The regulation of growth and development in Pisum sativum

Physiological changes within a plant during growth and development are under strict regulation, permitting an integrated response to multiple environmental and endogenous stimuli. Amongst these processes are the hormonal regulation of growth and the transition from a vegetative to reproductive state. The focus of this research was to investigate two developmental processes in pea (Pisum sativum): (i) the role of brassinosteroid (BR) phytohormones and (ii) flowering. BRs are essential regulators of plant growth and development that affect a host of molecular, cellular and physiological processes. There is a comprehensive knowledge of BR synthesis, whilst much less is known about the physiological and cellular effects of BRs. The promotive effect of BRs on cell elongation is profound and there is evidence in azuki bean and Arabidopsis that brassinolide can cause microtubule (MT) realignment and rescue MT organisation of BR mutants, respectively. This study augments these findings and has provided novel information on both cortical and epidermal cells. Immunofluorescence showed significant differences in the average MT orientation of cortical cells of mutants versus wild types. Strikingly, these mutants possessed abundant MTs, unlike the BR-deficient bull-1 mutant in Arabidopsis. Following the application of brassinolide to intact plants, both epidermal and cortical cells of BR-synthesis mutants lk and Ikb showed a significant shift in MT orientation towards more transverse, whereas the BR-receptor mutant, lka, showed a small non-significant response. Morphological and physiological studies showed that the photosynthetic abilities of BR mutants could be affected. Although the stomata of lk were smaller, the stomata of BR mutants were not physically blocked as in bull of Arabidopsis. Interestingly, the BR mutation was correlated with a reduced complexity of leaf epidermal cells. To investigate the elevated cell wall yield threshold of pea BR mutants as a potential explanation for the mechanism of BR-mediated cell elongation, a pea yieldin-like gene was cloned. Its expression was strongly regulated during development in lk, Ikb and lka as well as the wild types but no clear effect of a BR deficiency was observed. Pea has been frequently employed as a model legume to study flowering using genetic and physiological approaches, but compared to the detailed model known for Arabidopsis, there is a deficiency of information about the molecular regulation of flowering in pea. Combining molecular and physiological approaches is fundamental for revealing information about the role and interaction of the rapidly growing number of pea flowering genes. This study examined the expression patterns of seven flowering-related genes (COLa, SOC1a, FTL, TFL1aIDET, PIMIAP1, PM6/SEP1/2 and UNI/LFY) during early development of wild type pea seedlings. This information was related to parallel physiological experiments. FTL showed marked up-regulation under long days at the time of floral commitment, while up-regulation of PIM/AP1 and SEP1/2 occurred several days later. These results suggest that FTL is a suitable early marker of floral commitment and may be useful for diagnosing molecular defects in other flowering mutants.