Map-based cloning and functional analysis of spotted leaf 3 in rice

Abstract

A rice lesion mimic mutants (LMMs), which display spontaneous cell death on leaf surface without pathogen attack, have been reported in many plants, including maize, barley, Arabidopsis thaliana and rice. Thus, the molecular mechanisms underlying the control of lesion mimic and resistance were very intricate in plants. Mitogen-activated protein kinase (MAPK) is a significant signaling module for reacting to diverse extracellular stimuli in plants. MAPKs have been researched on signaling pathways containing hormone response, development, disease resistance, and abiotic stress. Abscisic acid (ABA) is an important plant hormone in various aspects of plant development. Lesion mimic mutant spotted leaf 3 (spl3) in rice, irradiated by gamma rays, develops cell death spontaneously in leaves. Histochemical analysis revealed that hydrogen peroxide and singlet oxygen were highly accumulated in the spl3 mutant leaves compared with the wild type. we identified one base pair deletion in the first exon of a candidate gene, LOC_Os03g06410, encoding MAPKKK, OsEDR1, which is the sequence ortholog of AtEDR1 (Arabidopsis Enhanced Disease Resistance 1), a putative MAPK kinase kinase. SPL3 transcripts were constantly suppressed in ABA- and SA-treated conditions until 24 HAT, whereas SPL3 transcripts were dramatically up-regulated in the drought-, sucrose- and NaCl-treated conditions with different time points for induction, although SPL3 transcript levels were reduced or not changed at the two HAT. We additionally examined the transcriptional response of SPL3 to cold conditions (4℃) and observed that SPL3 transcript was constantly suppressed as in SA. SPL3 transcripts accumulated highly in leaf sheath and leaf blade, suggesting the possible role of SPL3 gene in the protection of the cells from the generation of necrotic lesion in leaves. Although SPL3 transcripts were considerably altered in these stress conditions, no significant difference was observed in the phenotypic change to the stresses between the wild type and mutant. Under ABA-containing media spl3 mutant had significantly longer primary and adventitious roots in length and more adventitious roots in number compared to those of the wild type

the reduced growth retardation of spl3 mutant was more obvious in 10 µM than 5 µM ABA-containing media. Transcriptional regulations of ABI1 and ABL1 are less sensitive to exogenous ABA treatment in spl3 mutant, whereas ABI3 was hyper sensitive in the mutant. Interestingly, the transcript level of ABI4 was constantly lower in spl3 mutant than in the wild type irrespective of exogenous ABA treatment. Our results suggested that spl3 mutation led the plant response to be less sensitive to ABA by altering transcript levels of various ABI genes in ABA signaling cascades.