), are major plant regulators that control plant height. The genetic manipulations of genes related to GA biosynthesis are major targets to alter plant height (Coles et al.
, 1999; Hedden and Phillips 2000). Zhou et al., (2006) demonstrated the task of interfering RNA to suppress the expression of Os GLU1 gene encoding a membrane-bound endo-1, 4-b-D-glucanase gene in rice (Oryza sativa L.) which affected plant internode elongation causing structural changes in cell walls altering cell wall composition and leading to develop a dwarf phenotype. RNAi was used to suppress the expression of the OsGA20ox2 gene in rice (O.
sativa L.), which encodes the regulatory enzyme GA 20-oxidase, for the synthesis of biologically active gibberellins (GAs) in plants. A phenotypic series of transgenic lines were recovered expressing reduced content of endogenous biologically active GA 1, which decreased plant height and resulted in semi dwarf phenotype. The semi-dwarf lines exhibited shorter stem, lodging-resistant and better productivity as compared to the wild-type plants (Qiaoet al., 2007). In another study, Hu et al., (2009) showed that the expression of rice histone deacetylases (HDAC) genes displays specific expression patterns and divergent developmental functions compared with closely related homologs in Arabidopsis and most of them are responsive to drought or salt stresses.
Overexpression of several rice HDACs did not produce any visible phenotype, whereas down-regulation of a few HDAC genes affected different developmental aspects. Specifically, down-regulation of HDA703 by miRNA reduced rice peduncle elongation and fertility, while inactivation of a closely related homolog HDA710 by RNAi affected vegetative growth. HDA704 RNAi altered plant height and flag leaf morphology. Down regulation of HDT702 led to the production of narrowed leaves and stems.
The feasibility of manipulation of plant architecture using RNAi technology has a wide utility in flowering, ornamental, plantation crops, and forest trees, for example, appropriate easy access for leaves plucking in tea or mulberry plants, harvesting of fruits or seeds from tall trees, and absence of thorns in roses.