UC Riverside

Rice architecture has historically been manipulated to increase grain yields. Creating rice plants with an erect stature caused by upright leaves allows for higher-density planting. In rice, the plant hormone Brassinosteroid (BR) contributes to shoot architecture, mainly controlling plant height and leaf erectness. In Arabidopsis, LATERAL ORGAN BOUNDARIES (LOB) is expressed in organ boundaries and regulates organ separation by modulating BR accumulation. LOB is positively regulated by BRs and LOB activates expression of BAS1, which inactivates BR. This LOB-BR feedback loop limits growth in the organ boundaries. This dissertation focuses on characterizing LOB orthologs in rice and investigates the use of LOB and BAS1 to manipulate local BR accumulation in the rice, with a goal of creating plants with an erect stature.

Chapter 1 investigates the site-specific expression of AtLOB in rice to manipulate BR accumulation. In rice, the AtLOB promoter drove expression in organ boundaries and lamina joints and was used to drive expression of AtLOB to modulate BR accumulation. AtpLOB:LOB plants had altered leaf shape and decreased leaf inclination, characteristics associated with BR-deficiency. These results suggest that AtLOB expression in a distinct

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domain alters leaf morphology without other changes in shoot architecture caused by BR deficiency.

Chapter 2 examines the expression of AtBAS1 under the LOB promoter in rice to repress BR signaling. AtpLOB:BAS1 plants displayed characteristics associated with both reduced BR and enhanced BR signaling. AtpLOB:BAS1 plants had upright leaves as well as elongated inflorescences with increase branching and long, slender grains. These results suggest that localized AtBAS1 expression in rice alters leaf inclination and inflorescence architecture.

In Chapter 3, a genome-wide analysis of the rice LBD gene family was conducted to determine which OsLBDs are orthologous to AtLOB. Through phylogenetic analyses identified the OsLBD class Ia genes, OsRa2, OsIG1, OsLOB4, and OsIAL1, which had high sequence similarity to AtLOB protein sequence. All class Ia OsLBDs were expressed in lamina joints, inflorescences, and floral organs, suggesting that class Ia OsLBDs may have similar functions to AtLOB. Using BR hormone response data, class Ia OsLBDs were not BR regulated, suggesting they may not function in BR signaling.